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Non Native Apps Accessibility Support Best Practices

Here we provide best practices to support accessibility with custom view implementation.

With the practices, we will build a simple activity. In the activity, we draw four ImageView there using Canvas API. Those ImageViews are focusable and the accessibility focus could be navigating on those.

Create layout xml file

Create a layout file for the custom view activity.

  <?xml version="1.0" encoding="utf-8">
  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:orientation="vertical"
      android:layout_width="match_parent"
      android:layout_height="wrap_content">

    <LinearLayout xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".custom.CustomViewActivity">

      <com.example.nonnativebestpractice.SampleCustomView
          android:layout_width="match_parent"
          android:layout_height="wrap_content" />
    </LinearLayout>
  </LinearLayout>

We will define the com.example.nonnativebestpractice.SampleCustomView in next steps.

Define a simple activity

Create a simple activity to load the layout XML file.

Kotlin

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.example.android.tva11ydemo.R

class CustomViewBestPracticeActivity:AppCompatActivity() {

  protected fun onCreate(savedInstanceState:Bundle) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.custom_view_best_practices)
  }
}

Java

package com.example.android.tva11ydemo.custom.bestpractices;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import com.example.android.tva11ydemo.R;

public class CustomViewBestPracticeActivity extends AppCompatActivity{

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.custom_view_best_practices);
  }
}

Define the SampleCustomView class

Define the class and draw four image on that as below. However, with those, the accessibility focus cannot navigate on those yet.

Kotlin

package com.example.android.tva11ydemo.custom.bestpractices

import android.content.Context
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.graphics.Rect
import android.util.AttributeSet
import android.view.View
import androidx.annotation.Nullable
import androidx.core.view.ViewCompat
import java.util.HashMap

class SimplestCustomView:View {

  internal val virtualIdRectMap:Map<Int, VirtualRect> = HashMap<Int, VirtualRect>()
  private val rectangleId = 1

  internal class VirtualRect(rect:Rect, paint:Paint, id:Int) {
    val rect:Rect
    val paint:Paint
    val id:Int = 0
    init{
      this.rect = rect
      this.paint = paint
      this.id = id
    }
  }

  constructor(context:Context) : super(context) {
    init()
  }

  constructor(context:Context, @Nullable attrs:AttributeSet) : super(context, attrs) {
    init()
  }

  constructor(context:Context, @Nullable attrs:AttributeSet, defStyleAttr:Int) : super(context, attrs, defStyleAttr) {
    init()
  }

  private fun init() {
    setRectangle()
    ViewCompat.setAccessibilityDelegate(this,
                                        SimplestExploreByTouchHelper(this))
  }

  protected fun onDraw(canvas:Canvas) {
    super.onDraw(canvas)
    for (mapElement in virtualIdRectMap.entries)
    {
      val virtualRect = mapElement.value
      canvas.drawRect(virtualRect.rect, virtualRect.paint)
    }
  }

  private fun setRectangle() {
    val paint = Paint(Paint.ANTI_ALIAS_FLAG)
    paint.setColor(RECTANGLE_COLOR)

    val leftTopRectangle = Rect()
    leftTopRectangle.left = RECTANGLE_START_POINT_LEFT
    leftTopRectangle.right = leftTopRectangle.left + RECTANGLE_WIDTH
    leftTopRectangle.top = RECTANGLE_START_POINT_TOP
    leftTopRectangle.bottom = leftTopRectangle.top + RECTANGLE_HEIGHT
    virtualIdRectMap.put(rectangleId, VirtualRect(leftTopRectangle, paint, rectangleId++))

    val rightTopRectangle = Rect()
    rightTopRectangle.left = leftTopRectangle.right + HORISONTAL_PADDING
    rightTopRectangle.right = rightTopRectangle.left + RECTANGLE_WIDTH
    rightTopRectangle.top = leftTopRectangle.top
    rightTopRectangle.bottom = rightTopRectangle.top + RECTANGLE_HEIGHT
    virtualIdRectMap.put(rectangleId, VirtualRect(rightTopRectangle, paint, rectangleId++))

    val leftBottomRectangle = Rect()
    leftBottomRectangle.left = leftTopRectangle.left
    leftBottomRectangle.right = leftBottomRectangle.left + RECTANGLE_WIDTH
    leftBottomRectangle.top = leftTopRectangle.bottom + VERTICAL_PADDING
    leftBottomRectangle.bottom = leftBottomRectangle.top + RECTANGLE_HEIGHT
    virtualIdRectMap.put(rectangleId, VirtualRect(leftBottomRectangle, paint, rectangleId++))

    val rightBottomRectangle = Rect()
    rightBottomRectangle.left = leftBottomRectangle.right + HORISONTAL_PADDING
    rightBottomRectangle.right = rightBottomRectangle.left + RECTANGLE_WIDTH
    rightBottomRectangle.top = leftBottomRectangle.top
    rightBottomRectangle.bottom = rightBottomRectangle.top + RECTANGLE_HEIGHT
    virtualIdRectMap.put(rectangleId, VirtualRect(rightBottomRectangle, paint, rectangleId++))
  }

  companion object {
    private val RECTANGLE_START_POINT_LEFT = 50
    private val RECTANGLE_START_POINT_TOP = 100
    private val RECTANGLE_WIDTH = 200
    private val RECTANGLE_HEIGHT = 100
    private val HORISONTAL_PADDING = 50
    private val VERTICAL_PADDING = 100
    private val RECTANGLE_COLOR = Color.argb(100, 0, 163, 245)
  }
}

Java

package com.example.android.tva11ydemo.custom.bestpractices;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Rect;
import android.util.AttributeSet;
import android.view.View;
import androidx.annotation.Nullable;
import androidx.core.view.ViewCompat;
import java.util.HashMap;
import java.util.Map;

public class SimplestCustomView extends View {

  final Map<Integer, VirtualRect> virtualIdRectMap = new HashMap<>();

  private static final int RECTANGLE_START_POINT_LEFT = 50;
  private static final int RECTANGLE_START_POINT_TOP = 100;
  private static final int RECTANGLE_WIDTH = 200;
  private static final int RECTANGLE_HEIGHT = 100;
  private static final int HORISONTAL_PADDING = 50;
  private static final int VERTICAL_PADDING = 100;
  private static final int RECTANGLE_COLOR = Color.argb(100, 0, 163, 245);

  private int rectangleId = 1;

  static class VirtualRect {
    final Rect rect;
    final Paint paint;
    final int id;

    VirtualRect(Rect rect, Paint paint, int id) {
      this.rect = rect;
      this.paint = paint;
      this.id = id;
    }
  }

  public SimplestCustomView(Context context) {
    super(context);
    init();
  }

  public SimplestCustomView(Context context, @Nullable AttributeSet attrs) {
    super(context, attrs);
    init();
  }

  public SimplestCustomView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);
    init();
  }

  private void init() {
    setRectangle();
  }

  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    for (Map.Entry<Integer, VirtualRect> mapElement :
        virtualIdRectMap.entrySet()) {
      VirtualRect virtualRect = mapElement.getValue();
      canvas.drawRect(virtualRect.rect, virtualRect.paint);
    }
  }

  private void setRectangle() {
    Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
    paint.setColor(RECTANGLE_COLOR);
    Rect leftTopRectangle = new Rect();
    leftTopRectangle.left = RECTANGLE_START_POINT_LEFT;
    leftTopRectangle.right = leftTopRectangle.left + RECTANGLE_WIDTH;
    leftTopRectangle.top = RECTANGLE_START_POINT_TOP;
    leftTopRectangle.bottom = leftTopRectangle.top + RECTANGLE_HEIGHT;
    virtualIdRectMap.put(rectangleId, new VirtualRect(leftTopRectangle, paint, rectangleId++));

    Rect rightTopRectangle = new Rect();
    rightTopRectangle.left = leftTopRectangle.right + HORISONTAL_PADDING;
    rightTopRectangle.right = rightTopRectangle.left + RECTANGLE_WIDTH;
    rightTopRectangle.top = leftTopRectangle.top;
    rightTopRectangle.bottom = rightTopRectangle.top + RECTANGLE_HEIGHT;
    virtualIdRectMap.put(rectangleId, new VirtualRect(rightTopRectangle, paint, rectangleId++));

    Rect leftBottomRectangle = new Rect();
    leftBottomRectangle.left = leftTopRectangle.left;
    leftBottomRectangle.right = leftBottomRectangle.left + RECTANGLE_WIDTH;
    leftBottomRectangle.top = leftTopRectangle.bottom + VERTICAL_PADDING;
    leftBottomRectangle.bottom = leftBottomRectangle.top + RECTANGLE_HEIGHT;
    virtualIdRectMap.put(rectangleId, new VirtualRect(leftBottomRectangle, paint, rectangleId++));

    Rect rightBottomRectangle = new Rect();
    rightBottomRectangle.left = leftBottomRectangle.right + HORISONTAL_PADDING;
    rightBottomRectangle.right = rightBottomRectangle.left + RECTANGLE_WIDTH;
    rightBottomRectangle.top = leftBottomRectangle.top;
    rightBottomRectangle.bottom = rightBottomRectangle.top + RECTANGLE_HEIGHT;
    virtualIdRectMap.put(rectangleId, new VirtualRect(rightBottomRectangle, paint, rectangleId++));
  }
}

At this moment, if you run the app, you can see four light blue rectangle on screen. However, if you turn Talkback ON, the accessibility focus cannot navigate on the app yet.

Define ExploreByTouchHelper to expose AccessiblityNodeInfo

At first, we define a private class, SimplestExploreByTouchHelper, which inherit the ExploreByTouchHelper class and implement the required methods as below:

Kotlin

package com.example.android.tva11ydemo.custom.bestpractices

import android.graphics.Rect
import android.os.Bundle
import android.util.Log
import android.widget.ImageView
import androidx.annotation.NonNull
import androidx.annotation.Nullable
import androidx.core.view.accessibility.AccessibilityNodeInfoCompat
import androidx.core.view.accessibility.AccessibilityNodeInfoCompat.AccessibilityActionCompat
import androidx.customview.widget.ExploreByTouchHelper
import com.example.android.tva11ydemo.BuildConfig
import com.example.android.tva11ydemo.custom.bestpractices.SimplestCustomView.VirtualRect
import java.util.Objects

internal class SimplestExploreByTouchHelper(parentView:SimplestCustomView):ExploreByTouchHelper(parentView) {

  private val TAG = SimplestExploreByTouchHelper::class.java!!.getName()
  private val parentView:SimplestCustomView

  init{
    this.parentView = parentView
  }

  /**
   * Define AccessibilityNodeInfo objects with necessary attributes.
   * Set required attributes into the AccessibilityNodeInfo for the UI component whose virtualID is
   * the virtualViewId.
   * @param virtualViewId virtual ID for the component.
   * @param node The AccessibilityNodeInfoCompat used to set attribute here.
   */
  protected fun onPopulateNodeForVirtualView(
    virtualViewId:Int, @NonNull node:AccessibilityNodeInfoCompat) {
    val rectEle = parentView.virtualIdRectMap.get(virtualViewId)

    assert(rectEle != null)
    node.setContentDescription("Selected rect " + rectEle.id)
    node.setClassName(ImageView::class.java!!.getName())
    node.setPackageName(BuildConfig.APPLICATION_ID)
    node.setVisibleToUser(true)
    node.addAction(AccessibilityActionCompat.ACTION_CLICK)
    // *****************************************************************
    // It is very important to set rectangle area for each node. As a
    // result the accessibility service know where to set accessibility
    // focus on.
    // *****************************************************************
    node.setBoundsInParent(
      Objects.requireNonNull(parentView.virtualIdRectMap.get(
        virtualViewId)).rect)
  }

  protected fun getVirtualViewAt(x:Float, y:Float):Int {
    for (mapElement in parentView.virtualIdRectMap.entrySet())
    {
      val rect = mapElement.value.rect
      if ((rect.left <= x && x <= rect.right
           && rect.top <= y && y <= rect.bottom))
      {
        return mapElement.value.id
      }
    }

    return ExploreByTouchHelper.INVALID_ID
  }

  protected fun getVisibleVirtualViews(virtualViewIds:List<Int>) {
    for (mapElement in parentView.virtualIdRectMap.entrySet())
    {
      virtualViewIds.add(mapElement.key)
    }
  }

  protected fun onPerformActionForVirtualView(
    virtualViewId:Int, action:Int, @Nullable arguments:Bundle):Boolean {
    when (action) {
      // After turned Talkback on, if you want to handle
      // DPAD_CENTER key event with your own logic,
      // implement under ACTION_CLICK action.
      AccessibilityNodeInfoCompat.ACTION_CLICK -> {
        Log.e(TAG, "ACTION_CLICK action handled here.")
        return true
      }
    }

    // All the DPAD directional key will be handled by
    // Accessibility based on the AccessibilityNodeInfo Tree
    // you defined above.
    // fall through
    return false
  }
}

Java

package com.example.android.tva11ydemo.custom.bestpractices;

import android.graphics.Rect;
import android.os.Bundle;
import android.util.Log;
import android.widget.ImageView;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.core.view.accessibility.AccessibilityNodeInfoCompat;
import androidx.core.view.accessibility.AccessibilityNodeInfoCompat.AccessibilityActionCompat;
import androidx.customview.widget.ExploreByTouchHelper;
import com.example.android.tva11ydemo.BuildConfig;
import com.example.android.tva11ydemo.custom.bestpractices.SimplestCustomView.VirtualRect;
import java.util.List;
import java.util.Map;
import java.util.Objects;

class SimplestExploreByTouchHelper extends ExploreByTouchHelper {

  private final String TAG = SimplestExploreByTouchHelper.class.getName();

  private SimplestCustomView parentView;

  SimplestExploreByTouchHelper(SimplestCustomView parentView) {
    super(parentView);
    this.parentView = parentView;
  }

  /**
   * Define AccessibilityNodeInfo objects with necessary attributes.
   * Set required attributes into the AccessibilityNodeInfo for the UI component whose virtualID is
   * the virtualViewId.
   * @param virtualViewId virtual ID for the component.
   * @param node The AccessibilityNodeInfoCompat used to set attribute here.
   */
  @Override
  protected void onPopulateNodeForVirtualView(
      int virtualViewId, @NonNull AccessibilityNodeInfoCompat node) {
    VirtualRect rectEle = parentView.virtualIdRectMap.get(virtualViewId);

    assert rectEle != null;
    node.setContentDescription("Selected rect " + rectEle.id);
    node.setClassName(ImageView.class.getName());
    node.setPackageName(BuildConfig.APPLICATION_ID);
    node.setVisibleToUser(true);
    node.addAction(AccessibilityActionCompat.ACTION_CLICK);
    // *****************************************************************
    // It is very important to set rectangle area for each node. As a
    // result the accessibility service know where to set accessibility
    // focus on.
    // *****************************************************************
    node.setBoundsInParent(
        Objects.requireNonNull(parentView.virtualIdRectMap.get(
            virtualViewId)).rect);
  }

  @Override
  protected int getVirtualViewAt(float x, float y) {
    for (Map.Entry<Integer, VirtualRect> mapElement :
        parentView.virtualIdRectMap.entrySet()) {
      Rect rect = mapElement.getValue().rect;
      if (rect.left <= x && x <= rect.right
          && rect.top <= y && y <= rect.bottom) {
        return mapElement.getValue().id;
      }
    }
    return ExploreByTouchHelper.INVALID_ID;
  }

  @Override
  protected void getVisibleVirtualViews(List<Integer> virtualViewIds) {
    for (Map.Entry<Integer, VirtualRect> mapElement :
        parentView.virtualIdRectMap.entrySet()) {
      virtualViewIds.add(mapElement.getKey());
    }
  }

  @Override
  protected boolean onPerformActionForVirtualView(
      int virtualViewId, int action, @Nullable Bundle arguments) {
    switch (action) {
      // After turned Talkback on, if you want to handle
      // DPAD_CENTER key event with your own logic,
      // implement under ACTION_CLICK action.
      case AccessibilityNodeInfoCompat.ACTION_CLICK:
        Log.e(TAG, "ACTION_CLICK action handled here.");
        return true;
      // All the DPAD directional key will be handled by
      // Accessibility based on the AccessibilityNodeInfo Tree
      // you defined above.
      default:
        // fall through
    }

    return false;
  }
}

And then you should also invoke setAccessibilityDelegate() method in the init().

Kotlin

private fun init(@Nullable attrs:AttributeSet) {
  ...
  ViewCompat.setAccessibilityDelegate(this,
                                      SimplestExploreByTouchHelper(this))
}

Java

  private void init(@Nullable AttributeSet attrs) {
        ...
        ViewCompat.setAccessibilityDelegate(this,
        new SimplestExploreByTouchHelper(this));
    }

With those, if turn Talkback ON again, you can find that the accessibility focus could be navigate on those four light blue image now. And even if you click the DPAD_CENTER key on remote controller, you can see the ACTION_CLICK handling log in logcat.