حديقة يُوسُف الرقمية | yshalsager's Digital Garden

Unit 2 - Layouts

٢٢ مايو ٢٠٢٢21 دقائق للقراءة

Unit 2: Layouts

Get user input in an app

Classes and inheritance in Kotlin

  • Create a class hierarchy, that is a tree of classes where children inherit functionality from parent classes. Properties and functions are inherited by subclasses.
    • Class hierarchy. An arrangement where classes are organized in a hierarchy of parents and children. Hierarchy diagrams are usually drawn with the parents shown above children.
    • Child or subclass. Any class that is below another class in the hierarchy.
    • Parent or superclass or base class. Any class with one or more child classes.
    • Root or top-level class. The class at the top (or root) of the class hierarchy.
    • Inheritance. When a child class includes (or inherits) all the properties and methods of its parent class. This allows you to share and reuse code, which makes programs easier to understand and maintain.
  • Create an abstract class where some functionality is left to be implemented by its subclasses.
abstract class Dwelling(private var residents: Int) {
   
   abstract val buildingMaterial: String
   abstract val capacity: Int
    
   fun hasRoom(): Boolean {
       return residents < capacity
   }
}
  • An abstract class can therefore not be instantiated.
  • Create subclasses of an abstract class.
  • Use override keyword to override properties and functions in subclasses.
class SquareCabin(residents: Int) : Dwelling(residents) {
    override val buildingMaterial = "Wood"
    override val capacity = 6
}
  • Use the super keyword to reference functions and properties in the parent class.
override fun floorArea(): Double {
    return super.floorArea() * floors
}
  • Make a class open so that it can be subclassed.
open class RoundHut(residents: Int) : Dwelling(residents) {
   override val buildingMaterial = "Straw"
   override val capacity = 4
}
  • Make a propertyprivate, so it can only be used inside the class.
  • Use the with construct to make multiple calls on the same object instance.
with(roundHut) {
    println("\nRound Tower\n==========")
    println("Material: ${buildingMaterial}")
    println("Capacity: ${capacity}")
    println("Has room? ${hasRoom()}")
}
  • Import functionality from the kotlin.math library
import kotlin.math.PI
import kotlin.math.sqrt
 
fun floorArea(): Double {
    return PI * radius * radius
}
 
fun calculateMaxCarpetSize(): Double {
    val diameter = 2 * radius
    return sqrt(diameter * diameter / 2)
}

Create XML layouts for Android

  • XML (Extensible Markup Language) is a way of organizing text, made of tags, elements, and attributes.
  • Use XML to define the layout of an Android app.
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout  
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
 
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
 
</androidx.constraintlayout.widget.ConstraintLayout>
  • Use EditText to let the user input or edit text.
<EditText
    android:id="@+id/plain_text_input"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    android:inputType="text"/>
  • An EditText can have a hint to tell the user what is expected in that field.
    android:hint="Cost of Service"
  • Specify the android:inputType attribute to limit what type of text the user can input into an EditText field.
    android:inputType="numberDecimal"
  • Make a list of exclusive options with RadioButtons, grouped with a RadioGroup.
<RadioGroup
    android:id="@+id/tip_options"
    android:checkedButton="@id/option_twenty_percent"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:layout_constraintTop_toBottomOf="@id/service_question"
    app:layout_constraintStart_toStartOf="parent"
    android:orientation="vertical">
 
    <RadioButton
        android:id="@+id/option_twenty_percent"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Amazing (20%)" />
 
    <RadioButton
        android:id="@+id/option_eighteen_percent"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Good (18%)" />
 
</RadioGroup>
  • A RadioGroup can be vertical or horizontal, and you can specify which RadioButton should be selected initially.
    android:orientation="vertical"
    android:checkedButton="@id/option_twenty_percent"
  • Use a Switch to let the user toggle between two options.
<Switch
    android:id="@+id/round_up_switch"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:checked="true"
    android:text="Round up tip?"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@id/tip_options" />
  • You can add a label to a Switch without using a separate TextView.
    android:text="Round up tip?"
  • Each child of a ConstraintLayout needs to have vertical and horizontal constraints.
  • Use “start” and “end” constraints to handle both Left to Right (LTR) and Right to Left (RTL) languages.
  • Names of the constraint attributes follow the form layout_constraint<Source>_to<Target>Of.
    • layout_constraintLeft_toLeftOf
    • layout_constraintLeft_toRightOf
    • layout_constraintRight_toLeftOf
    • layout_constraintRight_toRightOf
    • layout_constraintTop_toTopOf
    • layout_constraintTop_toBottomOf
    • layout_constraintBottom_toTopOf
    • layout_constraintBottom_toBottomOf
    • layout_constraintBaseline_toBaselineOf
    • layout_constraintStart_toEndOf
    • layout_constraintStart_toStartOf
    • layout_constraintEnd_toStartOf
    • layout_constraintEnd_toEndOf
  • To make a View as wide as the ConstraintLayout it’s in, constrain the start and end to the start and end of the parent, and set the width to 0dp.
<Button
   android:id="@+id/calculate_button"
   android:layout_width="0dp"
   android:layout_height="wrap_content"
   android:text="Calculate"
   app:layout_constraintTop_toBottomOf="@id/round_up_switch"
   app:layout_constraintStart_toStartOf="parent"
   app:layout_constraintEnd_toEndOf="parent" />

Calculate the tip

  • View binding lets you more easily write code that interacts with the UI elements in your app.

Enable view binding

  • Open the app’s build.gradle file ( Gradle Scripts > build.gradle (Module: Tip_Time.app) )
  • In the android section, add the following lines:
buildFeatures {
    viewBinding true
}

Using view binding

  • Declare a top-level variable in the class for the binding object. It’s defined at this level because it will be used across multiple methods in MainActivity class.
lateinit var binding: ActivityMainBinding

  • The lateinit keyword is something new. It’s a promise that your code will initialize the variable before using it. If you don’t, your app will crash.

  • Initializes the binding object which you’ll use to access Views in the activity_main.xml layout.

binding = ActivityMainBinding.inflate(layoutInflater)
  • Set the content view of the activity. Instead of passing the resource ID of the layout, R.layout.activity_main, this specifies the root of the hierarchy of views in your app, binding.root.
setContentView(binding.root)
  • The full code in MainActivity now should be like:
class MainActivity : AppCompatActivity() {
 
    lateinit var binding: ActivityMainBinding
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)
    }
}
  • Now when you need a reference to a View in your app, you can get it from the binding object instead of calling findViewById(). Thebinding object automatically defines references for every View in your app that has an ID. Using view binding is so much more concise that often you won’t even need to create a variable to hold the reference for a View, just use it directly from the binding object.
// Old way with findViewById()
val myButton: Button = findViewById(R.id.my_button)
myButton.text = "A button"
 
// Better way with view binding
val myButton: Button = binding.myButton
myButton.text = "A button"
 
// Best way with view binding and no extra variable
binding.myButton.text = "A button"
  • The Double data type in Kotlin can store a decimal number. Kotlin provides a method for converting a String to a Double, called toDouble().
val cost = binding.costOfService.text.toString().toDouble()
  • Calling toDouble() on a string that is empty or a string that doesn’t represent a valid decimal number doesn’t work. Fortunately Kotlin also provides a method called toDoubleOrNull() which handles these problems. It returns a decimal number if it can, or it returns null if there’s a problem.
val cost = stringInTextField.toDoubleOrNull()
  • Use the checkedRadioButtonId attribute of a RadioGroup to find which RadioButton is selected.
val selectedId = binding.tipOptions.checkedRadioButtonId
val tipPercentage = when (selectedId) {
    R.id.option_twenty_percent -> 0.20
    R.id.option_eighteen_percent -> 0.18
    else -> 0.15
}
  • For a Switch element, you can check the isChecked attribute to see if the switch is “on”.
val roundUp = binding.roundUpSwitch.isChecked
  • To round a number you can use kotlin.math.ceil() method.
val tip = kotlin.math.ceil(tip)
  • Use NumberFormat.getCurrencyInstance() to get a formatter to use for formatting numbers as currency.
val formattedTip = NumberFormat.getCurrencyInstance().format(tip)
  • You can use string parameters like %s to create dynamic strings that can still be easily translated into other languages.
<string name="tip_amount">Tip Amount: %s</string>
  • You can set the string parameters by calling getString(R.string.tip_amount, formattedTip) and assign that to the text attribute of the tip result TextView.
binding.tipResult.text = getString(R.string.tip_amount, formattedTip)
  • When developing your app (and viewing the preview), it’s useful to have a placeholder for that TextView.
<!-- android:text="@string/tip_amount" -->
tools:text="Tip Amount: $10"
  • Testing is important!
  • You can use Logcat in Android Studio to troubleshoot problems like the app crashing.
  • A stack trace shows a list of methods that were called. This can be useful if the code generates an exception.
2020-06-24 10:09:41.564 24423-24423/com.example.tiptime E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.tiptime, PID: 24423
    java.lang.NumberFormatException: empty String
        at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842)
        at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
        at java.lang.Double.parseDouble(Double.java:538)
        at com.example.tiptime.MainActivity.calculateTip(MainActivity.kt:22)
        at com.example.tiptime.MainActivity$onCreate$1.onClick(MainActivity.kt:17)
  • Exceptions indicate a problem that code didn’t expect.
  • Null means “no value”.
  • Not all code can handle null values, so be careful using it.
  • Use Analyze > Inspect Code for suggestions to improve your code.

Change the app theme

  • Use the Material Color Tool to select colors for your app theme.
  • Alternatively, you can use the Material palette generator to help select a color palette.
  • Declare color resources in the colors.xml file to make it easier to reuse them.
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="purple_200">#FFBB86FC</color>
    <color name="purple_500">#FF6200EE</color>
    <color name="purple_700">#FF3700B3</color>
    <color name="teal_200">#FF03DAC5</color>
    <color name="teal_700">#FF018786</color>
    <color name="black">#FF000000</color>
    <color name="white">#FFFFFFFF</color>
</resources>
  • Use the colors in your theme in themes.xml:
<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Theme.TipTime" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/green</item>
        <item name="colorPrimaryVariant">@color/green_dark</item>
        <item name="colorOnPrimary">@color/white</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/blue</item>
        <item name="colorSecondaryVariant">@color/blue_dark</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>
</resources>
  • Dark theme can reduce power usage and make your app easier to read in low light. (themes.xml (night))
<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Application theme for dark theme. -->
    <style name="Theme.TipTime" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/green_light</item>
        <item name="colorPrimaryVariant">@color/green</item>
        <item name="colorOnPrimary">@color/black</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/blue_light</item>
        <item name="colorSecondaryVariant">@color/blue_light</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>
</resources>

Change the app icon

  • Place app icon files in the mipmap resource directories.
  • Provide different versions of an app icon bitmap image in each density bucket (mdpi, hdpi, xhdpi, xxhdpi, xxxhdpi) for backwards compatibility with older versions of Android.
    • mdpi - resources for medium-density screens (~160 dpi)
    • hdpi - resources for high-density screens (~240 dpi)
    • xhdpi - resources for extra-high-density screens (~320 dpi)
    • xxhdpi - resources for extra-extra-high-density screens (~480dpi)
    • xxxhdpi - resources for extra-extra-extra-high-density screens (~640dpi)
    • nodpi - resources that are not meant to be scaled, regardless of the screen’s pixel density
  • anydpi - resources that scale to any density
  • Add resource qualifiers onto resource directories to specify resources that should be used on devices with a certain configuration (e.g. v26).
  • Vector drawables are Android’s implementation of vector graphics. They are defined in XML as a set of points, lines, and curves along with associated color information. Vector drawables can be scaled for any density without loss of quality.
  • Adaptive icons were introduced to the Android platform in API 26. They are made up of a foreground and background layer that follow specific requirements, so that your app icon looks high-quality on a range of devices with different OEM masks.
<?xml version="1.0" encoding="utf-8"?>
<adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
    <background android:drawable="@drawable/ic_launcher_background" />
    <foreground android:drawable="@drawable/ic_launcher_foreground" />
</adaptive-icon>
  • Use Image Asset Studio in Android Studio to create legacy and adaptive icons for your app.

Create a more polished user experience

  • Use Material Design Components where possible to adhere to Material Design guidelines and allow for more customization.
  • In “ Add:
dependencies {
    ...
    implementation 'com.google.android.material:material:<version>'
}
  • To create a text field as shown above, use a TextInputLayout with an enclosed TextInputEditText from the MDC library. The Material text field can be easily customized to:
    • Display input text or a label that’s always visible
    • Display an icon in the text field
    • Display helper or error messages
<com.google.android.material.textfield.TextInputLayout
   android:id="@+id/cost_of_service"
   android:layout_width="160dp"
   android:layout_height="wrap_content"
   android:hint="@string/cost_of_service"
   app:layout_constraintStart_toStartOf="parent"
   app:layout_constraintTop_toTopOf="parent">
 
   <com.google.android.material.textfield.TextInputEditText
       android:layout_width="match_parent"
       android:layout_height="wrap_content"/>
 
</com.google.android.material.textfield.TextInputLayout>
  • To use SwitchMaterial, you must explicitly specify SwitchMaterial in your layout and use the fully qualified path name. In the activity_main.xml layout, change the XML tag from Switch to:
<com.google.android.material.switchmaterial.SwitchMaterial
    android:id="@+id/round_up_switch"
    android:layout_width="0dp"
    android:layout_height="wrap_content" ... />
  • it’s important to note that support for vector drawables on the Android platform wasn’t added until Android 5.0 (API level 21). in “ add:
android {
  defaultConfig {
    ...
    vectorDrawables.useSupportLibrary = true
   }
   ...
}
  • Add icons to give users visual cues about how parts of your app will function.
<ImageView
    android:id="@+id/icon_service_question"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:importantForAccessibility="no"
    app:srcCompat="@drawable/ic_service"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="@id/service_question"
    app:layout_constraintBottom_toBottomOf="@id/service_question" />
 
<TextView
    android:id="@+id/service_question"
    ...
    android:layout_marginTop="16dp"
    app:layout_constraintStart_toEndOf="@id/icon_service_question" />
  • Use ConstraintLayout to position elements in your layout.
  • A style is a collection of view attributes values for a single type of widget. For example, a TextView style can specify font color, font size, and background color, to name a few. By extracting these attributes into a style, you can easily apply the style to multiple views in the layout and maintain it in a single place (styles.xml).
<?xml version="1.0" encoding="utf-8"?>
<resources>
    <!-- TextView style -->
    <style name="Widget.TipTime.TextView" parent="Widget.MaterialComponents.TextView">
        <item name="android:minHeight">48dp</item>
        <item name="android:gravity">center_vertical</item>
        <item name="android:textAppearance">?attr/textAppearanceBody1</item>
    </style>
 
    <!-- Radio Button style -->
    <style name="Widget.TipTime.CompoundButton.RadioButton" parent="Widget.MaterialComponents.CompoundButton.RadioButton">
        <item name="android:paddingStart">8dp</item>
        <item name="android:textAppearance">?attr/textAppearanceBody1</item>
    </style>
 
    <!-- Switch style -->
    <style name="Widget.TipTime.CompoundButton.Switch" parent="Widget.MaterialComponents.CompoundButton.Switch">
        <item name="android:minHeight">48dp</item>
        <item name="android:gravity">center_vertical</item>
        <item name="android:textAppearance">?attr/textAppearanceBody1</item>
    </style>
</resources>
  • Test your app for edges cases (e.g. rotating your app in landscape mode) and make improvements where applicable.
  • You can add a ScrollView around the ConstraintLayout to prevent UI items from getting truncated in landscape mode.
<ScrollView
   xmlns:android="http://schemas.android.com/apk/res/android"
   xmlns:app="http://schemas.android.com/apk/res-auto"
   xmlns:tools="http://schemas.android.com/tools"
   android:layout_height="match_parent"
   android:layout_width="match_parent">
 
   <androidx.constraintlayout.widget.ConstraintLayout
       android:layout_width="match_parent"
       android:layout_height="wrap_content"
       android:padding="16dp"
       tools:context=".MainActivity">
 
       ...
   </ConstraintLayout>
 
</ScrollView>
  • Comment your code to help other people who are reading your code understand what your approach was.
  • Reformat your code and clean up your code to make it as concise as possible.

Hide keyboard on Enter key

It’s a bit cumbersome to manually hide the keyboard each time to better access the calculate button. Instead, make the keyboard automatically hide itself when the Enter key is pressed. The handleKeyEvent() is a private helper function that hides the onscreen keyboard if the keyCode input parameter is equal to KeyEvent.KEYCODE_ENTER. The InputMethodManager controls if a soft keyboard is shown, hidden, and allows the user to choose which soft keyboard is displayed. The method returns true if the key event was handled, and returns false otherwise.

private fun handleKeyEvent(view: View, keyCode: Int): Boolean {
   if (keyCode == KeyEvent.KEYCODE_ENTER) {
       // Hide the keyboard
       val inputMethodManager =
           getSystemService(Context.INPUT_METHOD_SERVICE) as InputMethodManager
       inputMethodManager.hideSoftInputFromWindow(view.windowToken, 0)
       return true
   }
   return false
}

The onKey() method takes in 3 input arguments: the view, the code for the key that was pressed, and a key event (which you won’t use, so you can call it “_”). When the onKey() method is called, you should call your handleKeyEvent() method and pass along the view and key code arguments. The syntax for writing this out is: view, keyCode, _ -> handleKeyEvent(view, keyCode).

override fun onCreate(savedInstanceState: Bundle?) {
   ...
 
   setContentView(binding.root)
 
   binding.calculateButton.setOnClickListener { calculateTip() }
 
   binding.costOfServiceEditText.setOnKeyListener { view, keyCode, _ -> handleKeyEvent(view, keyCode)
   }
}

Adjust the tint of the vector drawables

You can change the tint of the icons based on the primary color of the theme, so that the icons appear differently in light vs. dark theme.

One of the advantages of VectorDrawables versus bitmap images is the ability to scale and tint them. Below we have the XML representing the bell icon. There are two specific color attributes to take notice of: android:tint and android:fillColor.

<vector xmlns:android="http://schemas.android.com/apk/res/android"
   android:width="24dp"
   android:height="24dp"
   android:viewportWidth="24"
   android:viewportHeight="24"
   <!-- android:tint="?attr/colorControlNormal" -->
   android:tint="?attr/colorPrimary">
 <path
     android:fillColor="@android:color/white"
     android:pathData="M2,17h20v2L2,19zM13.84,7.79c0.1,-0.24 0.16,-0.51 0.16,-0.79 0,-1.1 -0.9,-2 -2,-2s-2,0.9 -2,2c0,0.28 0.06,0.55 0.16,0.79C6.25,8.6 3.27,11.93 3,16h18c-0.27,-4.07 -3.25,-7.4 -7.16,-8.21z"/>
</vector>

Display a scrollable list

Use Lists in Kotlin

Kotlin provides functionality to help you manage and manipulate collections of data more easily through the Kotlin Standard Library. A collection can be defined as a number of objects of the same data type. There are different basic collection types in Kotlin: lists, sets, and maps. This codelab focused specifically on lists, and you’ll learn more about sets and maps in future codelabs.

  • A list is an ordered collection of elements of a specific type, such as a list of Strings.
val numbers: List<Int> = listOf(1, 2, 3, 4, 5, 6)
  • The index is the integer position that reflects the position of the element (e.g. myList[2]).
println("Second element: ${numbers[1]}")
  • In a list, the first element is at index 0 (e.g. myList[0]), and the last element is at myList.size-1 (e.g. myList[myList.size-1] or myList.last()).
println("Last index: ${numbers.size - 1}")
println("Last element: ${numbers[numbers.size - 1]}")
  • Kotlin also supports first() and last() operations on a list.
println("First: ${numbers.first()}")
println("Last: ${numbers.last()}")
  • Another useful list operation is the contains() method to find out if a given element is in the list.
println("Contains 4? ${numbers.contains(4)}")
  • There are two types of lists: List and MutableList.
  • A List is read-only and cannot be modified once it has been initialized. However, you can apply operations such as sorted() and reversed() which return a new list without changing the original.
println("Sorted list: ${colors.sorted()}")
println("Reversed list: ${colors.reversed()}")
  • A MutableList can be modified after creation such as adding, removing, or modifying elements.
val entrees = mutableListOf<String>()
val entrees: MutableList<String> = mutableListOf()
println("Add noodles: ${entrees.add("noodles")}")
println("Remove spaghetti: ${entrees.remove("spaghetti")}")
println("Remove first element: ${entrees.removeAt(0)}")
entrees.clear()
  • You can add a list of items to a mutable list using addAll().
val moreItems = listOf("ravioli", "lasagna", "fettuccine")
println("Add list: ${entrees.addAll(moreItems)}")
  • Kotlin gives you a way to check if a list is empty using isEmpty() function.
println("Empty? ${entrees.isEmpty()}")
  • Use a while loop to execute a block of code until the expression evaluates to false and you exit the loop.
while (expression) {

// While the expression is true, execute this code block

}

val guestsPerFamily = listOf(2, 4, 1, 3)
var totalGuests = 0
var index = 0
while (index < guestsPerFamily.size) {
    totalGuests += guestsPerFamily[index]
    index++
}
println("Total Guest Count: $totalGuests")
  • Use a for loop to iterate over all items of a list:
for (item in myList) {

// Execute this code block for each element of the list

}

val names = listOf("Jessica", "Henry", "Alicia", "Jose")
for (name in names) {
    println("$name - Number of characters: ${name.length}")
}

Variations of for loop

Note: Here are some other variations of what you can do with for loops, including using them with ranges with specific steps (instead of incrementing by 1 each time).

for (item in list) print(item) // Iterate over items in a list
 
for (item in 'b'..'g') print(item) // Range of characters in an alphabet
 
for (item in 1..5) print(item) // Range of numbers
 
for (item in 5 downTo 1) print(item) // Going backward
 
for (item in 3..6 step 2) print(item) // Prints: 35
  • The vararg modifier allows you to pass in a variable number of arguments to a function or constructor.
class Vegetables(vararg val toppings: String) : Item("Vegetables", 5) {
    override fun toString(): String {
        return name + " " + toppings.joinToString()
    }
}
  • To specify a different separator other than a comma, pass in the desired separator string as an argument to the joinToString() method. Example: joinToString(" ") to separate each item with a space.

Use RecyclerView to display a scrollable list

  • Organizing your code logically helps you and other developers understand, maintain, and extend it. In the same way that you can organize paperwork into files and folders, you can organize your code into files and packages.
  • RecyclerView widget helps you display a list of data.
      • item - One data item of the list to display. Represents one Affirmation object in your app.
    • Adapter - Takes data and prepares it for RecyclerView to display.
    • ViewHolders - A pool of views for RecyclerView to use and reuse to display affirmations.
  • ConstraintLayout is ideal and flexible when you want to position multiple child views in a layout. Since your layout only has a single child view, RecyclerView, you can switch to a simpler ViewGroup called FrameLayout that should be used for holding a single child view.
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
 
    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:scrollbars="vertical"
        app:layoutManager="LinearLayoutManager" />
 
</FrameLayout>
  • RecyclerView uses the adapter pattern to adapt and display the data.
  • ViewHolder creates and holds the views for RecyclerView.
class ItemViewHolder(private val view: View) : RecyclerView.ViewHolder(view) {
    val textView: TextView = view.findViewById(R.id.item_title)
}
  • Each item in the RecyclerView has its own layout, which you define in a separate layout file. For example, list_item.xml.
<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/item_title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />
  • RecyclerView comes with built in LayoutManagers. RecyclerView delegates how items are laid out to LayoutManagers.
    app:layoutManager="LinearLayoutManager"
  • To be able to scroll through a vertical list of items that is longer than the screen, you need to add a vertical scrollbar.
    android:scrollbars="vertical"

Create the Affirmation data class

package com.example.affirmations.model
 
data class Affirmation(val stringResourceId: Int)

Create a class to be a data source

Data displayed in your app may come from different sources (e.g. within your app project or from an external source that requires connecting to the internet to download data). As a result, data may not be in the exact format that you need. The rest of the app should not concern itself with where the data originates from or in what format it is originally. You can and should hide away this data preparation in a separate Datasource class that prepares the data for the app. For example we used strings as data sources:

<resources>
    <string name="app_name">Affirmations</string>
    <string name="affirmation1">I am strong.</string>
    <string name="affirmation2">I believe in myself.</string>
    ...
    <string name="affirmation10">I will be present in all the moments that this day brings.</string>
</resources>

The Datasource class code should be like this:

package com.example.affirmations.data
 
import com.example.affirmations.R
import com.example.affirmations.model.Affirmation
 
 
class Datasource {
 
    fun loadAffirmations(): List<Affirmation> {
        return listOf<Affirmation>(
            Affirmation(R.string.affirmation1),
            Affirmation(R.string.affirmation2),
            ...
            Affirmation(R.string.affirmation10)
        )
    }
}

How To implement the adapter

  • Create a new class for the adapter, for example, ItemAdapter.
  • Create a custom ViewHolder class that represents a single list item view. Extend from RecyclerView.ViewHolder class.
  • Modify the ItemAdapter class to extend from the RecyclerView.Adapter class with the custom ViewHolder class.
  • Implement these methods within the adapter: getItemsCount(), onCreateViewHolder(), and onBindViewHolder().
package com.example.affirmations.adapter
 
import android.content.Context
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import com.example.affirmations.R
import com.example.affirmations.model.Affirmation
 
/**
 * Adapter for the [RecyclerView] in [MainActivity]. Displays [Affirmation] data object.
 */
class ItemAdapter(
    private val context: Context,
    private val dataset: List<Affirmation>
) : RecyclerView.Adapter<ItemAdapter.ItemViewHolder>() {
 
    // Provide a reference to the views for each data item
    // Complex data items may need more than one view per item, and
    // you provide access to all the views for a data item in a view holder.
    // Each data item is just an Affirmation object.
    class ItemViewHolder(private val view: View) : RecyclerView.ViewHolder(view) {
        val textView: TextView = view.findViewById(R.id.item_title)
    }
 
    /**
     * Create new views (invoked by the layout manager)
     */
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ItemViewHolder {
        // create a new view
        val adapterLayout = LayoutInflater.from(parent.context)
            .inflate(R.layout.list_item, parent, false)
 
        return ItemViewHolder(adapterLayout)
    }
 
    /**
     * Replace the contents of a view (invoked by the layout manager)
     */
    override fun onBindViewHolder(holder: ItemViewHolder, position: Int) {
        val item = dataset[position]
        holder.textView.text = context.resources.getString(item.stringResourceId)
    }
 
    /**
     * Return the size of your dataset (invoked by the layout manager)
     */
    override fun getItemCount() = dataset.size
}

Modify the MainActivity to use a RecyclerView

  1. Create an instance of Datasource, and call the loadAffirmations() method on it. Store the returned list of affirmations in a val named myDataset.
    val myDataset = Datasource().loadAffirmations()
  1. Create a variable called recyclerView and use findViewById() to find a reference to the RecyclerView within the layout.
    val recyclerView = findViewById<RecyclerView>(R.id.recycler_view)
  1. To tell the recyclerView to use the ItemAdapter class you created, create a new ItemAdapter instance. ItemAdapter expects two parameters: the context (this) of this activity, and the affirmations in myDataset.
  2. Assign the ItemAdapter object to the adapter property of the recyclerView.
    recyclerView.adapter = ItemAdapter(this, myDataset)
  1. Since the layout size of your RecyclerView is fixed in the activity layout, you can set the setHasFixedSize parameter of the RecyclerView to true. This setting is only needed to improve performance. Use this setting if you know that changes in content do not change the layout size of the RecyclerView.
    recyclerView.setHasFixedSize(true)
  1. When you are done, the code for MainActivity should be similar to the following.
package com.example.affirmations
 
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.RecyclerView
import com.example.affirmations.adapter.ItemAdapter
import com.example.affirmations.data.Datasource
 
class MainActivity : AppCompatActivity() {
 
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
 
        // Initialize data.
        val myDataset = Datasource().loadAffirmations()
 
        val recyclerView = findViewById<RecyclerView>(R.id.recycler_view)
        recyclerView.adapter = ItemAdapter(this, myDataset)
 
        // Use this setting to improve performance if you know that changes
        // in content do not change the layout size of the RecyclerView
        recyclerView.setHasFixedSize(true)
    }
}

Display a list of images using cards

  • To display additional content in a RecyclerView, modify the underlying data model class and data source. Then update the list item layout and adapter to set that data onto the views. Affirmation.kt
package com.example.affirmations.model
 
import androidx.annotation.DrawableRes
import androidx.annotation.StringRes
 
data class Affirmation(
   @StringRes val stringResourceId: Int,
   @DrawableRes val imageResourceId: Int
)

Datasource.kt

package com.example.affirmations.data
 
import com.example.affirmations.R
import com.example.affirmations.model.Affirmation
 
class Datasource() {
 
    fun loadAffirmations(): List<Affirmation> {
        return listOf<Affirmation>(
            Affirmation(R.string.affirmation1, R.drawable.image1),
            Affirmation(R.string.affirmation2, R.drawable.image2),
            ...
            Affirmation(R.string.affirmation10, R.drawable.image10)
        )
    }
}

list_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:padding="16dp">
 
    <ImageView
        android:layout_width="match_parent"
        android:layout_height="194dp"
        android:id="@+id/item_image"
        android:importantForAccessibility="no"
        android:scaleType="centerCrop" />
    
    <TextView
        android:id="@+id/item_title"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="16dp"
        android:textAppearance="?attr/textAppearanceHeadline6" />
 
</LinearLayout>

ItemAdapter.kt

class ItemViewHolder(private val view: View): RecyclerView.ViewHolder(view) {
    val textView: TextView = view.findViewById(R.id.item_title)
    val imageView: ImageView = view.findViewById(R.id.item_image)
}
    override fun onBindViewHolder(holder: ItemViewHolder, position: Int) {
        val item = dataset[position]
        holder.textView.text = context.resources.getString(item.stringResourceId)
        holder.imageView.setImageResource(item.imageResourceId)
    }
  • Use resource annotations to help ensure that the right type of resource ID is passed into a class constructor.
   @StringRes val stringResourceId: Int,
   @DrawableRes val imageResourceId: Int
  • Use the Material Components for Android library to have your app more easily follow the recommended Material Design guidelines.
  • Use MaterialCardView to display content in a Material card.
<?xml version="1.0" encoding="utf-8"?>
<com.google.android.material.card.MaterialCardView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="8dp">
 
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">
 
        <ImageView
            android:id="@+id/item_image"
            android:layout_width="match_parent"
            android:layout_height="194dp"
            android:importantForAccessibility="no"
            android:scaleType="centerCrop" />
 
        <TextView
            android:id="@+id/item_title"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:padding="16dp"
            android:textAppearance="?attr/textAppearanceHeadline6" />
 
    </LinearLayout>
 
</com.google.android.material.card.MaterialCardView>
  • Small visual tweaks to your app in terms of color and spacing can make the app look more polished and consistent.
  • Common Theme Attributes allows you to use predefined text appearance values.

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