Advanced Android in Kotlin 02.1: Creating Custom Views

1. Welcome

Introduction

Android offers a large set of View subclasses, such as Button, TextView, EditText, ImageView, CheckBox, or RadioButton. You can use these subclasses to construct a UI that enables user interaction and displays information in your app. If none of the View subclasses meet your needs, you can create a View subclass known as a custom view.

To create a custom view you can either extend an existing View subclass (such as a Button or EditText), or create your own subclass of View. By extending View directly, you can create an interactive UI element of any size and shape by overriding the onDraw() method for the View to draw it.

After you create a custom view, you can add it to your activity layouts in the same way you would add a TextView or Button.

This lesson shows you how to create a custom view from scratch by extending View.

What you should already know

• How to create an app with an Activity and run it using Android Studio.

What you'll learn

• How to extend View to create a custom view.
• How to draw a custom view that is circular in shape.
• How to use listeners to handle user interaction with the custom view.
• How to use a custom view in a layout.

What you'll do

• Extend View to create a custom view.
• Initialize the custom view with drawing and painting values.
• Override onDraw() to draw the view.
• Use listeners to provide the custom view's behavior.
• Add the custom view to a layout.

2. App overview

The CustomFanController app demonstrates how to create a custom view subclass by extending the View class. The new subclass is called DialView.

The app displays a circular UI element that resembles a physical fan control, with settings for off (0), low (1), medium (2), and high (3). When the user taps on the view, the selection indicator moves to the next position: 0-1-2-3, and back to 0. Also, if the selection is 1 or higher, the background color of the circular part of the view changes from gray to green (indicating that the fan power is on).

3. Concept: Understanding custom views

Views are the basic building blocks of an app's UI. The View class provides many subclasses, referred to as UI widgets, that cover many of the needs of a typical Android app's user interface.

UI building blocks such as Button and TextView are subclasses that extend the View class. To save time and development effort, you can extend one of these View subclasses. The custom view inherits the look and behavior of its parent, and you can override the behavior or aspect of the appearance that you want to change. For example, if you extend EditText to create a custom view, the view acts just like an EditText view, but could also be customized to show, for example, an X button that clears text from the text entry field.

You can extend any View subclass, such as EditText, to get a custom view—pick the one closest to what you want to accomplish. You can then use the custom view like any other View subclass in one or more layouts as an XML element with attributes.

To create your own custom view from scratch, extend the View class itself. Your code overrides View methods to define the view's appearance and functionality. Key to creating your own custom view is that you are responsible for drawing the entire UI element of any size and shape to the screen. If you subclass an existing view such as Button, that class handles drawing for you. (You'll learn more about drawing later in this codelab.)

To create a custom view follow these general steps:

• Create a custom view class that extends View, or extends a View subclass (such as Button or EditText).
• If you extend an existing View subclass, override only the behavior or aspects of the appearance that you want to change.
• If you extend the View class, draw the custom view's shape and control its appearance by overriding View methods such as onDraw() and onMeasure() in the new class.
• Add code to respond to user interaction and, if necessary, redraw the custom view.
• Use the custom view class as a UI widget in your activity's XML layout. You can also define custom attributes for the view, to provide customization for the view in different layouts.

4. Task. Create a custom view

In this task you will:

• Create an app with an ImageView as a temporary placeholder for the custom view.
• Extend View to create the custom view.
• Initialize the custom view with drawing and painting values.

Step 1: Create an app with an ImageView placeholder

1. Create a Kotlin app with the title CustomFanController using the Empty Activity template. Make sure the package name is com.example.android.customfancontroller.
2. Open activity_main.xml in the Text tab to edit the XML code.
3. Replace the existing TextView with this code. This text acts as a label in the activity for the custom view.

<TextView
       android:id="@+id/customViewLabel"
       android:textAppearance="@style/Base.TextAppearance.AppCompat.Display3"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:padding="16dp"
       android:textColor="@android:color/black"
       android:layout_marginStart="8dp"
       android:layout_marginEnd="8dp"
       android:layout_marginTop="24dp"
       android:text="Fan Control"
       app:layout_constraintLeft_toLeftOf="parent"
       app:layout_constraintRight_toRightOf="parent"
       app:layout_constraintTop_toTopOf="parent"/>

4. Add this ImageView element to the layout. This is a placeholder for the custom view you will create in this codelab.

<ImageView
       android:id="@+id/dialView"
       android:layout_width="200dp"
       android:layout_height="200dp"
       android:background="@android:color/darker_gray"
       app:layout_constraintTop_toBottomOf="@+id/customViewLabel"
       app:layout_constraintLeft_toLeftOf="parent"
       app:layout_constraintRight_toRightOf="parent"
       android:layout_marginLeft="8dp"
       android:layout_marginRight="8dp"
       android:layout_marginTop="8dp"/>

5. Extract string and dimension resources in both UI elements.
6. Click the Design tab. The layout should look like this:

Step 2. Create your custom view class

1. Create a new Kotlin class called DialView.
2. Modify the class definition to extend View. Import android.view.View when prompted.
3. Click on View and then click the red bulb. Choose Add Android View constructors using ‘@JvmOverloads'. Android Studio adds the constructor from the View class. The @JvmOverloads annotation instructs the Kotlin compiler to generate overloads for this function that substitute default parameter values.

class DialView @JvmOverloads constructor(
   context: Context,
   attrs: AttributeSet? = null,
   defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {

4. Above the DialView class definition, just below the imports, add a top-level enum to represent the available fan speeds. Note that this enum is of type Int because the values are string resources rather than actual strings. Android Studio will show errors for the missing string resources in each of these values; you'll fix that in a later step.

private enum class FanSpeed(val label: Int) {
   OFF(R.string.fan_off),
   LOW(R.string.fan_low),
   MEDIUM(R.string.fan_medium),
   HIGH(R.string.fan_high);
}

5. Below the enum, add these constants. You'll use these as part of drawing the dial indicators and labels.

private const val RADIUS_OFFSET_LABEL = 30      
private const val RADIUS_OFFSET_INDICATOR = -35

6. Inside the DialView class, define several variables you need in order to draw the custom view. Import android.graphics.PointF if requested.

private var radius = 0.0f                   // Radius of the circle.
private var fanSpeed = FanSpeed.OFF         // The active selection.
// position variable which will be used to draw label and indicator circle position
private val pointPosition: PointF = PointF(0.0f, 0.0f)

• The radius is the current radius of the circle. This value is set when the view is drawn on the screen.
• The fanSpeed is the current speed of the fan, which is one of the values in the FanSpeed enumeration. By default that value is OFF.
• Finally postPosition is an X, Y point that will be used for drawing several of the view's elements on the screen.

These values are created and initialized here instead of when the view is actually drawn, to ensure that the actual drawing step runs as fast as possible.

7. Also inside the DialView class definition, initialize a Paint object with a handful of basic styles. Import android.graphics.Paint and android.graphics.Typeface when requested. As previously with the variables, these styles are initialized here to help speed up the drawing step.

private val paint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
   style = Paint.Style.FILL
   textAlign = Paint.Align.CENTER
   textSize = 55.0f
   typeface = Typeface.create( "", Typeface.BOLD)
}

8. Open res/values/strings.xml and add the string resources for the fan speeds:

<string name="fan_off">off</string>
<string name="fan_low">1</string>
<string name="fan_medium">2</string>
<string name="fan_high">3</string>

5. Concept: Drawing custom views

Once you have created a custom view, you need to be able to draw it. When you extend a View subclass such as EditText, that subclass defines the view's appearance and attributes and draws itself on the screen. Consequently, you don't have to write code to draw the view. You can override methods of the parent to customize your view instead.

If you are creating your own view from scratch (by extending View), you are responsible for drawing the entire view each time the screen refreshes, and for overriding the View methods that handle drawing. In order to properly draw a custom view that extends View, you need to:

• Calculate the view's size when it first appears, and each time that view's size changes, by overriding the onSizeChanged() method.
• Override the onDraw() method to draw the custom view, using a Canvas object styled by a Paint object.
• Call the invalidate() method when responding to a user click that changes how the view is drawn to invalidate the entire view, thereby forcing a call to onDraw() to redraw the view.

The onDraw() method is called every time the screen refreshes, which can be many times a second. For performance reasons and to avoid visual glitches, you should do as little work as possible in onDraw(). In particular, don't place allocations in onDraw(), because allocations may lead to a garbage collection that may cause a visual stutter.

The Canvas and Paint classes offer a number of useful drawing shortcuts:

• Draw text using drawText(). Specify the typeface by calling setTypeface(), and the text color by calling setColor().
• Draw primitive shapes using drawRect(), drawOval(), and drawArc(). Change whether the shapes are filled, outlined, or both by calling setStyle().
• Draw bitmaps using drawBitmap().

You'll learn more about Canvas and Paint in a later codelab. To learn more about how Android draws views, see How Android Draws Views.

6. Task. Draw the custom view and add it to the layout

In this task you will draw the fan controller custom view onto the screen—the dial itself, the current position indicator, and the indicator labels—with the onSizeChanged() and onDraw() methods. You'll also create a helper method, computeXYForSpeed(), to calculate the current X,Y position of the indicator label on the dial.

Step 1. Calculate positions and draw the view

1. In the DialView class, below the initializations, override the onSizeChanged() method from the View class to calculate the size for the custom view's dial. Import kotlin.math.min when requested.

The onSizeChanged() method is called any time the view's size changes, including the first time it is drawn when the layout is inflated. Override onSizeChanged() to calculate positions, dimensions, and any other values related to your custom view's size, instead of recalculating them every time you draw. In this case you use onSizeChanged() to calculate the current radius of the dial's circle element.

override fun onSizeChanged(width: Int, height: Int, oldWidth: Int, oldHeight: Int) {
   radius = (min(width, height) / 2.0 * 0.8).toFloat()
}

2. Below onSizeChanged(), add this code to define a computeXYForSpeed() extension function for the PointF class. Import kotlin.math.cos and kotlin.math.sin when requested. This extension function on the PointF class calculates the X, Y coordinates on the screen for the text label and current indicator (0, 1, 2, or 3), given the current FanSpeed position and radius of the dial. You'll use this in onDraw().

private fun PointF.computeXYForSpeed(pos: FanSpeed, radius: Float) {
   // Angles are in radians.
   val startAngle = Math.PI * (9 / 8.0)   
   val angle = startAngle + pos.ordinal * (Math.PI / 4)
   x = (radius * cos(angle)).toFloat() + width / 2
   y = (radius * sin(angle)).toFloat() + height / 2
}

3. Override the onDraw() method to render the view on the screen with the Canvas and Paint classes. Import android.graphics.Canvas when requested. This is the skeleton override:

override fun onDraw(canvas: Canvas) {
   super.onDraw(canvas)
   
}

4. Inside onDraw(), add this line to set the paint color to gray (Color.GRAY) or green (Color.GREEN) depending on whether the fan speed is OFF or any other value. Import android.graphics.Color when requested.

// Set dial background color to green if selection not off.
paint.color = if (fanSpeed == FanSpeed.OFF) Color.GRAY else Color.GREEN

5. Add this code to draw a circle for the dial, with the drawCircle() method. This method uses the current view width and height to find the center of the circle, the radius of the circle, and the current paint color. The width and height properties are members of the View superclass and indicate the current dimensions of the view.

// Draw the dial.
canvas.drawCircle((width / 2).toFloat(), (height / 2).toFloat(), radius, paint)

6. Add this following code to draw a smaller circle for the fan speed indicator mark, also with the drawCircle() method This part uses the PointF.computeXYforSpeed() extension method to calculate the X,Y coordinates for the indicator center based on the current fan speed.

// Draw the indicator circle.
val markerRadius = radius + RADIUS_OFFSET_INDICATOR
pointPosition.computeXYForSpeed(fanSpeed, markerRadius)
paint.color = Color.BLACK
canvas.drawCircle(pointPosition.x, pointPosition.y, radius/12, paint)

7. Finally, draw the fan speed labels (0, 1, 2, 3) at the appropriate positions around the dial. This part of the method calls PointF.computeXYForSpeed() again to get the position for each label, and reuses the pointPosition object each time to avoid allocations. Use drawText() to draw the labels.

// Draw the text labels.
val labelRadius = radius + RADIUS_OFFSET_LABEL
for (i in FanSpeed.values()) {
   pointPosition.computeXYForSpeed(i, labelRadius)
   val label = resources.getString(i.label)
   canvas.drawText(label, pointPosition.x, pointPosition.y, paint)
}

The completed onDraw() method looks like this:

override fun onDraw(canvas: Canvas) {
   super.onDraw(canvas)
   // Set dial background color to green if selection not off.
   paint.color = if (fanSpeed == FanSpeed.OFF) Color.GRAY else Color.GREEN
   // Draw the dial.
   canvas.drawCircle((width / 2).toFloat(), (height / 2).toFloat(), radius, paint)
   // Draw the indicator circle.
   val markerRadius = radius + RADIUS_OFFSET_INDICATOR
   pointPosition.computeXYForSpeed(fanSpeed, markerRadius)
   paint.color = Color.BLACK
   canvas.drawCircle(pointPosition.x, pointPosition.y, radius/12, paint)
   // Draw the text labels.
   val labelRadius = radius + RADIUS_OFFSET_LABEL
   for (i in FanSpeed.values()) {
       pointPosition.computeXYForSpeed(i, labelRadius)
       val label = resources.getString(i.label)
       canvas.drawText(label, pointPosition.x, pointPosition.y, paint)
   }
}

Step 2. Add the view to the layout

To add a custom view to an app's UI, you specify it as an element in the activity's XML layout. Control its appearance and behavior with XML element attributes, as you would for any other UI element.

1. In activity_main.xml, change the ImageView tag for the dialView to com.example.android.customfancontroller.DialView, and delete the android:background attribute. Both DialView and the original ImageView inherit the standard attributes from the View class, so there is no need to change any of the other attributes. The new DialView element looks like this:

<com.example.android.customfancontroller.DialView
       android:id="@+id/dialView"
       android:layout_width="@dimen/fan_dimen"
       android:layout_height="@dimen/fan_dimen"
       app:layout_constraintTop_toBottomOf="@+id/customViewLabel"
       app:layout_constraintLeft_toLeftOf="parent"
       app:layout_constraintRight_toRightOf="parent"
       android:layout_marginLeft="@dimen/default_margin"
       android:layout_marginRight="@dimen/default_margin"
       android:layout_marginTop="@dimen/default_margin" />

2. Run the app. Your fan control view appears in the activity.

7. Task. Add view interactivity

The final task is to enable your custom view to perform an action when the user taps the view. Each tap should move the selection indicator to the next position: off-1-2-3 and back to off. Also, if the selection is 1 or higher, change the background from gray to green, indicating that the fan power is on.

To enable your custom view to be clickable, you:

• Set the view's isClickable property to true. This enables your custom view to respond to clicks.
• Implement the View class's performClick() to perform operations when the view is clicked.
• Call the invalidate() method. This tells the Android system to call the onDraw() method to redraw the view.

Normally, with a standard Android view, you implement OnClickListener() to perform an action when the user clicks that view. For a custom view, you implement the View class's performClick() method instead, and call super.performClick(). The default performClick() method also calls onClickListener(), so you can add your actions to performClick() and leave onClickListener() available for further customization by you or other developers that might use your custom view.

1. In DialView.kt, inside the FanSpeed enumeration, add an extension function next() that changes the current fan speed to the next speed in the list (from OFF to LOW, MEDIUM, and HIGH, and then back to OFF). The complete enumeration now looks like this:

private enum class FanSpeed(val label: Int) {
   OFF(R.string.fan_off),
   LOW(R.string.fan_low),
   MEDIUM(R.string.fan_medium),
   HIGH(R.string.fan_high);

   fun next() = when (this) {
       OFF -> LOW
       LOW -> MEDIUM
       MEDIUM -> HIGH
       HIGH -> OFF
   }
}

2. Inside the DialView class, just before the onSizeChanged() method, add an init{} block. Setting the view's isClickable property to true enables that view to accept user input.

init {
   isClickable = true
}

3. Below init{}, override the performClick() method with the code below.

override fun performClick(): Boolean {
   if (super.performClick()) return true

   fanSpeed = fanSpeed.next()
   contentDescription = resources.getString(fanSpeed.label)
  
   invalidate()
   return true
}

The call to super.performClick() must happen first, which enables accessibility events as well as calls onClickListener().

The next two lines increment the speed of the fan with the next() method, and set the view's content description to the string resource representing the current speed (off, 1, 2 or 3).

FInally, the invalidate() method invalidates the entire view, forcing a call to onDraw() to redraw the view. If something in your custom view changes for any reason, including user interaction, and the change needs to be displayed, call invalidate().

4. Run the app. Tap the DialView element to move the indicator from off to 1. The dial should turn green. With each tap, the indicator should move to the next position. When the indicator returns to off, the dial should turn gray again.

8. Task: Use custom attributes with your custom view

This example shows the basic mechanics of using custom attributes with your custom view. You define custom attributes for the DialView class with a different color for each fan dial position.

1. Create and open res/values/attrs.xml.
2. Inside <resources>, add a <declare-styleable> resource element.
3. Inside the <declare-styleable> resource element, add three attr elements, one for each attribute, with a name and format. The format is like a type, and in this case, it's color.

<?xml version="1.0" encoding="utf-8"?>
<resources>
       <declare-styleable name="DialView">
           <attr name="fanColor1" format="color" />
           <attr name="fanColor2" format="color" />
           <attr name="fanColor3" format="color" />
       </declare-styleable>
</resources>

4. Open the activity_main.xml layout file.
5. In the DialView, add attributes for fanColor1, fanColor2, and fanColor3, and set their values to the colors shown below. Use app: as the preface for the custom attribute (as in app:fanColor1) rather than android: because your custom attributes belong to the schemas.android.com/apk/res/your_app_package_name namespace rather than the android namespace.

app:fanColor1="#FFEB3B"
app:fanColor2="#CDDC39"
app:fanColor3="#009688"

In order to use the attributes in your DialView class, you need to retrieve them. They are stored in an AttributeSet, which is handed to your class upon creation, if it exists. You retrieve the attributes in init, and assign the attribute values to local variables for caching.

6. Open the DialView.kt class file.
7. Inside the DialView, declare variables to cache the attribute values.

private var fanSpeedLowColor = 0
private var fanSpeedMediumColor = 0
private var fanSpeedMaxColor = 0

8. In the init block, add the following code using the withStyledAttributes extension function. You supply the attributes and view, and set your local variables. Importing withStyledAttributes will also import the right getColor()function.

context.withStyledAttributes(attrs, R.styleable.DialView) {
   fanSpeedLowColor = getColor(R.styleable.DialView_fanColor1, 0)
   fanSpeedMediumColor = getColor(R.styleable.DialView_fanColor2, 0)
   fanSeedMaxColor = getColor(R.styleable.DialView_fanColor3, 0)
}

9. Use the local variables in onDraw()to set the dial color based on the current fan speed. Replace the line where the paint color is set (paint.color = if (fanSpeed == FanSpeed.OFF) Color.GRAY else Color.GREEN) with the code below.

paint.color = when (fanSpeed) {
   FanSpeed.OFF -> Color.GRAY
   FanSpeed.LOW -> fanSpeedLowColor
   FanSpeed.MEDIUM -> fanSpeedMediumColor
   FanSpeed.HIGH -> fanSeedMaxColor
} as Int

10. Run your app, click on the dial, and the color setting should be different for each position, as shown below.

To learn more about custom view attributes, see Creating a View Class.

9. Task: Add accessibility

Accessibility is a set of design, implementation, and testing techniques that enable your app to be usable by everyone, including people with disabilities.

Common disabilities that can affect a person's use of an Android device include blindness, low vision, color blindness, deafness or hearing loss, and restricted motor skills. When you develop your apps with accessibility in mind, you make the user experience better not only for users with these disabilities, but also for all of your other users.

Android provides several accessibility features by default in the standard UI views such as TextView and Button. When you create a custom view, however, you need to consider how that custom view will provide accessible features such as spoken descriptions of on-screen content.

In this task you'll learn about TalkBack, Android's screen reader, and modify your app to include speakable hints and descriptions for the DialView custom view.

Step 1. Explore TalkBack

TalkBack is Android's built-in screen reader. With TalkBack enabled, the user can interact with their Android device without seeing the screen, because Android describes screen elements aloud. Users with visual impairments might rely on TalkBack to use your app.

In this task, you enable TalkBack to understand how screen readers work and how to navigate apps.

1. On an Android device or emulator, navigate to Settings > Accessibility > TalkBack.
2. Tap the On/Off toggle button to turn on TalkBack.
3. Tap OK to confirm permissions.
4. Confirm your device password, if asked. If this is the first time you've run TalkBack, a tutorial launches. (The tutorial may not be available on older devices.)
5. It may be helpful to navigate the tutorial with your eyes closed. To open the tutorial again in the future, navigate to Settings > Accessibility > TalkBack > Settings > Launch TalkBack tutorial.
6. Compile and run the CustomFanController app, or open it with the Overview or Recents button on your device. With TalkBack on, notice that the name of the app is announced, as well as the text of the label TextView ("Fan Control"). However, if you tap on the DialView view itself, no information is spoken about either the state of the view (the current setting for the dial) or the action that will take place when you tap the view to activate it.

Step 2. Add content descriptions for dial labels

Content descriptions describe the meaning and purpose of the views in your app. These labels allow screen readers such as Android's TalkBack feature to explain the function of each element accurately. For static views such as ImageView, you can add the content description to the view in the layout file with the contentDescription attribute. Text views (TextView and EditText) automatically use the text in the view as the content description.

For the custom fan control view, you need to dynamically update the content description each time the view is clicked, to indicate the current fan setting.

1. At the bottom of the DialView class, declare a function updateContentDescription() with no arguments or return type.

fun updateContentDescription() {
}

2. Inside updateContentDescription(), change the contentDescription property for the custom view to the string resource associated with the current fan speed (off, 1, 2, or 3). These are the same labels as used in onDraw() when the dial is drawn on the screen.

fun updateContentDescription() {
   contentDescription = resources.getString(fanSpeed.label)
}

3. Scroll up to the init{} block, and at the end of that block add a call to updateContentDescription(). This initializes the content description when the view is initialized.

init {
   isClickable = true
   // ...

   updateContentDescription()
}

4. Add another call to updateContentDescription() in the performClick() method, just before invalidate().

override fun performClick(): Boolean {
   if (super.performClick()) return true
   fanSpeed = fanSpeed.next()
   updateContentDescription()
   invalidate()
   return true
}

5. Compile and run the app, and ensure TalkBack is turned on. Tap to change the setting for the dial view and notice that now that TalkBack announces the current label (off, 1, 2, 3) as well as the phrase "Double-tap to activate."

Step 3. Add more information for the click action

You could stop there and your view would be usable in TalkBack. But it would be helpful if your view could indicate not only that it can be activated ("Double-tap to activate") but also to explain what will happen when the view is activated ("Double-tap to change." or "Double-tap to reset.")

To do this, you add information about the view's action (here, a click or tap action) to an accessibility node info object, by way of an accessibility delegate. An accessibility delegate enables you to customize the accessibility-related features of your app through composition (rather than inheritance).

For this task you will use the accessibility classes in the Android Jetpack libraries (androidx.*), to ensure backwards compatibility.

1. In DialView.kt, in the init block, set an accessibility delegate on the view as a new AccessibilityDelegateCompat object. Import androidx.core.view.ViewCompat and androidx.core.view.AccessibilityDelegateCompat when requested. This strategy enables the greatest amount of backward compatibility in your app.

ViewCompat.setAccessibilityDelegate(this, object : AccessibilityDelegateCompat() {
   
})

2. Inside the AccessibilityDelegateCompat object, override the onInitializeAccessibilityNodeInfo() function with a AccessibilityNodeInfoCompat object, and call the super's method. Import androidx.core.view.accessibility.AccessibilityNodeInfoCompat when prompted.

ViewCompat.setAccessibilityDelegate(this, object : AccessibilityDelegateCompat() {
   override fun onInitializeAccessibilityNodeInfo(host: View, 
                            info: AccessibilityNodeInfoCompat) {
      super.onInitializeAccessibilityNodeInfo(host, info)

   }  
})

Every view has a tree of accessibility nodes, which may or may not correspond to the actual layout components of the view. Android's accessibility services navigates those nodes in order to find out information about the view (such as speakable content descriptions, or possible actions that can be performed on that view.) When you create a custom view you may also need to override the node information in order to provide custom information for accessibility. In this case you will be overriding the node info to indicate that there is custom information for the view's action.

3. Inside onInitializeAccessibilityNodeInfo(), create a new AccessibilityNodeInfoCompat.AccessibilityActionCompat object, and assign it to the customClick variable. Pass into the constructor the AccessibilityNodeInfo.ACTION_CLICK constant, and a placeholder string. Import AccessibilityNodeInfo when requested.

ViewCompat.setAccessibilityDelegate(this, object : AccessibilityDelegateCompat() {
   override fun onInitializeAccessibilityNodeInfo(host: View, 
                            info: AccessibilityNodeInfoCompat) {
      super.onInitializeAccessibilityNodeInfo(host, info)
      val customClick = AccessibilityNodeInfoCompat.AccessibilityActionCompat(
         AccessibilityNodeInfo.ACTION_CLICK,
        "placeholder"
      )
   }  
})

The AccessibilityActionCompat class represents an action on a view for accessibility purposes. A typical action is a click or tap, as you use here, but other actions can include gaining or losing the focus, a clipboard operation (cut/copy/paste) or scrolling within the view. The constructor for this class requires an action constant (here, AccessibilityNodeInfo.ACTION_CLICK), and a string that is used by TalkBack to indicate what the action is.

4. Replace the "placeholder" string with a call to context.getString() to retrieve a string resource. For the specific resource, test for the current fan speed. If the speed is currently FanSpeed.HIGH, the string is "Reset". If the fan speed is anything else, the string is "Change." You'll create these string resources in a later step.

ViewCompat.setAccessibilityDelegate(this, object : AccessibilityDelegateCompat() {
   override fun onInitializeAccessibilityNodeInfo(host: View, 
                            info: AccessibilityNodeInfoCompat) {
      super.onInitializeAccessibilityNodeInfo(host, info)
      val customClick = AccessibilityNodeInfoCompat.AccessibilityActionCompat(
         AccessibilityNodeInfo.ACTION_CLICK,
        context.getString(if (fanSpeed !=  FanSpeed.HIGH) R.string.change else R.string.reset)
      )
   }  
})

5. After the closing parentheses for the customClick definition, use the addAction() method to add the new accessibility action to the node info object.

ViewCompat.setAccessibilityDelegate(this, object : AccessibilityDelegateCompat() {
   override fun onInitializeAccessibilityNodeInfo(host: View, 
                            info: AccessibilityNodeInfoCompat) {
       super.onInitializeAccessibilityNodeInfo(host, info)
       val customClick = AccessibilityNodeInfoCompat.AccessibilityActionCompat(
           AccessibilityNodeInfo.ACTION_CLICK,
           context.getString(if (fanSpeed !=  FanSpeed.HIGH) 
                                 R.string.change else R.string.reset)
       )
       info.addAction(customClick)
   }
})

6. In res/values/strings.xml, add the string resources for "Change" and "Reset".

<string name="change">Change</string>
<string name="reset">Reset</string>

7. Compile and run the app and ensure TalkBack is turned on. Notice now that the phrase "Double-tap to activate" is now either "Double-tap to change" (if the fan speed is less than high or 3) or "Double-tap to reset" (if the fan speed is already at high or 3). Note that the prompt "Double-tap to..." is supplied by the TalkBack service itself.

10. Solution code

Download the code for the finished codelab..

$  git clone https://github.com/googlecodelabs/android-kotlin-drawing-custom-views

Alternatively you can download the repository as a Zip file, unzip it, and open it in Android Studio.

file_downloadDownload Zip

11. Summary

• To create a custom view that inherits the look and behavior of a View subclass such as EditText, add a new class that extends that subclass, and make adjustments by overriding some of the subclass's methods.
• To create a custom view of any size and shape, add a new class that extends View.
• Override View methods such as onDraw() to define the view's shape and basic appearance.
• Use invalidate() to force a draw or redraw of the view.
• To optimize performance, allocate variables and assign any required values for drawing and painting before using them in onDraw(), such as in the initialization of member variables.
• Override performClick() rather than OnClickListener() to the custom view to provide the view's interactive behavior. This enables your or other Android developers that may use your custom view class to use onClickListener() to provide further behavior.
• Add the custom view to an XML layout file with attributes to define its appearance, as you would with other UI elements.
• Create the attrs.xml file in the values folder to define custom attributes. You can then use the custom attributes for the custom view in the XML layout file.

12. Learn more

Udacity course:

• Developing Android Apps with Kotlin

Android developer documentation:

• Creating Custom Views
• @JvmOverloads
• Custom Components
• How Android Draws Views
• onMeasure()
• onSizeChanged()
• onDraw()
• Canvas
• Paint
• drawText()
• setTypeface()
• setColor()
• drawRect()
• drawOval()
• drawArc()
• drawBitmap()
• setStyle()
• invalidate()
• View
• Input Events
• Paint
• Kotlin extension library android-ktx
• withStyledAttributes
• Android KTX documentation
• Android KTX original announcement blog
• Make custom views more accessible
• AccessibilityDelegateCompat
• AccessibilityNodeInfoCompat
• AccessibilityNodeInfoCompat.AccessibilityActionCompat

Videos:

• Quick Intro to Creating a Custom View in Android
• Android Custom View Tutorial

13. Homework

This section lists possible homework assignments for students who are working through this codelab as part of a course led by an instructor. It's up to the instructor to do the following:

• Assign homework if required.
• Communicate to students how to submit homework assignments.
• Grade the homework assignments.

Instructors can use these suggestions as little or as much as they want, and should feel free to assign any other homework they feel is appropriate.

If you're working through this codelab on your own, feel free to use these homework assignments to test your knowledge.

Question 1

To calculate the positions, dimensions, and any other values when the custom view is first assigned a size, which method do you override?

▢ onMeasure()

▢ onSizeChanged()

▢ invalidate()

▢ onDraw()

Question 2

To indicate that you'd like your view to be redrawn with onDraw(), which method do you call from the UI thread, after an attribute value has changed?

▢ onMeasure()

▢ onSizeChanged()

▢ invalidate()

▢ getVisibility()

Question 3

Which View method should you override to add interactivity to your custom view?

▢ setOnClickListener()

▢ onSizeChanged()

▢ isClickable()

▢ performClick()

14. Next codelab

For links to other codelabs in this course, see the Advanced Android in Kotlin codelabs landing page.