Lesson 05 - App Architecture (UI Layer)

Lesson 05: App Architecture (UI Layer)¶

With Architecture Components you’ll have the power to design even the
most complicated app ideas. Combine ViewModels with LiveData to build
this super fun “Guess it” game.

• Many Android teams will follow an application architecture, which is a great upon set of design rules.
• Architecture provides the basic bones and structure of your app.
• A well-thought-out architecture can make your code maintainable for years and help your team collaborate.

Tour of the App¶

• MainActivity - This class does very little. It’s just a container for the NavHostFragment. The fragments that go in the NavHostFragment do most of the heavy lifting. This is similar to what we did in lesson 3
• res/navigation/main_navigation.xml - This is the navigation graph for this app. The navigation flow goes from the TitleFragment to the GameFragment to the ScoreFragment. From the ScoreFragment, you can play the game again by going back to the GameFragment. Note that the action from the GameFragment to the
  ScoreFragment has a Pop To attribute that removes the game from the backstack. This makes it so that you can never press the back button from the ScoreFragment to go back to a finished game. Instead, you’ll go to the title screen.
• TitleFragment - This is a simple fragment showing the title screen. Has a single button that takes you to the GameFragment.
• GameFragment - This fragment contains all the logic for the GuessIt game itself. It contains:
• word - A variable for the current word to guess.
• score - A variable for the current score.
• wordList - A variable for a mutable list of all the words you need to guess. This list is created and immediately shuffled using resetList() so that you get a new order of words every time.
• resetList() - Method that creates and shuffles the list of words.
• onSkip()/onCorrect() - Methods for when you press the Skip/Got It buttons. They modify the score and then go to the next word in your worldList.
• nextWord() - A method for moving to the next word to
  guess. If there are still words in your mutable list of words, remove the current word, and then set currentWord to whatever is next in the list. This will finish the game if there are no more words to guess.
• gameFinished() - A method that is called to finish the game. This passes your current score to the ScoreFragment using SafeArgs.
• ScoreFragment - This fragment gets the score passed in from the argument bundle and displays it. There’s also a button for playing again, that takes you back to the GameFragment.

What is Architecture¶

• A bloated class that does all sorts of different functions is a bad idea.

• Application architecture is a way of designing your app’s classes and the relationship between them, such that the code base is more organized, performative particular scenarios, and easier to work.

• There’s no one right way to architect an application much like there’s no empirically right way to architect a house.

• Architecture is heavily dependent on your circumstance needs and tastes.

• There are many different architectural styles for Android apps, each with different strengths and weaknesses. Some of these styles may even overlap. They fit different application needs team sizes team dynamics and more.

• For this lesson, you’re gonna be learning about a single, multi-purpose architectural pattern that we’re gonna build on for the rest of this course. It’s loosely based on an architecture called MVVM, which is model view viewmodel. The reason I’m choosing this style is because it’s officially endorsed by Google and it leverages the lifecycle classes.

Separation of Concerns¶

Divide your code into classes, each with separate, well-defined responsibilities.

Our App Architecture¶

• Architecture gives you guidelines to figure out which classes should have what responsibilities in your app.

• We’re going to be working with three different classes: the UI Controller, the ViewModel and LiveData.

The first class is the UI Controller:

• UI Controller is the word that I’m using to describe what activities and fragments are.

• UI Controller is responsible for any user interface related tasks like displaying views and capturing user input.

• By design, UI Controllers execute all of the draw commands that put our views on the screen.

• Also when an operating system event happens like when a user presses a button it’s the UI Controller that get notified first.

• You should limit UI Controller to only user interface related tasks and take any sort of decision-making power out of UI Controller so while UI Controller is responsible for drawing a text to you to the screen. It is not responsible for the calculations or processing that decides what actual text to draw.

• While UI Controller is what knows that a button has been pressed it’s going to immediately pass that information along to the second class in our architecture the ViewModel.

ViewModel will do the actual decision-making.

• The purpose of the ViewModel is to hold the specific data needed to display the fragment or activity it’s associated with.

• Also, ViewModels may do simple calculations and transformations on that data so that it’s ready to be displayed by the UI Controller.

• The ViewModel class will contain instances of a third-class LiveData.

LiveData classes are crucial for communicating information for the ViewModel to UI Controller that it should update and redraw the screen.

In our case, the UI Controllers will be our three fragments. Let’s take game fragment as an example:

• Game fragment will be responsible for drawing the game elements to the screen and knowing when the user presses the buttons nothing more.

• When the buttons are pressed game, fragment will pass that information to the game ViewModel.

• The game ViewModel will hold data like the score value, the list of words, and the current word to be displayed because that’s the data that is needed to know what to display on the screen. It’ll also be in charge of simple calculations to decide the current state of the data. For example what the current word is in the list of words and what the current score should be.

ViewModel¶

• ViewModel is an abstract class that you will extend and then implement. It holds your apps UI data and survives configuration changes.

• Instead of having the UI data the fragment move it to your ViewModel and have the fragment reference the ViewModel.

• ViewModel survives configuration changes so while the fragment is destroyed and then remade all of that juicy data that you need to display in the fragment like the score the current word and so on remain in the ViewModel.

• If you reconnect your recreated fragment to the same ViewModel all of the data is just right there for you.

• Unlike the onSavedInstanceState bundle the ViewModel has no restrictions on size so you can store lots of data in here without worrying

Adding A ViewModel¶

1. Add lifecycle-extensions gradle dependency:

In the Module: app build.gradle file, add the lifecycle-extensions
dependency. You can find the most current version of the dependency here.

// Lifecycles  
implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0'  

2. Create the GameViewModel class, extending ViewModel:
   Create a new file called GameViewModel.kt in the java/com.example.android.guesstheword/game package. Then in this file, create a class GameViewModel that extends ViewModel:

class GameViewModel : ViewModel()  

3. Add init block and override onCleared. Add log statements to both:

Make an init block that prints out a log saying “GameViewModel
created!”.

init {  
   Log.i("GameViewModel", "GameViewModel created!")  
}  

Then override onCleared so you can track the lifetime of this ViewModel. You can use the keyboard shortcut Ctrl + O to do the override. Then add the log statement saying “GameViewModel destroyed!” to onCleared.

override fun onCleared() {  
   super.onCleared()  
   Log.i("GameViewModel", "GameViewModel destroyed!")  
}  

4. Create and initialize a GameViewModel, using ViewModelProvider. Add a log statement:

Back in GameFragment use lateinit to create a field for GameViewModel called viewModel.

private lateinit var viewModel: GameViewModel  

Then in onCreateView, request the current GameViewModel using the ViewModelProvider class:

// Get the viewmodel  
Log.i("GameFragment", "Called ViewModelProvider")  
viewModel = ViewModelProvider(this).get(GameViewModel::class.java)  

Note 1: Sharing UI-related data between destinations with ViewModel :
You can save ViewModel state between fragments with navigation by initializing it scoped to a navigation graph.

val viewModel: MyViewModel by navGraphViewModels(R.id.my_graph)  

Note 2: :: Usage in Kotlin:

• The keyword :: is used for Reflection in Kotlin:

1. Class Reference val myClass = MyClass::class
2. Function Reference list::isEmpty()
3. Property Reference ::someVal.isInitialized
4. Constructor Reference ::MyClass

What should be moved to the ViewModel?¶

• UI Controller only displays and gets user/OS events. It doesn’t make decisions.
• The ViewModel is a stable place to store the data to display in the associated UI controller.
• The Fragment draws the data on screen and captures input events. It
  should not decide what to display on screen or process what happens during an input event.
• The ViewModel never contains references to activities, fragments, or views.

So, in our app, The score and word field, wordList field, and resetList and nextWord methods should be moved to the ViewModel.

1. Move the word, score, and wordList variables to the GameViewModel:

In the GameFragment find the word, score, and wordList variables, and move them to GameViewModel. Make sure you delete the versions of these variables in GameFragment.

class GameViewModel : ViewModel() {  
       // The current word  
   var word = ""  

   // The current score  
   var score = 0  

   // The list of words - the front of the list is the next word to guess  
   private lateinit var wordList: MutableList<String>  

Remember, do not move the binding! This is because the binding contains references to views.

2. Move methods resetList, nextWord, onSkip, and onCorrect to the GameViewModel:

In GameFragment, find each of the methods: resetList, nextWord, onSkip and onCorrect. Move them to GameViewModel.

Remove the private modifier onSkip and onCorrect methods so they can be called from the GameFragment.

/** Methods for buttons presses **/  

fun onSkip() {  
   score--  
   nextWord()  
}  

fun onCorrect() {  
   score++  
   nextWord()  
}  

3. Move the initialization methods to the GameViewModel:

Initialization in the GameFragment involved calling resetList and nextWord. Now that they are both in the GameViewModel, call them in the GameViewModel when it is created.

init {  
   resetList()  
   nextWord()  
}  

4. Update the onClickListeners to refer to call methods in the ViewModel, and then update the UI:

Now that onSkip and onCorrect have been moved to the GameViewModel, the OnClickListeners in the GameFragment, refer to method that aren’t there. Update the OnClickListeners to call the methods in the GameViewModel. Then in the OnClickListeners, update the score and word texts so that they have the newest data.

binding.correctButton.setOnClickListener {  
   viewModel.onCorrect()  
   updateScoreText()  
   updateWordText()  
}  
binding.skipButton.setOnClickListener {  
   viewModel.onSkip()  
   updateScoreText()  
   updateWordText()  
}  

5. Update the methods to get word and score from the viewModel:

In the GameViewModel, remove the private modifier from word and score.

In the GameFragment, update gameFinished, updateWordText and updateScoreText to get the data from the gameViewModel.

/**  

* Called when the game is finished  
*/  
private fun gameFinished() {  
   val action = GameFragmentDirections.actionGameToScore(viewModel.score)  
   findNavController(this).navigate(action)  
}  

/** Methods for updating the UI **/  

private fun updateWordText() {  
   binding.wordText.text = viewModel.word  

}  

private fun updateScoreText() {  
   binding.scoreText.text = viewModel.score.toString()  
}  

6. Do final cleanup in the GameViewModel:

Once you’ve copied over the variables and methods, remove any code that refers to the GameFragment. In the GameViewModel, comment out the reference to gameFinished in nextWord. You’ll deal with this later. You can also clean up the log statements from the prior step.

The Benefits of a Good Architecture¶

The separation of concerns design principle helps us in two ways:

• The code is more organized, manageable, and debuggable.
• By moving code to the ViewModel you protect yourself from having to worry about lifecycle problems and bugs.

Another aspect of this design is:

• The code is very modular.
  UI Controller knows about the ViewModel but the ViewModel doesn’t know about the UI Controller.
  One of the goals of this architecture is to keep the number of references between classes small. This modularity allows us to swap out a single class for a different implementation without needing to update a bunch of other classes in your app. For example, you could redesign the UI and you wouldn’t necessarily need to make changes to the ViewModel

• ViewModel contains no references to activities fragments or views.

This happens to be helpful for testing.
Testing on Android differentiates between tests that can be run without emulating the android framework which is in the test folder and tests that are more heavyweight that require emulating the android framework. these tests can be found in the Android test folder.

By designing your app so that these Android classes aren’t referenced in the ViewModel you can run pure lightweight unit tests that don’t depend on the android framework code and therefore run faster and are easier to write.

The Power and Limits of the ViewModel¶

By adding the ViewModel you’ve actually fixed the rotation issue. The ViewModel does preserve data with the configuration changes, but the data is still lost after the app is shut down by the OS.

The proper way to preserve data even if the app is terminated usually involves using a combination of onSavedInstance and data persistence.

The second issue is that we have related functions that should be combined in one file, but since we are separating the logic and UI controller it’s okay to keep it like this.

Both of these issues boil down to the fact that there’s no way to communicate back to the fragment from the view. And since you should not have references to the fragment in your view we need another thing to deal with a situation like this.

LiveData¶

We need a way to communicate for the ViewModel back to the UI controller without having the ViewModel store references to any views activities or fragments.
For example, we need the ViewModel to communicate when data like the score has changed so that the game fragment knows to actually redraw the score.
What would be really great is if we could change the data in the ViewModel and then just have the screen magically know when to update itself. Fortunately, live data can do this for us.

LiveData is an observable data holder class that is lifecycle-aware.

• LiveData holds data this means that it wraps around some data. For example, we’ll have live data wrap around the current score.

• LiveData is also observable.

The Observer pattern is where you have an object called a Subject. The subject keeps track of a list of other objects known as Observers. Observers watch or observe the subject when the status of the subject changes. it notifies the observers by calling a method in the observer.
In LiveData’s case, the subject is the live data object and the observers are the UI controllers. The state change is whenever the data wrapped inside of live data changes.

By setting up this observer relationship you could have the ViewModel communicate data changes back to the UI Controller.

The LiveData object should be val as it will always stay the same. Although the
data is stored within it might change.

Adding LiveData to GameViewModel¶

1. Wrap word and score in MutableLiveData:

Since MutableLiveData is generic we need to specify the type.

val word = MutableLiveData<String>()  
val score = MutableLiveData<Int>()  

2. Initialize score.value to 0 because MutableLiveData is nullable.

init {  
   resetList()  
   nextWord()  
   // Initialize score.value to 0  
   score.value = 0  
}  

3. Change references to score and word to score.value and word.value and add the required null safety checks:

Check the onSkip() and onCorrect() methods and change references.
Add null safety checks, then call the minus and plus functions, respectively.

fun onSkip() {  
   // score--  
   score.value = (score.value)?.minus(1)  
   nextWord()  
}  

fun onCorrect() {  
   // score++  
   score.value = (score.value)?.plus(1)  
   nextWord()  
}  

4. Set up the observation relationship for the score and word LiveDatas:

Move over to GameFragment. UI controllers are where you’ll set up the observation relationship. Get the LiveData from your viewModel and call the observe method. Make sure to pass in this and then an observer lambda. Move the code to update the score TextView and the word TextView to your Observers. Here’s the code to set up an observation relationship for the score. This should be in GameFragment.onCreate:

// Setup the LiveData observation relationship by getting the LiveData from your  
// ViewModel and calling observe. Make sure to pass in *this* and then an Observer lambda  

/** Setting up LiveData observation relationship **/  
viewModel.word.observe(viewLifecycleOwner, { newWord ->  
   binding.wordText.text = newWord  
})  

viewModel.score.observe(viewLifecycleOwner, { newScore ->  
   binding.scoreText.text = newScore.toString()  
})  

And now you can remove updateWordText() and updateScoreText() any references to them completely.

5. Add a null safety check when passing currentScore in the gameFinished method:

viewModel.score.value can possibly be null, so add a null safety check in the gameFinished method:

val currentScore = viewModel.score.value ?: 0  

Lifecycle-Awareness for LiveData¶

• LiveData is aware of its associated Ul controller lifecycle state.

• Ul controller off-screen, no updates. (Only updates when it’s on the foreground)

• Ul controller back on-screen, get current data. (Immediately send the freshest data when it comes back to foreground)

• New Ul controller observes, get current data. (Immediately send the freshest data when a new Ul controller observes it)

• Ul controller destroyed, it cleans up connection after itself.

Adding LiveData Encapsulation to GameViewModel¶

• Encapsulation is the notion of restricting the direct access to objects fields. This way you could expose a public set of methods that modify the private internal fields and you can control the exact ways outside classes can and can’t manipulate these internal fields.

• You should try to restrict edit access to your LiveData. In this case, only the ViewModel should be updating the score and word fields. But for this observation code to work, it’s important that you could still get some access to the LiveData so it can’t be completely private.

• Being able to write and read from LiveData is where the distinction between MutableLiveData and just plain old LiveData comes. MutableLiveData is a LiveData that can be mutated or modified, whereas LiveData on the other hand is LiveData that you could read but you cannot call set value on.

• Inside the ViewModel we want the LiveData to be MutableLiveData, but outside the ViewModel we want the MutableLiveData to only be exposed as LiveData. To do that we can use a concept in Kotlin known as a backing property.

• A backing property allows you to return something for a getter other than the exact object.

1. Make internal and external versions of word and score:

Open up GameViewModel. The internal version should be a
MutableLiveData, has an underscore in front of its name, and be private. The underscore is our convention for marking the variable as the internal version of a variable.

The external version should be a LiveData and not have an underscore in front of its name.

2. Make a backing property for the external version that returns the internal MutableLiveData as a LiveData:

Kotlin automatically makes getters and setters for your fields. If you want to override the getter for score, you can do so using a backing property. You’ve actually already defined your backing properties (_score and _word). Now you can take your public versions of the variables (score and word) and override get to return the backing properties.

// Make an internal and external version of the word and score  
// The internal version should be a MutableLiveData, have an underscore in front of its' name and be private  
// The external version should be a LiveData  

// The current word  
private val _word = MutableLiveData<String>()  
val word: LiveData<String>  
   // Make a backing property for the external version that  
   // returns the internal MutableLiveData as a LiveData  
   get() = _word  

// The current score  
private val _score = MutableLiveData<Int>()  
val score: LiveData<Int>  
   get() = _score  

By making the return type LiveData rather than MutableLiveData, you’ve exposed only score and word as LiveData.

3. In the view model, use the internal, mutable versions of score and word:

In GameViewModel, update the code so that you use the mutable versions, _score and _word, throughout the view model.

Event vs. State¶

LiveData keeps track of data State, like button color and score value. But Navigating to another screen is an example of an Event.

An Event happens once and it’s done until it’s triggered again. For example:

• A notification
• A sound playing when a button is pressed
• Navigating to a different screen

LiveData And Events¶

1. Make a properly encapsulated LiveData called eventGameFinish in the GameViewModel that holds a boolean and will represent game end Event:

// Event which triggers the end of the game  
private val _eventGameFinish = MutableLiveData<Boolean>()  
val eventGameFinish: LiveData<Boolean>  
   get() = _eventGameFinish  

Also, initialize its value to false.

_eventGameFinish.value = false  

2. Make the function onGameFinishComplete which makes the value of eventGameFinish false:

This function simply sets the value of _eventGameFinish to false. This is to signal that you’ve handled the game finish event and that you don’t need to handle it again. In this specific example,
it’s a way to say you’ve done the navigation.

/** Methods for completed events **/  
fun onGameFinishComplete() {  
   _eventGameFinish.value = false  
}  

3. Set eventGameFinish to true, to signify that the game is over:

In the GameViewModel, find the condition where the wordList is empty. If it’s empty, the game is over, so signal that by setting the value of eventGameFinish to true.

if (wordList.isEmpty()) {  
   // Set eventGameFinish to true, to signify that the game is over  
   _eventGameFinish.value = true  
} else {  
   _word.value = wordList.removeAt(0)  
}  

4. Add an observer of eventGameFinish:

Back in GameFragment, add an observer of eventGameFinish.

In the observer lambda you should:

1. Make sure that the boolean holding the current value of eventGameFinished is true. This means the game has finished.

2. If the game has finished, call gameFinished.

3. Tell the view model that you’ve handled the game finished event by calling onGameFinishComplete.

// Add an observer of eventGameFinish which, when eventGameFinish is true, calls gameFinished()  
// Make sure to call onGameFinishCompete to tell your viewmodel that the game finish event was dealt with  

// Sets up event listening to navigate the player when the game is finished  
viewModel.eventGameFinish.observe(viewLifecycleOwner, {isFinished ->  
   if (isFinished) {  
       gameFinished()  
       viewModel.onGameFinishComplete()  
   }  
})  

Adding a Timer¶

Where should the timer go? GameViewModel

We put the code for tracking and counting downtime in the fragment that would get destroyed whenever you rotated the phone. The logic for counting down a timer should go in the game view model. The only thing the fragment should worry about is updating this textview as the timer ticks.

1. Copy the provided companion object with the timer constants:

In GameViewModel, copy the following companion object code. This companion object has constants for our timer:

companion object {  
   // These represent different important times  
   // This is when the game is over  
   const val DONE = 0L  
   // This is the number of milliseconds in a second  
   const val ONE_SECOND = 1000L  
   // This is the total time of the game  
   const val COUNTDOWN_TIME = 60000L  
}  

Feel free to change the COUNTDOWN_TIME constant so that the game doesn’t last a whole minute. This can be helpful for running the app to check whether it’s working.

2. Create a properly encapsulated LiveData for the current time called currentTime:

Use the same method as you did earlier to encapsulate LiveData for score and word. The type of currentTime should be of type Long.

// The current time  
private val _currentTime = MutableLiveData<Long>()  
val currentTime: LiveData<Long>  
   get() = _currentTime  

3. Create a timer field of type CountDownTimer in the GameViewModel:
   You don’t need to worry about initializing it yet. Just declare it and ignore the error for now.

private val timer: CountDownTimer  

4. Copy over the CountDownTimer code and then update currentTime and eventGameFinish appropriately as the timer ticks and finishes:

In the init of GameViewModel, copy the CountDownTimer code below:

// Creates a timer which triggers the end of the game when it finishes  
timer = object : CountDownTimer(COUNTDOWN_TIME, ONE_SECOND) {  
   override fun onTick(millisUntilFinished: Long) {  
       _currentTime.value = (millisUntilFinished / ONE_SECOND)  
   }  
   override fun onFinish() {  
       _currentTime.value = DONE  
       _eventGameFinish.value = true  
   }  
}  
timer.start()  

5. Update the logic in the nextWord function so that it doesn’t end the game:

The game should finish when the timer runs out not when there are no words left in the list. If there are no words in the list, you should add the words back to the list and re-shuffle the list.

You can do this using resetList. Update the code so that it doesn’t end the game, but instead calls resetList.

private fun nextWord() {  
   if (wordList.isEmpty()) {  
       // Update this logic so that the game doesn't finish;  
       // Instead the list is reset and re-shuffled when you run out of words  
       resetList()  
   }  
   _word.value = wordList.removeAt(0)  
}  

6. Cancel the timer in onCleared:

To avoid memory leaks, you should always cancel a CountDownTimer if you no longer need it. To do that, you can call:

// Cancel the timer in onCleared  
override fun onCleared() {  
   super.onCleared()  
   timer.cancel()  
}  

7. Update the UI:

You want the timerText on the screen to show the appropriate time. Figure out how to use LiveData from the GameViewModel to do this - remember, you’ve done something similar for score and word. You’ll need to convert the Long for the timer into a String. You can use the DateUtils tool to do that.

// Setup an observer relationship to update binding.timerText  
// You can use DateUtils.formatElapsedTime to correctly format the long to a time string  
viewModel.currentTime.observe(viewLifecycleOwner, { newTime ->  
   binding.timerText.text = DateUtils.formatElapsedTime(newTime)  
})  

ViewModelFactory¶

A class that knows how to create ViewModels.

There’s one challenge with the score fragment and that is that you get the score data passed in from this arguments bundle, you want to display this immediately so this should probably be given to the ViewModel when it’s initialized.

There are two ways to do this. One is to make a setter for the score variable in the ViewModel and then to call it from onCreateView within the score fragment. The other is to add a constructor for the ViewModel.

ViewModel: Adding a constructor¶

• Create a ViewModel that takes in a constructor parameter
• Make a ViewModel Factory for ViewModel
• Have factory construct ViewModel with constructor parameter
• Add ViewModel Factory when using ViewModel Providers

Adding a ViewModelFactory¶

In this exercise, you’ll pass data into a ViewModel. You’ll create a view model factory that allows you to define a custom constructor for a ViewModel that gets called when you use ViewModelProvider.

1. Create the ScoreViewModel class and have it take in an integer constructor parameter called finalScore:

Make sure to create the ScoreViewModel class file in the same package as the ScoreFragment.

class ScoreViewModel(finalScore: Int) : ViewModel() {  

}  

2. Copy over ScoreViewModelFactory:

Create ScoreViewModel factory in the same package as the ScoreFragment. You can use this code later if you ever need to create a view model factory.

Note that the constructor of your view model factory should take any parameters you want to pass into your ScoreViewModel. In this case, it takes in the final score.

In the overridden create method, construct and return an instance of ScoreViewModel, passing in finalScore:

The create method’s purpose is to create and return your view model.
So you should construct a new ScoreViewModel and return it. You’ll also need to deal with the generics, so the statement will be:

return ScoreViewModel(finalScore) as T  

And the full code will be:

class ScoreViewModelFactory(private val finalScore: Int) : ViewModelProvider.Factory {  
   @Suppress("unchecked_cast")  
   override fun <T : ViewModel?> create(modelClass: Class<T>): T {  
       if (modelClass.isAssignableFrom(ScoreViewModel::class.java)) {  
           return ScoreViewModel(finalScore) as T  
       }  
       throw IllegalArgumentException("Unknown ViewModel class")  
   }  
}  

3. Create and construct a ScoreViewModelFactory:

In ScoreFragment, create viewModelFactory from ScoreViewModelFactory.

// Get args using by navArgs property delegate  
val scoreFragmentArgs by navArgs<ScoreFragmentArgs>()  
// Create and construct a ScoreViewModelFactory  
viewModelFactory = ScoreViewModelFactory(scoreFragmentArgs.score)  

4. Create ScoreViewModel by using ViewModelProvider as usual, except you’ll also pass in your ScoreViewModelFactory:

viewModel = ViewModelProvider(this, viewModelFactory)  
       .get(ScoreViewModel::class.java)  

By passing in the ViewModel factory, you’re telling ViewModelProvider to use this factory to create ScoreViewModel.

Note : Sharing UI-related data between destinations with ViewModel using ViewModelFactory:

private val winFragmentArgs by navArgs<WinFragmentArgs>()  
private val viewModel: WinViewModel by navGraphViewModels(R.id.navigation) {  
    WinViewModelFactory(  
        winFragmentArgs.winnerPlayer  
    )  
}  

5. Add a LiveData for the score and the play again event:

Create LiveData for score and eventPlayAgain using the best practices for encapsulation and event handling that you’ve learned. Make sure to initialize score’s value to the finalScore you pass into the view model.

private val _eventPlayAgain = MutableLiveData<Boolean>()  
val eventPlayAgain: LiveData<Boolean>  
   get() = _eventPlayAgain  

private val _score = MutableLiveData<Int>()  
val score: LiveData<Int>  
   get() = _score  

init {  
   _score.value = finalScore  
}  

6. Convert ScoreFragment to properly observe and use ScoreViewModel to update the UI:

In ScoreFragment, add observers for score and eventPlayAgain LiveData. Use them to update the UI.

// Add observer for score  
viewModel.score.observe(viewLifecycleOwner, { newScore ->  
   binding.scoreText.text = newScore.toString()  
})  

// Navigates back to title when button is pressed  
viewModel.eventPlayAgain.observe(viewLifecycleOwner, { playAgain ->  
   if (playAgain) {  
       findNavController().navigate(ScoreFragmentDirections.actionRestart())  
       viewModel.onPlayAgainComplete()  
   }  
})  

In ScoreViewModel, helper functions to work LiveData Events.

fun onPlayAgain() {  
   _eventPlayAgain.value = true  
}  

fun onPlayAgainComplete() {  
   _eventPlayAgain.value = false  
}  

Adding ViewModel to Data Binding¶

In this exercise, you’re going to use data binding in the layout XML code to communicate directly with the ViewModel. In particular, we’re going to tell the ViewModel when various buttons are clicked!

1. Add a GameViewModel data binding variable to GameFragment layout:

In the layout xml file for the GameFragment (game_fragment.xml), create a gameViewModel variable inside the layout.

<data>  
   <variable  
       name="gameViewModel"  
       type="com.example.android.guesstheword.screens.game.GameViewModel" />  
</data>  

Then pass the GameViewModel into the data binding:

In the GameFragment.onCreate, pass in the view model to the GameFragmentBinding.

// Pass the GameViewModel into the data binding - then you can remove the  
binding.gameViewModel = viewModel  

2. In the GameFragment layout, use the view model variable and data binding to handle clicking:

In XML, you can define an onClick attribute for buttons. Using data binding, you can define a data binding expression which is a Listener binding. Essentially this means that you define the OnClickListener in the XML. You also have your view model variable available via data binding. So to create an onClick attribute that will call onSkip in the view model, you can use:

android:onClick="@{() -> gameViewModel.onSkip()}"  

Now you can (and should) remove the OnClickListener setup from the GameFragment. Everything should work just as before.

3. Add a ScoreViewModel data binding variable to ScoreFragment layout:

In score_fragment.xml, repeat the process you used for game_fragment.xml in Step 1.

4. In the ScoreFragment layout, use the view model variable and data binding to handle clicking.

5. Pass the ScoreViewModel into the data binding and remove OnClickListener setup for playAgainButton.

Now instead of defining OnClickListener code in the fragment, you’re using data binding. Run your app and see how all the buttons still work.

Adding LiveData Data Binding¶

In this step, you’ll use LiveData to automagically update your layout via data binding. This will allow you to remove all of your observation lambdas for simple UI updates.

1. Call binding.setLifecycleOwner to make the data binding lifecycle aware:

Open GameFragment. To make your data binding lifecycle aware and to have it play nicely with LiveData, you need to set binding.setLifecycleOwner to this – which refers to GameFragment. This looks like:

// Specify the current activity as the lifecycle owner of the binding. This is used so that  
// the binding can observe LiveData updates  
binding.lifecycleOwner = this  

2. For score_text and word_text use the LiveData from GameViewModel to set the text attribute:

Open up the game_fragment layout. You can use the word LiveData to set the text for the word_text and score_text TextViews. For example, for word_text:

android:text="@{gameViewModel.word}"  

You can also use text formatting.

android:text="@{@string/quote_format(gameViewModel.word)}"  

android:text="@{@string/score_format(gameViewModel.score)}"  

3. Remove the score and word observers:

Note that we’ll fix the currentTime observation in the next exercise. Once you’ve removed the score and word observers, you should be able to run your app and see that the score and word text still updates.

4. Use the LiveData from ScoreViewModel to set the score_text text attribute:

This is very similar to what you just did with word_text and score_text. Repeat in score_fragment.xml. Note that since score is an Int, you’ll need to use String.valueOf() in your binding expression to convert the Int to a String.

5. Call binding.setLifecycleOwner and remove the score observer.

Repeat steps 2 and 3 in the ScoreFragment code. Run your app again to make sure it compiles and runs.

LiveData Map Transformation¶

One of the easiest ways to do simple data manipulations to LiveData such as changing an integer to a string is by using a method called Transformation.map().

The map function takes the output of one LiveData which I’ll call LiveData A and does some sort of conversion on it and then outputs the result to another LiveData which I’ll call LiveData B.

Observers can then observe LiveData B if they want. The conversion is defined in a function. So in our case LiveData A can output along representing how much time has passed. And then we could do a conversion function on it to format it as a string showing the elapsed time, and then that string would be output from LiveData B which in turn would update the game fragment.

In this step you’ll use a Tranformations.map to convert the current time into a formatted String.

1. In GameViewModel create a new LiveData called currentTimeString and Use Transformation.map to take currentTime to a String output from currentTimeString:

This will store the String version of currentTime.

What you want is to use DateUtils to convert the currentTime number output into a String. Then we want to emit that from the
currentTimeString LiveData.

// Create a new LiveData called currentTimeString.  
// Use Transformation.map to take the number output from currentTime, and transform  
// it into a String using DateUtils.  

// The String version of the current time  
val currentTimeString = Transformations.map(currentTime) { time ->  
   DateUtils.formatElapsedTime(time)  
}  

2. Set timer_text to the value of currentTimeString:

In the game_fragment.xml, find the timer_text view and set the text attribute to the value of currentTimeString (not currentTime!) in a binding expression.

android:text="@{gameViewModel.currentTimeString}"  

3. Delete the observer for currentTime from GameFragment:

We don’t need it anymore! As always, run your code!

Optional Exercise: Adding the Buzzer¶

You’ll need some logic that determines when the phone should buzz. Where should that code go? ViewModel

Is this an Event or a State? Event

Let’s add a buzzer! Before you start, check the reference documentation on how to use Vibration on Android. This will mostly be a self-guided exercise, but we’ll give you a few steps to get started.

1. Add the Vibrate permission:

In the AndroidManifest.xml file, above the application tag, add the following tag:

<!-- Add the Vibrate permission -->  
<uses-permission android:name="android.permission.VIBRATE" />  

This provides a permission that lets us vibrate the phone. We will describe Permissions in greater detail later in this course - suffice it to say that without this, our app cannot cause the phone to vibrate on its own.

2. You can also optionally lock the screen to landscape:

While you’re in the manifest, you can also optionally lock the phone to landscape mode. That is done by adding the following lines to the MainActivity tag:

<activity  
   android:name=".MainActivity"  
   android:configChanges="keyboardHidden|orientation|screenSize"  
   android:screenOrientation="landscape">  

3. Copy over the different buzz pattern Long array constants:

Vibration is controlled by passing in an array representing the number of milliseconds each interval of buzzing and non-buzzing takes. So the array [0, 200, 100, 300] will wait 0 milliseconds, then buzz for 200ms, then wait 100ms, then buzz fo 300ms. Here are some example buzz patterns you can copy over:

private val CORRECT_BUZZ_PATTERN = longArrayOf(100, 100, 100, 100, 100, 100)  
private val PANIC_BUZZ_PATTERN = longArrayOf(0, 200)  
private val GAME_OVER_BUZZ_PATTERN = longArrayOf(0, 2000)  
private val NO_BUZZ_PATTERN = longArrayOf(0)  

Put these in the GameViewModel, above the class.

4. Make an enum called BuzzType in GameViewModel.

This enum will represent the different types of buzzing that can occur:

enum class BuzzType(val pattern: LongArray) {  
   CORRECT(CORRECT_BUZZ_PATTERN),  
   GAME_OVER(GAME_OVER_BUZZ_PATTERN),  
   COUNTDOWN_PANIC(PANIC_BUZZ_PATTERN),  
   NO_BUZZ(NO_BUZZ_PATTERN)  
}  

5. Create a properly encapsulated LiveData for a buzz event - its’ type should be BuzzType.

// Event that triggers the phone to buzz using different patterns, determined by BuzzType  
private val _eventBuzz = MutableLiveData<BuzzType>()  
val eventBuzz: LiveData<BuzzType>  
   get() = _eventBuzz  

6. Set the value of buzz event to the correct BuzzType when the buzzer should fire. This should happen when the game is over when the user gets a correct answer, and on each tick when countdown buzzing starts.

override fun onTick(millisUntilFinished: Long) {  
   _currentTime.value = (millisUntilFinished / ONE_SECOND)  
   if (millisUntilFinished / ONE_SECOND <= COUNTDOWN_PANIC_SECONDS) {  
   // COUNTDOWN_PANIC buzz  
       _eventBuzz.value = BuzzType.COUNTDOWN_PANIC  
   }  
}  

override fun onFinish() {  
   _currentTime.value = DONE  
   // GAME_OVER buzz  
   _eventBuzz.value = BuzzType.GAME_OVER  
   _eventGameFinish.value = true  
}  

7. Add a function onBuzzComplete for telling the view model when the buzz event has completed.

fun onBuzzComplete() {  
    _eventBuzz.value = BuzzType.NO_BUZZ  
}  

8. Copy over the buzz method.

Given a pattern, this method will actually perform the buzz. It uses the activity to get a system service, so you should put this in your GameFragment:

private fun buzz(pattern: LongArray) {  
   val buzzer = activity?.getSystemService<Vibrator>()  

   buzzer?.let {  
       if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {  
           buzzer.vibrate(VibrationEffect.createWaveform(pattern, -1))  
       } else {  
           //deprecated in API 26  
           buzzer.vibrate(pattern, -1)  
       }  
   }  
}  

9. Created an observer for the buzz event which calls the buzz method with the correct pattern. Remember to call onBuzzComplete!

// Buzzes when triggered with different buzz events  
viewModel.eventBuzz.observe(viewLifecycleOwner, { buzzType ->  
   if (buzzType != GameViewModel.BuzzType.NO_BUZZ) {  
       buzz(buzzType.pattern)  
       viewModel.onBuzzComplete()  
   }  
})  

Extra: How to force reload the layout?¶

You can force reload the layout by calling binding.invalidateAll().

Summary¶

• The Android app architecture guidelines recommend separating classes that have different responsibilities.
• A UI controller is UI-based class like Activity or Fragment. UI controllers should only contain logic that handles UI and operating system interactions; they shouldn’t contain data to be displayed in the UI. Put that data in a ViewModel.
• The ViewModel class stores and manages UI-related data. The ViewModel class allows data to survive configuration changes such as screen rotations.
• ViewModel is one of the recommended Android Architecture Components.
• ViewModelProvider.Factory is an interface you can use to create a ViewModel object.

The table below compares UI controllers with the ViewModel instances that hold data for them:

LiveData¶

• LiveData is an observable data holder class that is lifecycle-aware, one of the Android Architecture Components.
• You can use LiveData to enable your UI to update automatically when the data updates.
• LiveData is observable, which means that an observer like an activity or an fragment can be notified when the data held by the LiveData object changes.
• LiveData holds data; it is a wrapper that can be used with any data.
• LiveData is lifecycle-aware, meaning that it only updates observers that are in an active lifecycle state such as STARTED or RESUMED.

To add LiveData¶

• Change the type of the data variables in ViewModel to LiveData or MutableLiveData.

MutableLiveData is a LiveData object whose value can be changed. MutableLiveData is a generic class, so you need to specify the type of data that it holds.

• To change the value of the data held by the LiveData, use the setValue() method on the LiveData variable.

To encapsulate LiveData¶

• The LiveData inside the ViewModel should be editable. Outside the ViewModel, the LiveData should be readable. This can be implemented using a Kotlin backing property.
• A Kotlin backing property allows you to return something from a getter other than the exact object.
• To encapsulate the LiveData, use private MutableLiveData inside the ViewModel and return a LiveData backing property outside the ViewModel.

Observable LiveData¶

• LiveData follows an observer pattern. The “observable” is the LiveData object, and the observers are the methods in the UI controllers, like fragments. Whenever the data wrapped inside LiveData changes, the observer methods in the UI controllers are notified.
• To make the LiveData observable, attach an observer object to the LiveData reference in the observers (such as activities and fragments) using the observe() method.
• This LiveData observer pattern can be used to communicate from the ViewModel to the UI controllers.

Data binding with ViewModel and LiveData¶

• The Data Binding Library works seamlessly with Android Architecture Components like ViewModel and LiveData.
• The layouts in your app can bind to the data in the Architecture Components, which already help you manage the UI controller’s lifecycle and notify about changes in the data.

ViewModel data binding¶

• You can associate a ViewModel with a layout by using data binding.
• ViewModel objects hold the UI data. By passing ViewModel objects into the data binding, you can automate some of the communication between the views and the ViewModel objects.

How to associate a ViewModel with a layout:

• In the layout file, add a data-binding variable of the type ViewModel.

<data>  
   <variable  
       name="gameViewModel"  
       type="com.example.android.guesstheword.screens.game.GameViewModel" />  
</data>  

• In the GameFragment file, pass the GameViewModel into the data binding.

binding.gameViewModel = viewModel  

Listener bindings¶

• Listener bindings are binding expressions in the layout that run when click events such as onClick() are triggered.
• Listener bindings are written as lambda expressions.
• Using listener bindings, you replace the click listeners in the UI controllers with listener bindings in the layout file.
• Data binding creates a listener and sets the listener on the view.

android:onClick="@{() -> gameViewModel.onSkip()}"  

Adding LiveData to data binding¶

• LiveData objects can be used as a data-binding source to automatically notify the UI about changes in the data.
• You can bind the view directly to the LiveData object in the ViewModel. When the LiveData in the ViewModel changes, the views in the layout can be automatically updated, without the observer methods in the UI controllers.

android:text="@{gameViewModel.word}"  

• To make the LiveData data binding work, set the current activity (the UI controller) as the lifecycle owner of the binding variable in the UI controller.

binding.lifecycleOwner = this  

String formatting with data binding¶

• Using data binding, you can format a string resource with placeholders like %s for strings and %d for integers.
• To update the text attribute of the view, pass in the LiveData object as an argument to the formatting string.

android:text="@{@string/quote_format(gameViewModel.word)}"  

LiveData transformations¶

Transforming LiveData

• Sometimes you want to transform the results of LiveData. For example, you might want to format a Date string as “hours:mins:seconds,” or return the number of items in a list rather than returning the list itself. To perform transformations on LiveData, use helper methods in the Transformations class.
• The Transformations.map() method provides an easy way to perform data manipulations on the LiveData and return another LiveData object. The recommended practice is to put data-formatting logic that uses the Transformations class in the ViewModel along with the UI data.

Displaying the result of a transformation in a TextView

• Make sure the source data is defined as LiveData in the ViewModel.
• Define a variable, for example newResult. Use Transformation.map() to perform the transformation and return the result to the variable.

val newResult = Transformations.map(someLiveData) { input ->  
   // Do some transformation on the input live data  
   // and return the new value  
}  

• Make sure the layout file that contains the TextView declares a <data> variable for the ViewModel.

<data>  
   <variable  
       name="MyViewModel"  
       type="com.example.android.something.MyViewModel" />  
</data>  

• In the layout file, set the text attribute of the TextView to the binding of the newResult of the ViewModel. For example:

android:text="@{SomeViewModel.newResult}"  

Formatting dates

• The DateUtils.formatElapsedTime() utility method takes a long number of milliseconds and formats the number to use a MM:SS string format.

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