Build accessible custom views

If your application requires a custom view component, you must do some additional work to ensure that your custom view is accessible. These are the main tasks for ensuring the accessibility of your view:

Handle directional controller clicks

On most devices, clicking a view using a directional controller sends a KeyEvent with KEYCODE_DPAD_CENTER to the view currently in focus. All standard Android views already handle KEYCODE_DPAD_CENTER appropriately. When building a custom View control, make sure this event has the same effect as touching the view on the touchscreen.

Your custom control should also treat the KEYCODE_ENTER event the same as KEYCODE_DPAD_CENTER. This approach makes interaction from a full keyboard much easier for users.

Implement accessibility API methods

Accessibility events are messages about users interaction with visual interface components in your application. These messages are handled by Accessibility Services, which use the information in these events to produce supplemental feedback and prompts. In Android 4.0 (API Level 14) and higher, the methods for generating accessibility events have been expanded to provide more detailed information than the AccessibilityEventSource interface introduced in Android 1.6 (API Level 4). The expanded accessibility methods are part of the View class as well as the View.AccessibilityDelegate class. The methods are as follows:

sendAccessibilityEvent()
(API Level 4) This method is called when a user takes action on a view. The event is classified with a user action type such as TYPE_VIEW_CLICKED. You typically do not need to implement this method unless you are creating a custom view.
sendAccessibilityEventUnchecked()
(API Level 4) This method is used when the calling code needs to directly control the check for accessibility being enabled on the device (AccessibilityManager.isEnabled()). If you do implement this method, you must perform the call as if accessibility is enabled, regardless of the actual system setting. You typically do not need to implement this method for a custom view.
dispatchPopulateAccessibilityEvent()
(API Level 4) The system calls this method when your custom view generates an accessibility event. As of API Level 14, the default implementation of this method calls onPopulateAccessibilityEvent() for this view and then the dispatchPopulateAccessibilityEvent() method for each child of this view. In order to support accessibility services on revisions of Android prior to 4.0 (API Level 14) you must override this method and populate getText() with descriptive text for your custom view, which is spoken by accessibility services, such as TalkBack.
onPopulateAccessibilityEvent()
(API Level 14) This method sets the spoken text prompt of the AccessibilityEvent for your view. This method is also called if the view is a child of a view which generates an accessibility event.

Note: Modifying additional attributes beyond the text within this method potentially overwrites properties set by other methods. While you can modify attributes of the accessibility event with this method, you should limit these changes to text content, and use the onInitializeAccessibilityEvent() method to modify other properties of the event.

Note: If your implementation of this event completely overrides the output text without allowing other parts of your layout to modify its content, then do not call the super implementation of this method in your code.

onInitializeAccessibilityEvent()
(API Level 14) The system calls this method to obtain additional information about the state of the view, beyond text content. If your custom view provides interactive control beyond a simple TextView or Button, you should override this method and set the additional information about your view into the event using this method, such as password field type, checkbox type or states that provide user interaction or feedback. If you do override this method, you must call its super implementation and then only modify properties that have not been set by the super class.
onInitializeAccessibilityNodeInfo()
(API Level 14) This method provides accessibility services with information about the state of the view. The default View implementation has a standard set of view properties, but if your custom view provides interactive control beyond a simple TextView or Button, you should override this method and set the additional information about your view into the AccessibilityNodeInfo object handled by this method.
onRequestSendAccessibilityEvent()
(API Level 14) The system calls this method when a child of your view has generated an AccessibilityEvent. This step allows the parent view to amend the accessibility event with additional information. You should implement this method only if your custom view can have child views and if the parent view can provide context information to the accessibility event that would be useful to accessibility services.

In order to support these accessibility methods for a custom view, you should take one of the following approaches:

  • If your application targets Android 4.0 (API level 14) and higher, override and implement the accessibility methods listed above directly in your custom view class.
  • If your custom view is intended to be compatible with Android 1.6 (API Level 4) and above, add the Android Support Library, revision 5 or higher, to your project. Then, within your custom view class, call the ViewCompat.setAccessibilityDelegate() method to implement the accessibility methods above. For an example of this approach, see the Android Support Library (revision 5 or higher) sample AccessibilityDelegateSupportActivity in (<sdk>/extras/android/support/v4/samples/Support4Demos/)

In either case, you should implement the following accessibility methods for your custom view class:

For more information about implementing these methods, see Populate Accessibility Events.

Send accessibility events

Depending on the specifics of your custom view, it may need to send AccessibilityEvent objects at a different times or for events not handled by the default implementation. The View class provides a default implementation for these event types:

Note: Hover events are associated with the Explore by Touch feature, which uses these events as triggers for providing audible prompts for user interface elements.

In general, you should send an AccessibilityEvent whenever the content of your custom view changes. For example, if you are implementing a custom slider bar that allows a user to select a numeric value by pressing the left or right arrows, your custom view should emit an event of type TYPE_VIEW_TEXT_CHANGED whenever the slider value changes. The following sample code demonstrates the use of the sendAccessibilityEvent() method to report this event.

@Override
public boolean onKeyUp (int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_DPAD_LEFT) {
        mCurrentValue--;
        sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_TEXT_CHANGED);
        return true;
    }
    ...
}

Populate accessibility events

Each AccessibilityEvent has a set of required properties that describe the current state of the view. These properties include things such as the view's class name, content description and checked state. The specific properties required for each event type are described in the AccessibilityEvent reference documentation. The View implementation provides default values for these properties. Many of these values, including the class name and event timestamp, are provided automatically. If you are creating a custom view component, you must provide some information about the content and characteristics of the view. This information may be as simple as a button label, but may also include additional state information that you want to add to the event.

The minimum requirement for providing information to accessibility services with a custom view is to implement dispatchPopulateAccessibilityEvent(). This method is called by the system to request information for an AccessibilityEvent and makes your custom view compatible with accessibility services on Android 1.6 (API Level 4) and higher. The following example code demonstrates a basic implementation of this method.

@Override
public boolean dispatchPopulateAccessibilityEvent(AccessibilityEvent event) {
    // Call the super implementation to populate its text to the event, which
    // calls onPopulateAccessibilityEvent() on API Level 14 and up.
    boolean completed = super.dispatchPopulateAccessibilityEvent(event);

    // In case this is running on a API revision earlier that 14, check
    // the text content of the event and add an appropriate text
    // description for this custom view:
    CharSequence text = getText();
    if (!TextUtils.isEmpty(text)) {
        event.getText().add(text);
        return true;
    }
    return completed;
}

For Android 4.0 (API Level 14) and higher, use the onPopulateAccessibilityEvent() and onInitializeAccessibilityEvent() methods to populate or modify the information in an AccessibilityEvent. Use the onPopulateAccessibilityEvent() method specifically for adding or modifying the text content of the event, which is turned into audible prompts by accessibility services such as TalkBack. Use the onInitializeAccessibilityEvent() method for populating additional information about the event, such as the selection state of the view.

In addition, implement the onInitializeAccessibilityNodeInfo() method. The AccessibilityNodeInfo objects populated by this method are used by accessibility services to investigate the view hierarchy that generated an accessibility event after receiving that event, to obtain a more detailed context information and provide appropriate feedback to users.

The example code below shows how override these three methods by using ViewCompat.setAccessibilityDelegate(). Note that this sample code requires that the Android Support Library for API Level 4 (revision 5 or higher) is added to your project.

ViewCompat.setAccessibilityDelegate(new AccessibilityDelegateCompat() {
    @Override
    public void onPopulateAccessibilityEvent(View host, AccessibilityEvent event) {
        super.onPopulateAccessibilityEvent(host, event);
        // We call the super implementation to populate its text for the
        // event. Then we add our text not present in a super class.
        // Very often you only need to add the text for the custom view.
        CharSequence text = getText();
        if (!TextUtils.isEmpty(text)) {
            event.getText().add(text);
        }
    }
    @Override
    public void onInitializeAccessibilityEvent(View host, AccessibilityEvent event) {
        super.onInitializeAccessibilityEvent(host, event);
        // We call the super implementation to let super classes
        // set appropriate event properties. Then we add the new property
        // (checked) which is not supported by a super class.
        event.setChecked(isChecked());
    }
    @Override
    public void onInitializeAccessibilityNodeInfo(View host,
            AccessibilityNodeInfoCompat info) {
        super.onInitializeAccessibilityNodeInfo(host, info);
        // We call the super implementation to let super classes set
        // appropriate info properties. Then we add our properties
        // (checkable and checked) which are not supported by a super class.
        info.setCheckable(true);
        info.setChecked(isChecked());
        // Quite often you only need to add the text for the custom view.
        CharSequence text = getText();
        if (!TextUtils.isEmpty(text)) {
            info.setText(text);
        }
    }
}

You can implement these methods directly in your custom view class. For another example of this approach, see the Android Support Library (revision 5 or higher) sample AccessibilityDelegateSupportActivity in (<sdk>/extras/android/support/v4/samples/Support4Demos/).

Provide a customized accessibility context

In Android 4.0 (API Level 14), the framework was enhanced to allow accessibility services to inspect the containing view hierarchy of a user interface component that generates an accessibility event. This enhancement allows accessibility services to provide a much richer set of contextual information with which to aid users.

There are some cases where accessibility services cannot get adequate information from the view hierarchy. An example of this is a custom interface control that has two or more separately clickable areas, such as a calendar control. In this case, the services cannot get adequate information because the clickable subsections are not part of the view hierarchy.

Figure 1. A custom calendar view with selectable day elements.

In the example shown in Figure 1, the entire calendar is implemented as a single view, so if you do not do anything else, accessibility services do not receive enough information about the content of the view and the user's selection within the view. For example, if a user clicks on the day containing 17, the accessibility framework only receives the description information for the whole calendar control. In this case, the TalkBack accessibility service would simply announce "Calendar" or, only slightly better, "April Calendar" and the user would be left to wonder what day was selected.

To provide adequate context information for accessibility services in situations like this, the framework provides a way to specify a virtual view hierarchy. A virtual view hierarchy is a way for application developers to provide a complementary view hierarchy to accessibility services that more closely matches the actual information on screen. This approach allows accessibility services to provide more useful context information to users.

Another situation where a virtual view hierarchy may be needed is a user interface containing a set of controls (views) that have closely related functions, where an action on one control affects the contents of one or more elements, such as a number picker with separate up and down buttons. In this case, accessibility services cannot get adequate information because action on one control changes content in another and the relationship of those controls may not be apparent to the service. To handle this situation, group the related controls with a containing view and provide a virtual view hierarchy from this container to clearly represent the information and behavior provided by the controls.

In order to provide a virtual view hierarchy for a view, override the getAccessibilityNodeProvider() method in your custom view or view group and return an implementation of AccessibilityNodeProvider. For an example implementation of this accessibility feature, see AccessibilityNodeProviderActivity in the ApiDemos sample project. You can implement a virtual view hierarchy that is compatible with Android 1.6 and later by using the Support Library with the ViewCompat.getAccessibilityNodeProvider() method and providing an implementation with AccessibilityNodeProviderCompat.

Handle custom touch events

Custom view controls may require non-standard touch event behavior. For example, a custom control may use the onTouchEvent(MotionEvent) listener method to detect the ACTION_DOWN and ACTION_UP events and trigger a special click event. In order to maintain compatibility with accessibility services, the code that handles this custom click event must do the following:

  1. Generate an appropriate AccessibilityEvent for the interpreted click action.
  2. Enable accessibility services to perform the custom click action for users who are not able to use a touch screen.

To handle these requirements in an efficient way, your code should override the performClick() method, which must call the super implementation of this method and then execute whatever actions are required by the click event. When the custom click action is detected, that code should then call your performClick() method. The following code example demonstrates this pattern.

class CustomTouchView extends View {

    public CustomTouchView(Context context) {
        super(context);
    }

    boolean mDownTouch = false;

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        super.onTouchEvent(event);

        // Listening for the down and up touch events
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mDownTouch = true;
                return true;

            case MotionEvent.ACTION_UP:
                if (mDownTouch) {
                    mDownTouch = false;
                    performClick(); // Call this method to handle the response, and
                                    // thereby enable accessibility services to
                                    // perform this action for a user who cannot
                                    // click the touchscreen.
                    return true;
                }
        }
        return false; // Return false for other touch events
    }

    @Override
    public boolean performClick() {
        // Calls the super implementation, which generates an AccessibilityEvent
        // and calls the onClick() listener on the view, if any
        super.performClick();

        // Handle the action for the custom click here

        return true;
    }
}

The pattern shown above makes sure that the custom click event is compatible with accessibility services by using the performClick() method to both generate an accessibility event and provide an entry point for accessibility services to act on behalf of a user to perform this custom click event.

Note: If your custom view has distinct clickable regions, such as a custom calendar view, you must implement a virtual view hierarchy by overriding getAccessibilityNodeProvider() in your custom view in order to be compatible with accessibility services.

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