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Lesson 06 - App Architecture (Persistence)

٢٢ مايو ٢٠٢٢47 دقائق للقراءة

Lesson 06: App Architecture (Persistence)

This lesson is all about data persistence, there are many ways to storing data permanently, but with Room things have become even easier than ever before! We will be building Track My Sleep Quality app.

Most apps have data that needs to be kept, even after the user closes the app. For example, the app might store a playlist, an inventory of game items, records of expenses and income, a catalog of constellations, or sleep data over time. Commonly, you would use a database to store persistent data.Most apps have data that needs to be kept, even after the user closes the app. For example, the app might store a playlist, an inventory of game items, records of expenses and income, a catalog of constellations, or sleep data over time. Commonly, you would use a database to store persistent data.

Tour of the App

  • The app will show the UI for the SleepTrackerFragment, but no data, and the buttons do not respond to clicks.
  • Notice the provided dependencies in module gradle filefor all the Android Jetpack libraries, including Room, and the dependencies for coroutines.
// Room and Lifecycle dependencies
implementation "androidx.room:room-runtime:$version_room"
kapt "androidx.room:room-compiler:$version_room"
implementation "androidx.lifecycle:lifecycle-extensions:$version_lifecycle_extensions"
 
// Coroutines
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$version_coroutine"
implementation "org.jetbrains.kotlinx:kotlinx-coroutines-android:$version_coroutine"
 
// Kotlin Extensions and Coroutines support for Room
implementation "androidx.room:room-ktx:$version_room"
  • The app is structured by functionality. There is one package for all database code. In the starter app, it contains a placeholder file for defining sleep data.
  • The sleepquality and sleeptracker packages contain the provided fragments, and you will add most of your code in those two packages.
  • The Util.kt file contains functions to help display sleep quality data. Some code is commented out because it references a ViewModel, which you will create later.
  • androidTest folder/SleepDatabaseTest.kt will be used to verify that the database is working as intended.

SQL

This course assumes that you are familiar with databases in general, SQL databases in particular, and the SQL language used to interact with them. This page is a refresher and quick reference about SQLite.

Designing Entities

In Android, data is represented in data classes, and the data is accessed and modified using function calls. However, in the database world, you need entities and queries.

Entity:

  • An entity represents an object or concept, and its properties, to store in the database. An entity class defines a table, and each instance of that class represents a row in the table.
  • Each property defines a column. In your app, the entity is going to hold information about a night of sleep.

Query:

  • A query is a request for data or information from a database table or combination of tables, or a request to perform an action on the data.
  • Common queries are for getting, inserting, and updating entities. For example, you could query for all the sleep nights on record, sorted by start time.

Room does all the hard work for you to get from Kotlin data classes to entities that can be stored in SQLite tables, and from function declarations to SQL queries.

You must define each entity as an annotated data class, and the interactions as an annotated interface, a data access object (DAO). Room uses these annotated classes to create tables in the database, and queries that act on the database.

DAO.png

  1. In the database package, find and open the SleepNight.kt file.
  2. Create the SleepNight data class with parameters for an ID, start time and end time in milliseconds, and a numerical sleep quality rating:
data class SleepNight  (
    var nightId: Long = 0L,
    val startTimeMilli: Long =  System.currentTimeMillis(),
    var endTimeMilli: Long = startTimeMilli,
    var sleepQuality: Int = -1
)

Note: currentTimeMillis method returns the current time in milliseconds. It can be formatted using SimpleDateFormat. For example:

SimpleDateFormat("EEEE MMM-dd-yyyy' Time: 'HH:mm").format(systemTime).toString()
  1. Annotate the data class with @Entity, and name the table daily_sleep_quality_table. (Remember to perform the necessary imports for this and all the following annotations.)
@Entity(tableName = "daily_sleep_quality_table")
  1. Identify the nightId as the primary key by annotating it with @PrimaryKey, and set the autoGenerate parameter to true:
@PrimaryKey(autoGenerate = true)
  1. Annotate the remaining properties with ColumnInfo @ColumnInfo and customize their names as shown below.
import androidx.room.ColumnInfo
import androidx.room.Entity
import androidx.room.PrimaryKey
 
@Entity(tableName = "daily_sleep_quality_table")
data class SleepNight(
    @PrimaryKey(autoGenerate = true)
    var nightId: Long = 0L,
 
    @ColumnInfo(name = "start_time_milli")
    val startTimeMilli: Long = System.currentTimeMillis(),
 
    @ColumnInfo(name = "end_time_milli")
    var endTimeMilli: Long = startTimeMilli,
 
    @ColumnInfo(name = "quality_rating")
    var sleepQuality: Int = -1
)

Data Access Object (DAO)

  • When you use a Room database, you query the database by defining and calling Kotlin functions in your code. These Kotlin functions map to SQL queries. You define those mappings in a DAO using annotations, and Room creates the necessary code.
  • Think of a DAO as defining a custom interface for accessing your database.
  • For common database operations, the Room library provides convenience annotations, such as @Insert, @Delete, and @Update. For everything else, there is the @Query annotation. You can write any query that’s supported by SQLite.
  • As an added bonus, as you create your queries in Android Studio, the compiler checks your SQL queries for syntax errors.

For the sleep-tracker database of sleep nights, you need to be able to do the following:

  • Insert new nights.
  • Update an existing night to update an end time and a quality rating.
  • Get a specific night based on its key.
  • Get all nights, so you can display them.
  • Get the most recent night. when we don’t have a key that matches.
  • Delete all entries in the database.

Creating the SleepDatabase DAO

  1. In the database package, open SleepDatabaseDao.kt. Create an interface SleepDatabaseDao and annotate it with @Dao.
@Dao
interface SleepDatabaseDao {}
  1. Inside the body of the interface, add an @Insert annotation. Below the @Insert, add an insert() function that takes an instance of the Entity class SleepNight as its argument. Room will generate all the necessary code to insert the passed-in SleepNight into the database. Note that you can call the function anything you want.
@Insert
fun insert(night: SleepNight)
  1. Add an @Update annotation with an update() function for one SleepNight. The entity that’s updated is the entity that has the same key as the one that’s passed in. You can update some or all of the entity’s other properties.
@Update
fun update(night: SleepNight)
  1. Add a @Query annotation with a get() function that takes a Long key argument and returns a nullable SleepNight. Add a parameter to @Query. Make it a String that is a SQLite query that selects all columns from the daily_sleep_quality_table WHERE the nightId matches the :key argument.
@Query("SELECT * from daily_sleep_quality_table WHERE nightId = :key")
fun get(key: Long): SleepNight?
  1. Add another @Query with a clear() function and a SQLite query to DELETE everything from the daily_sleep_quality_table. This query does not delete the table itself.
@Query("DELETE FROM daily_sleep_quality_table")
fun clear()

Note: The @Delete annotation deletes one item, and you could use @Delete and supply a list of nights to delete. The drawback is that you need to fetch or know what’s in the table. The @Delete annotation is great for deleting specific entries, but not efficient for clearing all entries from a table.

  1. Add a @Query with a getTonight() function. Make the SleepNight returned by getTonight() nullable, so that the function can handle the case where the table is empty. (The table is empty at the beginning, and after the data is cleared.)
    • You get tonight by writing a SQLite query that returns the first element of a list of results ordered by nightId in descending order. Use LIMIT 1 to return only one element.
@Query("SELECT * FROM daily_sleep_quality_table ORDER BY nightId DESC LIMIT 1")
fun getTonight(): SleepNight?
  1. Add a @Query with a getAllNights() function:
    • The SQLite query should return all columns from the daily_sleep_quality_table, ordered in descending order. Let getAllNights() return a list of SleepNight as LiveData. Room keeps this LiveData updated for us, and we don’t have to specify an observer for it.
    • You may need to import LiveData from androidx.lifecycle.LiveData.
@Query("SELECT * FROM daily_sleep_quality_table ORDER BY nightId DESC")
fun getAllNights(): LiveData<List<SleepNight>>
Suspend functions and LiveData in DAO

Note that functions that return LiveData in the DAO can’t use suspend keyword because it will cause compile-time errors.

Creating a Room Database

You need to create an abstract database holder class, annotated with @Database. This class has one method that either creates an instance of the database if the database doesn’t exist, or returns a reference to an existing database.

Getting a Room database is a bit involved, so here’s the general process before you start with the code:

  • Create a public abstract class that extends RoomDatabase. This class is to act as a database holder. The class is abstract, because Room creates the implementation for you.
  • Annotate the class with @Database. In the arguments, declare the entities for the database and set the version number.
  • Inside a companion object, define an abstract method or property that returns a SleepDatabaseDao. Room will generate the body for you.
  • You only need one instance of the Room database for the whole app, so make the RoomDatabase a singleton.
  • Use Room’s database builder to create the database only if the database doesn’t exist. Otherwise, return the existing database.

Tip: The code will be much the same for any Room database, so you can use this code as a template.

  1. In the database package, open SleepDatabase.kt and create an abstract class called SleepDatabase that extends RoomDatabase.Annotate the class with @Database and supply the SleepNight as the only item with the list of entities, set the version as 1, and set exportSchema to false, so as not to keep schema version history backups. Note: Whenever you change the schema, you’ll have to increase the version number.
@Database(entities = [SleepNight::class], version = 1, exportSchema = false)
abstract class SleepDatabase : RoomDatabase() {
}
  1. The database needs to know about the DAO. Inside the body of the class, declare an abstract value that returns the SleepDatabaseDao. You can have multiple DAOs.
abstract val sleepDatabaseDao: SleepDatabaseDao
  1. Below that, define a companion object. The companion object allows clients to access the methods for creating or getting the database without instantiating the class. Since the only purpose of this class is to provide a database, there is no reason to ever instantiate it.
companion object {}
  1. Inside the companion object, declare a private nullable variable INSTANCE for the database and initialize it to null. The INSTANCE variable will keep a reference to the database, once one has been created. This helps you avoid repeatedly opening connections to the database, which is expensive.
  • Annotate INSTANCE with @Volatile. The value of a volatile variable will never be cached, and all writes and reads will be done to and from the main memory. This helps make sure the value of INSTANCE is always up-to-date and the same to all execution threads. It means that changes made by one thread to INSTANCE are visible to all other threads immediately, and you don’t get a situation where, say, two threads each update the same entity in a cache, which would create a problem.
@Volatile
private var INSTANCE: SleepDatabase? = null
  1. Below INSTANCE, still inside the companion object, define a getInstance()method with a Context parameter that the database builder will need. Return a type SleepDatabase. You’ll see an error because getInstance() isn’t returning anything yet.
fun getInstance(context: Context): SleepDatabase {}
  1. Inside getInstance(), add a synchronized{} block. Pass in this so that you can access the context.

Note: Multiple threads can potentially ask for a database instance at the same time, resulting in two databases instead of one. This problem is not likely to happen in this sample app, but it’s possible for a more complex app. Wrapping the code to get the database into synchronized means that only one thread of execution at a time can enter this block of code, which makes sure the database only gets initialized once.

synchronized(this) {}
  1. Inside the synchronized block, copy the current value of INSTANCE to a local variable instance. This is to take advantage of smart cast, which is only available to local variables.
var instance = INSTANCE
  1. Inside the synchronized block, return instance at the end of the synchronized block. Ignore the return type mismatch error; you won’t ever return null once you are done.
return instance
  1. Above the return statement, add an if statement to check whether instance is null, that is, there is no database yet. If instance is null, use the database builder to get a database. In the body of the if statement, invoke Room.databaseBuilder and supply the context that you passed in, the database class, and a name for the database, sleep_history_database. To remove the error, you’ll have to add a migration strategy and build() in the following steps.
instance = Room.databaseBuilder(
        context.applicationContext,
        SleepDatabase::class.java,
        "sleep_history_database"
)
  1. Add the required migration strategy to the builder. Use .fallbackToDestructiveMigration(). Normally, you would have to provide a migration object with a migration strategy for when the schema changes. A migration object is an object that defines how you take all rows with the old schema and convert them to rows in the new schema, so that no data is lost. Migration is beyond the scope of this course. A simple solution is to destroy and rebuild the database, which means that the data is lost.
.fallbackToDestructiveMigration()
  1. Finally, call .build().
.build()
  1. Assign INSTANCE = instance as the final step inside the if statement.
INSTANCE = instance

Your final code should look like this:

import android.content.Context
import androidx.room.Database
import androidx.room.Room
import androidx.room.RoomDatabase
 
/**
 * A database that stores SleepNight information.
 * And a global method to get access to the database.
 *
 * This pattern is pretty much the same for any database,
 * so you can reuse it.
 */
@Database(entities = [SleepNight::class], version = 1, exportSchema = false)
abstract class SleepDatabase : RoomDatabase() {
 
    /**
     * Connects the database to the DAO.
     */
    abstract val sleepDatabaseDao: SleepDatabaseDao
 
    /**
     * Define a companion object, this allows us to add functions on the SleepDatabase class.
     *
     * For example, clients can call `SleepDatabase.getInstance(context)` to instantiate
     * a new SleepDatabase.
     */
    companion object {
        /**
         * INSTANCE will keep a reference to any database returned via getInstance.
         *
         * This will help us avoid repeatedly initializing the database, which is expensive.
         *
         *  The value of a volatile variable will never be cached, and all writes and
         *  reads will be done to and from the main memory. It means that changes made by one
         *  thread to shared data are visible to other threads.
         */
        @Volatile
        private var INSTANCE: SleepDatabase? = null
 
        /**
         * Helper function to get the database.
         *
         * If a database has already been retrieved, the previous database will be returned.
         * Otherwise, create a new database.
         *
         * This function is threadsafe, and callers should cache the result for multiple database
         * calls to avoid overhead.
         *
         * This is an example of a simple Singleton pattern that takes another Singleton as an
         * argument in Kotlin.
         *
         * To learn more about Singleton read the wikipedia article:
         * https://en.wikipedia.org/wiki/Singleton_pattern
         *
         * @param context The application context Singleton, used to get access to the filesystem.
         */
        fun getInstance(context: Context): SleepDatabase {
            // Multiple threads can ask for the database at the same time, ensure we only initialize
            // it once by using synchronized. Only one thread may enter a synchronized block at a
            // time.
            synchronized(this) {
                // Copy the current value of INSTANCE to a local variable so Kotlin can smart cast.
                // Smart cast is only available to local variables.
                var instance = INSTANCE
                // If instance is `null` make a new database instance.
                if (instance == null) {
                    instance = Room.databaseBuilder(
                            context.applicationContext,
                            SleepDatabase::class.java,
                            "sleep_history_database"
                    )
                            // Wipes and rebuilds instead of migrating if no Migration object.
                            // Migration is not part of this lesson. You can learn more about
                            // migration with Room in this blog post:
                            // https://medium.com/androiddevelopers/understanding-migrations-with-room-f01e04b07929
                            .fallbackToDestructiveMigration()
                            .build()
                    // Assign INSTANCE to the newly created database.
                    INSTANCE = instance
                }
                // Return instance; smart cast to be non-null.
                return instance
            }
        }
    }
}

You now have all the building blocks for working with your Room database. This code compiles and runs, but you have no way of telling if it actually works. So, this is a good time to add some basic tests.

Testing the Room Database

In this step, you run provided tests to verify that your database works. This helps ensure that the database works before you build onto it. The provided tests are basic. For a production app, you would exercise all of the functions and queries in all the DAOs.

The starter app contains an androidTest folder. This androidTest folder contains unit tests that involve Android instrumentation, which is a fancy way of saying that the tests need the Android framework, so you need to run the tests on a physical or virtual device. Of course, you can also create and run pure unit tests that do not involve the Android framework.

In the androidTest folder, open the SleepDatabaseTest file. Uncomment the code. Right-click on the test file in the Project view and choose Run. After the app builds and runs, verify in the SleepDatabaseTest pane that all the tests have passed.

Here’s a quick run-through of the testing code, because it’s another piece of code that you can reuse:

  • SleepDabaseTest is a test class.
  • The @RunWith annotation identifies the test runner, which is the program that sets up and executes the tests.
  • During setup, the function annotated with @Before is executed, and it creates an in-memory SleepDatabase with the SleepDatabaseDao. “In-memory” means that this database is not saved on the file system and will be deleted after the tests run.
  • Also when building the in-memory database, the code calls another test-specific method, allowMainThreadQueries. By default, you get an error if you try to run queries on the main thread. This method allows you to run tests on the main thread, which you should only do during testing.
  • In a test method annotated with @Test, you create, insert, and retrieve a SleepNight, and assert that they are the same. If anything goes wrong, throw an exception. In a real test, you would have multiple @Test methods.
  • When testing is done, the function annotated with @After executes to close the database.

Displaying Sleep Data

Open activity_main.xml. This layout contains the nav_host_fragment. Also, notice the merge tag. The merge tag can be used to eliminate redundant layouts when including layouts, and it’s a good idea to use it. You can find more information about it here.

<merge xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
 
    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:navGraph="@navigation/navigation" />
 
</merge>

Adding A ViewModel

Now that you have a database and a UI, you need to collect data, add the data to the database, and display the data. All this work is done in the view model. Your sleep-tracker view model will handle button clicks, interact with the database via the DAO, and provide data to the UI via LiveData. All database operations will have to be run away from the main UI thread, and you’ll do that using coroutines.

  1. In the sleeptracker package, open SleepTrackerViewModel.kt. Inspect the provided SleepTrackerViewModel class definition that extends AndroidViewModel(). This class is the same as ViewModel, but it takes the application context as a parameter and makes it available as a property. You are going to need this later on to access resources. The ViewModel needs access to the data in the database, so pass in an instance of the SleepDatabaseDao. And then pass this up to the super class as well.
class SleepTrackerViewModel(
       val database: SleepDatabaseDao,
       application: Application) : AndroidViewModel(application) {
}
  1. In the sleeptracker package, open **SleepTrackerViewModelFactory.kt.

  • The provided SleepTrackerViewModelFactory takes the same argument as the ViewModel and extends ViewModelProvider.Factory.

  • Inside the factory, the code overrides create(), which takes any class type as an argument and returns a ViewModel.

  • In the body of create(), the code checks that there is a SleepTrackerViewModel class available, and if there is, returns an instance of it. Otherwise, the code throws an exception.

Tip: This is mostly boilerplate code, so you can reuse the code for future view-model factories.

class SleepTrackerViewModelFactory(
       private val dataSource: SleepDatabaseDao,
       private val application: Application) : ViewModelProvider.Factory {
   @Suppress("unchecked_cast")
   override fun <T : ViewModel?> create(modelClass: Class<T>): T {
       if (modelClass.isAssignableFrom(SleepTrackerViewModel::class.java)) {
           return SleepTrackerViewModel(dataSource, application) as T
       }
       throw IllegalArgumentException("Unknown ViewModel class")
   }
}
  1. In the SleepTrackerFragment, get a reference to the application context. Put the reference in onCreateView(), below binding. You need a reference to the app that this fragment is attached to, to pass into the view-model factory provider.
val application = requireNotNull(this.activity).application

Note: The requireNotNull Kotlin function throws an IllegalArgumentException if the value is null.

  1. You need a reference to your data source via a reference to the DAO. In onCreateView(), before the return, define a dataSource. To get a reference to the DAO of the database, use SleepDatabase.getInstance(application).sleepDatabaseDao.
val dataSource = SleepDatabase.getInstance(application).sleepDatabaseDao
  1. In onCreateView(), before the return, create an instance of the viewModelFactory. You need to pass it dataSource and the application. Now that you have a factory, get a reference to the SleepTrackerViewModel. The SleepTrackerViewModel::class.java parameter refers to the runtime Java class of this object.
val viewModelFactory = SleepTrackerViewModelFactory(dataSource, application)
val sleepTrackerViewModel = ViewModelProvider(this, viewModelFactory).get(SleepTrackerViewModel::class.java)

Your finished code should look like this:

// Create an instance of the ViewModel Factory.
val dataSource = SleepDatabase.getInstance(application).sleepDatabaseDao
val viewModelFactory = SleepTrackerViewModelFactory(dataSource, application)
 
// Get a reference to the ViewModel associated with this fragment.
val sleepTrackerViewModel =
       ViewModelProvider(
               this, viewModelFactory).get(SleepTrackerViewModel::class.java)

And the onCreateView() method so far should look like:

/**
    * Called when the Fragment is ready to display content to the screen.
    *
    * This function uses DataBindingUtil to inflate R.layout.fragment_sleep_quality.
    */
override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?,
                            savedInstanceState: Bundle?): View {
 
    // Get a reference to the binding object and inflate the fragment views.
    val binding: FragmentSleepTrackerBinding = DataBindingUtil.inflate(
            inflater, R.layout.fragment_sleep_tracker, container, false)
 
    val application = requireNotNull(this.activity).application
 
    val dataSource = SleepDatabase.getInstance(application).sleepDatabaseDao
 
    val viewModelFactory = SleepTrackerViewModelFactory(dataSource, application)
 
    val sleepTrackerViewModel =
            ViewModelProvider(
                    this, viewModelFactory).get(SleepTrackerViewModel::class.java)
 
    return binding.root
}

Data Binding for ViewModel With the basic ViewModel in place, you need to finish setting up data binding in the SleepTrackerFragment to connect the ViewModel with the UI.

  1. In fragment_sleep_tracker.xml: Inside the <data> block, create a <variable> that references the SleepTrackerViewModel class.
<data>
   <variable
       name="sleepTrackerViewModel"
       type="com.example.android.trackmysleepquality.sleeptracker.SleepTrackerViewModel" />
</data>
  1. In SleepTrackerFragment:

  • Set the current activity as the lifecycle owner of the binding. Add this code inside the onCreateView() method, before the return statement.

  • Assign the sleepTrackerViewModel binding variable to the sleepTrackerViewModel. Put this code inside onCreateView(), below the code that creates the SleepTrackerViewModel

binding.sleepTrackerViewModel = sleepTrackerViewModel
binding.lifecycleOwner = this

Multithreading and Coroutines

To use processors more efficiently, the operating system can enable an application to create more than one thread of execution within a process.

There is a lot of infrastructure to manage all those threads needing to run, sometimes in a given order and finished to:

  • Scheduler: which takes into account things such as priorities and makes sure all the threads get to run and finish.
  • Dispatcher: which sets up threads, and specifies a context for that to happen in.
  • Context: You can think of the context as a separate specialized reading room. Some contexts are best for user interface stuff, and some are specialized to deal with input, output operations.

A user-facing application usually has a main thread that runs in a foreground and can dispatch other threads that may run into background.

  • On Android, the main thread is a single thread that handles all updates to the UI. The main thread is also the thread that calls all click handlers and other UI and life cycle callbacks.
  • The UI thread is the default thread, meaning unless you explicitly switched threads or use a class that runs on a different thread, everything you do is on the main thread.

It’s essential to avoid blocking the UI thread. Blocking in this context means the UI thread is waiting, not doing anything at all for an action to be done. For example, something like a database to be done updating. Many common tasks take longer than 16 milliseconds, such as fetching data from the internet, reading a large file, or writing data to a database. Therefore, calling code like this from the main thread can cause the app to pause, stutter or even freeze.

Coroutines

Callbacks

  • One pattern for performing long-running tasks without blocking the main thread is to use callbacks. See the Multi-threading & callbacks primer for an introduction to multi-threading and callbacks.

  • By using callbacks, you can start long-running tasks on a background thread.

  • When a task completes, the callback supplied as an argument is called to inform you of the result on the main thread.

Callbacks have a few drawbacks:

  • Codes that heavily uses callbacks can become hard to read and harder to reason about.
  • Because while the code looks sequential, the callback code will run at some asynchronous time in the future.
  • In addition, callbacks don’t allow the use of some language features, such as exceptions.

Coroutines

In Kotlin, coroutines are the way to handle long-running tasks elegantly and efficiently. Kotlin coroutines let you convert callback-based code to sequential code. Code written sequentially is typically easier to read and can even use language features such as exceptions. In the end, coroutines and callbacks do the same thing: they wait until a result is available from a long-running task and continue execution.

Coroutines have the following properties:

Coroutines are asynchronous.

  • A coroutine runs independently from the main execution steps of your program.
  • This could be in parallel or on a separate processor. It could also be that while the rest of the app is waiting for input, you sneak in a bit of processing.
  • One of the important aspects of async is that you cannot expect that the result is available, until you explicitly wait for it.

For example, let’s say you have a question that requires research, and you ask a colleague to find the answer. They go off and work on it, which is like they’re doing the work “asynchronously” and “on a separate thread.” You can continue to do other work that doesn’t depend on the answer, until your colleague comes back and tells you what the answer is.

Coroutines are non-blocking. Non-blocking means that a coroutine does not block the main or UI thread. So with coroutines, users always have the smoothest possible experience, because the UI interaction always has priority.

Coroutines use suspend functions to make asynchronous code sequential. The keyword suspend is Kotlin’s way of marking a function, or function type, as being available to coroutines. When a coroutine calls a function marked with suspend, instead of blocking until the function returns like a normal function call, the coroutine suspends execution until the result is ready. Then the coroutine resumes where it left off, with the result.

While the coroutine is suspended and waiting for a result, it unblocks the thread that it’s running on. That way, other functions or coroutines can run.

The suspend keyword doesn’t specify the thread that the code runs on. A suspend function may run on a background thread, or on the main thread.

Tip: The difference between blocking and suspending is that if a thread is blocked, no other work happens. If the thread is suspended, other work happens until the result is available.

non-blocking.png

To use coroutines in Kotlin, you need three things:

Job: Basically, a job is anything that can be canceled. Every coroutine has a job, and you can use the job to cancel the coroutine. Jobs can be arranged into parent-child hierarchies. Canceling a parent job immediately cancels all the job’s children, which is a lot more convenient than canceling each coroutine manually.

Dispatcher: The dispatcher sends off coroutines to run on various threads. For example, Dispatchers.Main runs tasks on the main thread, and Dispatchers.IO offloads blocking I/O tasks to a shared pool of threads.

Scope: A coroutine’s scope defines the context in which the coroutine runs. A scope combines information about a coroutine’s job and dispatchers. Scopes keep track of coroutines. When you launch a coroutine, it’s “in a scope,” which means that you’ve indicated which scope will keep track of the coroutine.

Kotlin coroutines with Architecture components

CoroutineScope: A CoroutineScope keeps track of all your coroutines, helps you to manage when your coroutines should run. It can also cancel all of the coroutines started in it. Each asynchronous operation or a coroutine runs within a particular scope.

Architecture components provide first-class support for coroutines for logical scopes in your app. Architecture components define the following built-in scopes that you can use in your app. The built-in coroutine scopes are in the KTX extensions for each corresponding Architecture component. Be sure to add the appropriate dependencies when using these scopes.

ViewModelScope: A ViewModelScope is defined for each ViewModel in your app. Any coroutine launched in this scope is automatically canceled if the ViewModel is cleared. In this codelab you will use ViewModelScope to initiate the database operations.

Room and Dispatcher

When using the Room library to perform a database operation, Room uses a Dispatchers.IO for you to perform the database operations in the background. You don’t have to explicitly specify the Dispatchers.

Implementing Coroutines

Old method

  1. Open the SleepTrackerViewModel.kt file. Define viewModelJob and assign it an instance of Job.
private var viewModelJob = Job()
  1. Override onCleared() and cancel all coroutines.
override fun onCleared() {
    super.onCleared()
    viewModelJob.cancel()
}
  1. Define a uiScope for the coroutines:
private val uiScope = CoroutineScope(Dispatchers.Main +  viewModelJob)
  1. continue from Step 2.3.

Step 1: Mark DAO functions as suspend functions

Open database/SleepDatabaseDao.kt, add suspend keyword to all the methods except for getAllNights(). The complete SleepDatabaseDao class will look like this.

@Dao
interface SleepDatabaseDao {
 
   @Insert
   suspend fun insert(night: SleepNight)
 
   @Update
   suspend fun update(night: SleepNight)
 
   @Query("SELECT * from daily_sleep_quality_table WHERE nightId = :key")
   suspend fun get(key: Long): SleepNight?
 
   @Query("DELETE FROM daily_sleep_quality_table")
   suspend fun clear()
 
   @Query("SELECT * FROM daily_sleep_quality_table ORDER BY nightId DESC LIMIT 1")
   suspend fun getTonight(): SleepNight?
 
   @Query("SELECT * FROM daily_sleep_quality_table ORDER BY nightId DESC")
   fun getAllNights(): LiveData<List<SleepNight>>
}

Step 2: Set up coroutines for database operations

When the Start button in the Sleep Tracker app is tapped, you want to call a function in the SleepTrackerViewModel to create a new instance of SleepNight and store the instance in the database.

Tapping any of the buttons triggers a database operation, such as creating or updating a SleepNight. For this reason and others, you use coroutines to implement click handlers for the app’s buttons.

  1. Open the app-level build.gradle file. Under the dependencies section, you need these dependencies, which were added for you.
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0"
 
// Kotlin Extensions and Coroutines support for Room
implementation "androidx.room:room-ktx:$room_version"
  1. Open the SleepTrackerViewModel file. Define a variable called tonight to hold the current night. Make the variable MutableLiveData, because you need to be able to observe the data and change it.
private var tonight = MutableLiveData<SleepNight?>()
  1. To initialize the tonight variable as soon as possible, create an init block below the definition of tonight and call initializeTonight(). You define initializeTonight() in the next step.
init {
    initializeTonight()
}
  1. Below the init block, implement initializeTonight(). Use the viewModelScope.launch to start a coroutine in the ViewModelScope. Inside the curly braces, get the value for tonight from the database by calling getTonightFromDatabase(), and assign the value to tonight.value. You define getTonightFromDatabase() in the next step.
private fun initializeTonight() {
   viewModelScope.launch {
       tonight.value = getTonightFromDatabase()
   }
}
  1. Implement getTonightFromDatabase(). Define it as a private suspend function that returns a nullable SleepNight, if there is no current started SleepNight. This leaves you with an error, because the function has to return something.
private suspend fun getTonightFromDatabase(): SleepNight? { }
  1. Inside the function body of getTonightFromDatabase(), get tonight (the newest night) from the database. If the start and end times are not the same, meaning that the night has already been completed, return null. Otherwise, return the night.
    var night = database.getTonight()
    if (night?.endTimeMilli != night?.startTimeMilli) {
        night = null
    }
    return night

Your completed getTonightFromDatabase() suspend function should look like this.

/**
*  Handling the case of the stopped app or forgotten recording,
*  the start and end times will be the same.
*
*  If the start time and end time are not the same, 
*  then we do not have an unfinished recording.
*/
private suspend fun getTonightFromDatabase(): SleepNight? {
    var night = database.getTonight()
    if (night?.endTimeMilli != night?.startTimeMilli) {
        night = null
    }
    return night
}

Step 3: Add the click handler for the Start button

Now you can implement onStartTracking(), the click handler for the Start button. You need to create a new SleepNight, insert it into the database, and assign it to tonight. The structure of onStartTracking() is going to look very much like initializeTonight().

  1. Start with the function definition for onStartTracking(). You can put the click handlers above onCleared() in the SleepTrackerViewModel file.
fun onStartTracking() {}
  1. Inside onStartTracking(), launch a coroutine in the viewModelScope, because you need this result to continue and update the UI.
viewModelScope.launch {}
  1. Inside the coroutine launch, create a new SleepNight, which captures the current time as the start time.
val newNight = SleepNight()
  1. Still inside the coroutine launch, call insert() to insert newNight into the database. You will see an error, because you have not defined this insert() suspend function yet. (This is not the DAO function of the same name.)
insert(newNight)
  1. Also inside the coroutine launch, update tonight.
tonight.value = getTonightFromDatabase()
  1. Below onStartTracking(), define insert() as a private suspend function that takes a SleepNight as its argument.
private suspend fun insert(night: SleepNight) {}
  1. Within the insert() method, use the DAO to insert night into the database. Note that a coroutine with Room uses Dispatchers.IO, so this will not happen on the main thread.
database.insert(night)

The full function code should be:

/**
* Executes when the START button is clicked.
*/
fun onStartTracking() {
    viewModelScope.launch {
        // Create a new night, which captures the current time,
        // and insert it into the database.
        val newNight = SleepNight()
 
        insert(newNight)
 
        tonight.value = getTonightFromDatabase()
    }
}
  1. In the fragment_sleep_tracker.xml layout file, add the click handler for onStartTracking() to the start_button using the magic of data binding that you set up earlier. The @{() -> function notation creates a lambda function that takes no arguments and calls the click handler in the sleepTrackerViewModel.
android:onClick="@{() -> sleepTrackerViewModel.onStartTracking()}"
The pattern

Now you can see a pattern:

  1. Launch a coroutine that runs on the main or UI thread, because the result affects the UI. You can access the CoroutineScope of a ViewModel through the viewModelScope property of the ViewModel, as shown in the following example:
  2. Call a suspend function to do the long-running work, so that you don’t block the UI thread while waiting for the result.
  3. The long-running work has nothing to do with the UI. Switch to the I/O dispatcher,(if using Room, this code is generated for you) so that the work can run in a thread pool that’s optimized and set aside for these kinds of operations.
  4. Then call the long running function to do the work.

The pattern is shown below.

fun someWorkNeedsToBeDone {
   viewModelScope.launch {
        suspendFunction()
   }
}
 
suspend fun suspendFunction() {
   withContext(Dispatchers.IO) {
       longrunningWork()
   }
}

Using Room

// Using Room
fun someWorkNeedsToBeDone {
   viewModelScope.launch {
        suspendDAOFunction()
   }
}
 
suspend fun suspendDAOFunction() {
   // No need to specify the Dispatcher, Room uses Dispatchers.IO.
   longrunningDatabaseWork()
}

Step 4: Display the data

In the SleepTrackerViewModel, the nights variable references LiveData because getAllNights() in the DAO returns LiveData.

It is a Room feature that every time the data in the database changes, the LiveData nights is updated to show the latest data. You never need to explicitly set the LiveData or update it. Room updates the data to match the database.

However, if you display nights in a text view, it will show the object reference. To see the contents of the object, transform the data into a formatted string. Use a Transformation map that’s executed every time nights receives new data from the database.

  1. Open the Util.kt file and uncomment the code for the definition of formatNights()and the associated import statements. To uncomment code in Android Studio, select all the code marked with // and press Cmd+/ or Control+/.
  2. Notice that formatNights() returns a type Spanned, which is an HTML-formatted string.
  3. Open strings.xml. Notice the use of CDATA to format the string resources for displaying the sleep data.
  4. Open SleepTrackerViewModel. In the SleepTrackerViewModel class, define a variable called nights. Get all the nights from the database and assign them to the nights variable.
// Define a variable, nights. Then getAllNights() from the database
// and assign to the nights variable.
private val nights = database.getAllNights()
  1. Right below the definition of nights, add code to transform nights into a nightsString. Use the formatNights() function from Util.kt. Pass nights into the map() function from the Transformations class. To get access to your string resources, define the mapping function as calling formatNights(). Supply nights and a Resources object.
/**
* Converted nights to Spanned for displaying.
*/
val nightsString = Transformations.map(nights) { nights ->
   formatNights(nights, application.resources)
}
  1. Open the fragment_sleep_tracker.xml layout file. In the TextView, in the android:text property, you can now replace the resource string with a reference to nightsString.
android:text="@{sleepTrackerViewModel.nightsString}"

Step 5: Add the click handler for the Stop button

Using the same pattern as in the previous step, implement the click handler for the Stop button in SleepTrackerViewModel.

  1. Add onStopTracking() to the ViewModel. Launch a coroutine in the viewModelScope. If the end time hasn’t been set yet, set the endTimeMilli to the current system time and call update() with the night data. In Kotlin, the return@label syntax specifies the function from which this statement returns, among several nested functions.
fun onStopTracking() {
    viewModelScope.launch {
        // In Kotlin, the return@label syntax is used for specifying which function among
        // several nested ones this statement returns from.
        // In this case, we are specifying to return from launch(),
        // not the lambda.
        val oldNight = tonight.value ?: return@launch
 
        // Update the night in the database to add the end time.
        oldNight.endTimeMilli = System.currentTimeMillis()
 
        update(oldNight)
    }
}
  1. Implement update() using the same pattern as you used to implement insert().
private suspend fun update(night: SleepNight) {
    database.update(night)
}
  1. To connect the click handler to the UI, open the fragment_sleep_tracker.xml layout file and add the click handler to the stop_button.
android:onClick="@{() -> sleepTrackerViewModel.onStopTracking()}"

Step 6: Add the click handler for the Clear button

  1. Similarly, implement onClear() and clear().
/**
* Executes when the CLEAR button is clicked.
*/
fun onClear() {
    viewModelScope.launch {
        // Clear the database table.
        clear()
 
        // And clear tonight since it's no longer in the database
        tonight.value = null
    }
}
 
private suspend fun clear() {
    database.clear()
}
  1. To connect the click handler to the UI, open fragment_sleep_tracker.xml and add the click handler to the clear_button.
android:onClick="@{() -> sleepTrackerViewModel.onClear()}"

  • Now to give users a way to enter the sleep quality, we are going to use a fragment with touchable icons.

  • We press “START” to start the sleep night. Then when we press “STOP” and we navigate back to the sleep quality tracker fragment, where we can press a pretty smiley icon, which takes us right back and updates the sleep quality.

In more pragmatic terms, our basic workflow is the user presses, “STOP” is:

  • On stop tracking we navigate to this SleepQualityFragment.
  • We use safeArgs to pass the correct night’s key so that we can update the correct sleep night.
  • Once the user taps a quality icon, we update the current night and return to the SleepTrackerFragment.
  • As an additional challenge, we also need to handle the back button.
  • We want the navigation to happen in the fragment, but the click handler stays in the ViewModel processing the data.
  • We do this by creating a navigation event variable in a ViewModel, observe the variable, when the variable changes navigate immediately, and create a navigation event variable to signify we have finished navigating.

Recording Sleep Quality

Add navigation

The navigation graph already includes the paths from the SleepTrackerFragment to the SleepQualityFragment and back again. However, the click handlers that implement the navigation from one fragment to the next are not coded yet. You add that code now in the ViewModel.

In the click handler, you set a LiveData that changes when you want the app to navigate to a different destination. The fragment observes this LiveData. When the data changes, the fragment navigates to the destination and tells the view model that it’s done, which resets the state variable.

  1. Open SleepTrackerViewModel. You need to add navigation so that when the user taps the Stop button, the app navigates to the SleepQualityFragment to collect a quality rating.
  2. In SleepTrackerViewModel, create a LiveData that changes when you want the app to navigate to the SleepQualityFragment. Use encapsulation to only expose a gettable version of the LiveData to the ViewModel. You can put this code anywhere at the top level of the class body.
/**
* Variable that tells the Fragment to navigate to a specific [SleepQualityFragment]
*
* This is private because we don't want to expose setting this value to the Fragment.
*/
 
private val _navigateToSleepQuality = MutableLiveData<SleepNight>()
val navigateToSleepQuality: LiveData<SleepNight>
   get() = _navigateToSleepQuality
  1. Add a doneNavigating() function that resets the variable that triggers navigation.
/**
* Call this immediately after navigating to [SleepQualityFragment]
*
* It will clear the navigation request, so if the user rotates their phone it won't navigate
* twice.
*/
fun doneNavigating() {
    _navigateToSleepQuality.value = null
}
  1. In the click handler for the Stop button, onStopTracking(), trigger the navigation to the SleepQualityFragment. Set the navigateToSleepQuality variable at the end of the function as the last thing inside the launch{} block. Note that this variable is set to the night. When this variable has a value, the app navigates to the SleepQualityFragment, passing along the night.
_navigateToSleepQuality.value = oldNight
  1. The SleepTrackerFragment needs to observe navigateToSleepQuality so that the app knows when to navigate. In the SleepTrackerFragment, in onCreateView(), add an observer for navigateToSleepQuality(). Note that the import for this is ambiguous and you need to import androidx.lifecycle.Observer.
  2. Inside the observer block, navigate and pass along the ID of the current night, and then call doneNavigating(). If your import is ambiguous, import androidx.navigation.fragment.findNavController.
// Add an Observer on the state variable for Navigating when STOP button is pressed.
sleepTrackerViewModel.navigateToSleepQuality.observe(viewLifecycleOwner, Observer { night ->
    night?.let {
        // We need to get the navController from this, because button is not ready, and it
        // just has to be a view. For some reason, this only matters if we hit stop again
        // after using the back button, not if we hit stop and choose a quality.
        // Also, in the Navigation Editor, for Quality -> Tracker, check "Inclusive" for
        // popping the stack to get the correct behavior if we press stop multiple times
        // followed by back.
        // Also: https://stackoverflow.com/questions/28929637/difference-and-uses-of-oncreate-oncreateview-and-onactivitycreated-in-fra
        this.findNavController().navigate(
                SleepTrackerFragmentDirections
                        .actionSleepTrackerFragmentToSleepQualityFragment(night.nightId))
        // Reset state to make sure we only navigate once, even if the device
        // has a configuration change.
        sleepTrackerViewModel.doneNavigating()
    }
})
Kotlin let

  • let takes the object it is invoked upon as the parameter and returns the result of the lambda expression.

  • Kotlin let is a scoping function wherein the variables declared inside the expression cannot be used outside.

  • Additionally, let is useful for checking Nullable properties as shown below.

var name : String? = "Kotlin let null check"
name?.let { println(it) } //prints Kotlin let null check
name = null
name?.let { println(it) } //nothing happens

Record the sleep quality

Step 1: Create a ViewModel and a ViewModelFactory

  1. In the sleepquality package, create or open SleepQualityViewModel.kt.
  2. Create a SleepQualityViewModel class that takes a sleepNightKey and database as arguments. Just as you did for the SleepTrackerViewModel, you need to pass in the database from the factory. You also need to pass in the sleepNightKey from the navigation.
/**
* ViewModel for SleepQualityFragment.
*
* @param sleepNightKey The key of the current night we are working on.
*/
class SleepQualityViewModel(
    private val sleepNightKey: Long = 0L,
    val database: SleepDatabaseDao) : ViewModel() {
}
  1. To navigate back to the SleepTrackerFragment using the same pattern as above, declare _navigateToSleepTracker. Implement navigateToSleepTracker and doneNavigating().
/**
* Variable that tells the fragment whether it should navigate to [SleepTrackerFragment].
*
* This is `private` because we don't want to expose the ability to set [MutableLiveData] to
* the [Fragment]
*/
private val _navigateToSleepTracker = MutableLiveData<Boolean?>()
 
/**
* When true immediately navigate back to the [SleepTrackerFragment]
*/
val navigateToSleepTracker: LiveData<Boolean?>
    get() = _navigateToSleepTracker
 
 
/**
* Call this immediately after navigating to [SleepTrackerFragment]
*/
fun doneNavigating() {
    _navigateToSleepTracker.value = null
}
  1. Create one click handler, onSetSleepQuality(), for all the sleep-quality images to use. Use the same coroutine pattern:

  • Launch a coroutine in the viewModelScope

  • Get tonight using the sleepNightKey.

  • Set the sleep quality.

  • Update the database.

  • Trigger navigation.

Notice that the code sample below does all the work in the click handler, instead of factoring out the database operation in the different context.

/**
* Sets the sleep quality and updates the database.
*
* Then navigates back to the SleepTrackerFragment.
*/
fun onSetSleepQuality(quality: Int) {
    viewModelScope.launch {
            val tonight = database.get(sleepNightKey) ?: return@launch
            tonight.sleepQuality = quality
            database.update(tonight)
 
        // Setting this state variable to true will alert the observer and trigger navigation.
        _navigateToSleepTracker.value = true
    }
}
  1. In the sleepquality package, create or open SleepQualityViewModelFactory.kt and add the SleepQualityViewModelFactory class, as shown below. This class uses a version of the same boilerplate code you’ve seen before. Inspect the code before you move on.
/**
 * This is pretty much boiler plate code for a ViewModel Factory.
 *
 * Provides the key for the night and the SleepDatabaseDao to the ViewModel.
 */
class SleepQualityViewModelFactory(
        private val sleepNightKey: Long,
        private val dataSource: SleepDatabaseDao) : ViewModelProvider.Factory {
    @Suppress("unchecked_cast")
    override fun <T : ViewModel?> create(modelClass: Class<T>): T {
        if (modelClass.isAssignableFrom(SleepQualityViewModel::class.java)) {
            return SleepQualityViewModel(sleepNightKey, dataSource) as T
        }
        throw IllegalArgumentException("Unknown ViewModel class")
    }
}

Step 2: Update the SleepQualityFragment

  1. Open SleepQualityFragment.kt.
  2. In onCreateView(), after you get the application, you need to get the arguments that came with the navigation. These arguments are in SleepQualityFragmentArgs. You need to extract them from the bundle.
val arguments = SleepQualityFragmentArgs.fromBundle(arguments!!)
  1. Next, get the dataSource.
val dataSource = SleepDatabase.getInstance(application).sleepDatabaseDao
  1. Create a factory, passing in the dataSource and the sleepNightKey.
    val viewModelFactory = SleepQualityViewModelFactory(arguments.sleepNightKey, dataSource)
  1. Get a ViewModel reference.
// Get a reference to the ViewModel associated with this fragment.
val sleepQualityViewModel =
        ViewModelProvider(
                this, viewModelFactory).get(SleepQualityViewModel::class.java)
  1. Add the ViewModel to the binding object. (If you see an error with the binding object, ignore it for now.)
binding.sleepQualityViewModel = sleepQualityViewModel
  1. Add the observer. When prompted, import androidx.lifecycle.Observer.
// Add an Observer to the state variable for Navigating when a Quality icon is tapped.
sleepQualityViewModel.navigateToSleepTracker.observe(viewLifecycleOwner, Observer {
    if (it == true) { // Observed state is true.
        this.findNavController().navigate(
                SleepQualityFragmentDirections.actionSleepQualityFragmentToSleepTrackerFragment())
        // Reset state to make sure we only navigate once, even if the device
        // has a configuration change.
        sleepQualityViewModel.doneNavigating()
    }
})

Step 3: Update the layout file and run the app.

  1. Open the fragment_sleep_quality.xml layout file. In the <data> block, add a variable for the SleepQualityViewModel.
<data>
    <variable
        name="sleepQualityViewModel"
        type="com.example.android.trackmysleepquality.sleepquality.SleepQualityViewModel" />
</data>
  1. For each of the six sleep-quality images, add a click handler like the one below. Match the quality rating to the image.
android:onClick="@{() -> sleepQualityViewModel.onSetSleepQuality(5)}"

Button States

Now your app works great. The user can tap Start and Stop as many times as they want. When the user taps Stop, they can enter a sleep quality. When the user taps Clear, all the data is cleared silently in the background. However, all the buttons are always enabled and clickable, which does not break the app, but it does allow users to create incomplete sleep nights.

The idea is to set the button state so that in the beginning, only the Start button is enabled, which means it is clickable.

After the user taps Start, the Stop button becomes enabled and Start is not. The Clear button is only enabled when there is data in the database.

  1. Open the fragment_sleep_tracker.xml layout file.
  2. Add the android:enabled property to each button. The android:enabled property is a boolean value that indicates whether or not the button is enabled. (An enabled button can be tapped; a disabled button can’t.) Give the property the value of a state variable that you’ll define in a moment.

start_button:

android:enabled="@{sleepTrackerViewModel.startButtonVisible}"

stop_button:

android:enabled="@{sleepTrackerViewModel.stopButtonVisible}"

clear_button:

android:enabled="@{sleepTrackerViewModel.clearButtonVisible}"
  1. Open SleepTrackerViewModel and create three corresponding variables. Assign each variable a transformation that tests it.
  • The Start button should be enabled when tonight is null.
  • The Stop button should be enabled when tonight is not null.
  • The Clear button should only be enabled if nights, and thus the database, contains sleep nights.
/**
* If tonight has not been set, then the START button should be visible.
*/
val startButtonVisible = Transformations.map(tonight) {
    null == it
}
 
/**
* If tonight has been set, then the STOP button should be visible.
*/
val stopButtonVisible = Transformations.map(tonight) {
    null != it
}
 
/**
* If there are any nights in the database, show the CLEAR button.
*/
val clearButtonVisible = Transformations.map(nights) {
    it?.isNotEmpty()
}

Tip: Setting the appearance of a disabled View

The enabled attribute is not the same as the visibility attribute. The enabled attribute only determines whether the View is enabled, not whether the View is visible.

The meaning of “enabled” varies by the subclass. A user can edit the text in an enabled EditText, but not in a disabled EditText. A user can tap an enabled Button, but can’t tap a disabled one.

A default style is applied to a disabled View to visually represent that the View is not active.

However, if the View has a background attribute or a textColor attribute, the values of those attributes are used when the View is displayed, even if the View is disabled.

To define which colors to use for enabled and disabled states, use a ColorStateList for the text color and a StateListDrawable for the background color.

SnackBar

After the user clears the database, show the user a confirmation using the Snackbar widget. A snackbar provides brief feedback about an operation through a message at the bottom of the screen. A snackbar disappears after a timeout, after a user interaction elsewhere on the screen, or after the user swipes the snackbar off the screen.

Showing the snackbar is a UI task, and it should happen in the fragment. Deciding to show the snackbar happens in the ViewModel. To set up and trigger a snackbar when the data is cleared, you can use the same technique as for triggering navigation.

  1. In the SleepTrackerViewModel, create the encapsulated event.
/**
* Request a toast by setting this value to true.
*
* This is private because we don't want to expose setting this value to the Fragment.
*/
private var _showSnackbarEvent = MutableLiveData<Boolean>()
 
/**
* If this is true, immediately `show()` a toast and call `doneShowingSnackbar()`.
*/
val showSnackBarEvent: LiveData<Boolean>
    get() = _showSnackbarEvent
  1. Then implement doneShowingSnackbar().
/**
* Call this immediately after calling `show()` on a toast.
*
* It will clear the toast request, so if the user rotates their phone it won't show a duplicate
* toast.
*/
 
fun doneShowingSnackbar() {
    _showSnackbarEvent.value = false
}
  1. In the SleepTrackerFragment, in onCreateView(), add an observer:
sleepTrackerViewModel.showSnackBarEvent.observe(viewLifecycleOwner, Observer {})
  1. Inside the observer block, display the snackbar and immediately reset the event.
if (it == true) { // Observed state is true.
    Snackbar.make(
            requireActivity().findViewById(android.R.id.content),
            getString(R.string.cleared_message),
            Snackbar.LENGTH_SHORT // How long to display the message.
    ).show()
    // Reset state to make sure the snackbar is only shown once, even if the device
    // has a configuration change.
    sleepTrackerViewModel.doneShowingSnackbar()
}
  1. In SleepTrackerViewModel, trigger the event in the onClear() method. To do this, set the event value to true inside the launch block:
// Show a snackbar message, because it's friendly.
_showSnackbarEvent.value = true

Summary

Create a Room Database

  • Define your tables as data classes annotated with @Entity. Define properties annotated with @ColumnInfo as columns in the tables.
  • Define a data access object (DAO) as an interface annotated with @Dao. The DAO maps Kotlin functions to database queries.
  • Use annotations to define @Insert, @Delete, and @Update functions.
  • Use the @Query annotation with an SQLite query string as a parameter for any other queries.
  • Create an abstract class that has a getInstance() function that returns a database.
  • Use instrumented tests to test that your database and DAO are working as expected. You can use the provided tests as a template.

Coroutines and Room

  • Use ViewModel, ViewModelFactory, and data binding to set up the UI architecture for the app.
  • To keep the UI running smoothly, use coroutines for long-running tasks, such as all database operations.
  • Coroutines are asynchronous and non-blocking. They use suspend functions to make asynchronous code sequential.
  • When a coroutine calls a function marked with suspend, instead of blocking until that function returns like a normal function call, it suspends execution until the result is ready. Then it resumes where it left off with the result.
  • The difference between blocking and suspending is that if a thread is blocked, no other work happens. If the thread is suspended, other work happens until the result is available.

To implement click handlers that trigger database operations, follow this pattern:

  1. Launch a coroutine that runs on the main or UI thread, because the result affects the UI.
  2. Call a suspend function to do the long-running work, so that you don’t block the UI thread while waiting for the result.
  3. The long-running work has nothing to do with the UI, so switch to the I/O context. That way, the work can run in a thread pool that’s optimized and set aside for these kinds of operations.
  4. Then call the long running function to do the work.

Use a Transformations map to create a string from a LiveData object every time the object changes.

Use LiveData to control button states

Implementing sleep quality tracking in this app is like playing a familiar piece of music in a new key. While details change, the underlying pattern of what you did in previous codelabs in this lesson remains the same. Being aware of these patterns makes coding much faster, because you can reuse code from existing apps. Here are some of the patterns used in this course so far:

  • Create a ViewModel and a ViewModelFactory and set up a data source.
  • Trigger navigation. To separate concerns, put the click handler in the view model and the navigation in the fragment.
  • Use encapsulation with LiveData to track and respond to state changes.
  • Use transformations with LiveData.
  • Create a singleton database.
  • Set up coroutines for database operations.

Triggering navigation

You define possible navigation paths between fragments in a navigation file. There are some different ways to trigger navigation from one fragment to the next. These include:

  • Define onClick handlers to trigger navigation to a destination fragment.
  • Alternatively, to enable navigation from one fragment to the next:
  • Define a LiveData value to record if navigation should occur.
  • Attach an observer to that LiveData value.
  • Your code then changes that value whenever navigation needs to be triggered or is complete.

Setting the android:enabled attribute

  • The android:enabled attribute is defined in TextView and inherited by all subclasses, including Button.
  • The android:enabled attribute determines whether or not a View is enabled. The meaning of “enabled” varies by subclass. For example, a non-enabled EditText prevents the user from editing the contained text, and a non-enabled Button prevents the user from tapping the button.
  • The enabled attribute is not the same as the visibility attribute.
  • You can use transformation maps to set the value of the enabled attribute of buttons based on the state of another object or variable.

Other points covered in this codelab:

  • To trigger notifications to the user, you can use the same technique as you use to trigger navigation.
  • You can use a Snackbar to notify the user.

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