Implementing Accessibility on Android

When it comes to reaching as wide a userbase as possible, it’s important to pay attention to accessibility in your Android application. Cues in your user interface that may work for a majority of users, such as a visible change in state when a button is pressed, can be less optimal if the user is visually impaired.

This class shows you how to make the most of the accessibility features built into the Android framework. It covers how to optimize your app for accessibility, leveraging platform features like focus navigation and content descriptions. It also covers how to build accessibility services, that can facilitate user interaction with any Android application, not just your own.

Lesson 1: Developing Accessible Applications

Android has several accessibility-focused features baked into the platform, which make it easy to optimize your application for those with visual or physical disabilities. However, it’s not always obvious what the correct optimizations are, or the easiest way to leverage the framework toward this purpose. This lesson shows you how to implement the strategies and platform features that make for a great accessibility-enabled Android application.

Add Content Descriptions

A well-designed user interface (UI) often has elements that don’t require an explicit label to indicate their purpose to the user. A checkbox next to an item in a task list application has a fairly obvious purpose, as does a trash can in a file manager application. However, to your users with vision impairment, other UI cues are needed.

Fortunately, it’s easy to add labels to UI elements in your application that can be read out loud to your user by a speech-based accessibility service like TalkBack. If you have a label that’s likely not to change during the lifecycle of the application (such as “Pause” or “Purchase”), you can add it via the XML layout, by setting a UI element’s android:contentDescription attribute, like in this example:

<Button
android:id=”@+id/pause_button”
android:src=”@drawable/pause”
android:contentDescription=”@string/pause”/>

However, there are plenty of situations where it’s desirable to base the content description on some context, such as the state of a toggle button, or a piece selectable data like a list item. To edit the content description at runtime, use the setContentDescription() method, like this:

String contentDescription = “Select ” + strValues[position];
label.setContentDescription(contentDescription);

This addition to your code is the simplest accessibility improvement you can make to your application, but one of the most useful. Try to add content descriptions wherever there’s useful information, but avoid the web-developer pitfall of labelling everything with useless information. For instance, don’t set an application icon’s content description to “app icon”. That just increases the noise a user needs to navigate in order to pull useful information from your interface.

Try it out! Download TalkBack (an accessibility service published by Google) and enable it in Settings > Accessibility > TalkBack. Then navigate around your own application and listen for the audible cues provided by TalkBack.

Design for Focus Navigation

Your application should support more methods of navigation than the touch screen alone. Many Android devices come with navigation hardware other than the touchscreen, like a D-Pad, arrow keys, or a trackball. In addition, later Android releases also support connecting external devices like keyboards via USB or bluetooth.

In order to enable this form of navigation, all navigational elements that the user should be able to navigate to need to be set as focusable. This modification can be done at runtime using the View.setFocusable() method on that UI control, or by setting the android:focusable attrubute in your XML layout files.

Also, each UI control has 4 attributes, android:nextFocusUp, android:nextFocusDown, android:nextFocusLeft, and android:nextFocusRight, which you can use to designate the next view to receive focus when the user navigates in that direction. While the platform determines navigation sequences automatically based on layout proximity, you can use these attributes to override that sequence if it isn’t appropriate in your application.

For instance, here’s how you represent a button and label, both focusable, such that pressing down takes you from the button to the text view, and pressing up would take you back to the button.

<Button android:id=”@+id/doSomething”
android:focusable=”true”
android:nextFocusDown=”@id/label”
… />
<TextView android:id=”@+id/label”
android:focusable=”true”
android:text=”@string/labelText”
android:nextFocusUp=”@id/doSomething”
… />

Verify that your application works intuitively in these situations. The easiest way is to simply run your application in the Android emulator, and navigate around the UI with the emulator’s arrow keys, using the OK button as a replacement for touch to select UI controls.

Fire Accessibility Events

If you’re using the view components in the Android framework, an AccessibilityEvent is created whenever you select an item or change focus in your UI. These events are examined by the accessibility service, enabling it to provide features like text-to-speech to the user.

If you write a custom view, make sure it fires events at the appropriate times. Generate events by calling sendAccessibilityEvent(int), with a parameter representing the type of event that occurred. A complete list of the event types currently supported can be found in the AccessibilityEvent reference documentation.

As an example, if you want to extend an image view such that you can write captions by typing on the keyboard when it has focus, it makes sense to fire an TYPE_VIEW_TEXT_CHANGED event, even though that’s not normally built into image views. The code to generate that event would look like this:

public void onTextChanged(String before, String after) {

if (AccessibilityManager.getInstance(mContext).isEnabled()) {
sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_TEXT_CHANGED);
}

}

Test Your Application

Be sure to test the accessibility functionality as you add it to your application. In order to test the content descriptions and Accessibility events, install and enable an accessibility service. One option is Talkback, a free, open source screen reader available on Google Play. With the service enabled, test all the navigation flows through your application and listen to the spoken feedback.

Also, attempt to navigate your application using a directional controller, instead of the touch screen. You can use a physical device with a d-pad or trackball if one is available. If not, use the Android emulator and it’s simulated keyboard controls.

Between the service providing feedback and the directional navigation through your application, you should get a sense of what your application is like to navigate without any visual cues. Fix problem areas as they appear, and you’ll end up with with a more accessible Android application.

Lesson 2: Developing an Accessibility Service

Accessibility services are a feature of the Android framework designed to provide alternative navigation feedback to the user on behalf of applications installed on Android devices. An accessibility service can communicate to the user on the application’s behalf, such as converting text to speech, or haptic feedback when a user is hovering on an important area of the screen. This lesson covers how to create an accessibility service, process information received from the application, and report that information back to the user.

Create Your Accessibility Service

An accessibility service can be bundled with a normal application, or created as a standalone Android project. The steps to creating the service are the same in either situation. Within your project, create a class that extends AccessibilityService.

package com.example.android.apis.accessibility;

import android.accessibilityservice.AccessibilityService;

public class MyAccessibilityService extends AccessibilityService {

@Override
public void onAccessibilityEvent(AccessibilityEvent event) {
}

@Override
public void onInterrupt() {
}


}

Like any other service, you also declare it in the manifest file. Remember to specify that it handles the android.accessibilityservice intent, so that the service is called when applications fire an AccessibilityEvent.

<application …>

<service android:name=”.MyAccessibilityService”>
<intent-filter>
<action android:name=”android.accessibilityservice.AccessibilityService” />
</intent-filter>
. . .
</service>

</application>

If you created a new project for this service, and don’t plan on having an application, you can remove the starter Activity class (usually called MainActivity.java) from your source. Remember to also remove the corresponding activity element from your manifest.

Configure Your Accessibility Service

Setting the configuration variables for your accessibility service tells the system how and when you want it to run. Which event types would you like to respond to? Should the service be active for all applications, or only specific package names? What different feedback types does it use?

You have two options for how to set these variables. The backwards-compatible option is to set them in code, using setServiceInfo(android.accessibilityservice.AccessibilityServiceInfo). To do that, override the onServiceConnected() method and configure your service in there.

@Override
public void onServiceConnected() {
// Set the type of events that this service wants to listen to.  Others
// won’t be passed to this service.
info.eventTypes = AccessibilityEvent.TYPE_VIEW_CLICKED |
AccessibilityEvent.TYPE_VIEW_FOCUSED;

// If you only want this service to work with specific applications, set their
// package names here.  Otherwise, when the service is activated, it will listen
// to events from all applications.
info.packageNames = new String[]
{“com.example.android.myFirstApp”, “com.example.android.mySecondApp”};

// Set the type of feedback your service will provide.
info.feedbackType = AccessibilityServiceInfo.FEEDBACK_SPOKEN;

// Default services are invoked only if no package-specific ones are present
// for the type of AccessibilityEvent generated.  This service *is*
// application-specific, so the flag isn’t necessary.  If this was a
// general-purpose service, it would be worth considering setting the
// DEFAULT flag.

// info.flags = AccessibilityServiceInfo.DEFAULT;

info.notificationTimeout = 100;

this.setServiceInfo(info);

}

Starting with Android 4.0, there is a second option available: configure the service using an XML file. Certain configuration options like canRetrieveWindowContent are only available if you configure your service using XML. The same configuration options above, defined using XML, would look like this:

<accessibility-service
android:accessibilityEventTypes=”typeViewClicked|typeViewFocused”
android:packageNames=”com.example.android.myFirstApp, com.example.android.mySecondApp”
android:accessibilityFeedbackType=”feedbackSpoken”
android:notificationTimeout=”100″
android:settingsActivity=”com.example.android.apis.accessibility.TestBackActivity”
android:canRetrieveWindowContent=”true”
/>

If you go the XML route, be sure to reference it in your manifest, by adding a <meta-data> tag to your service declaration, pointing at the XML file. If you stored your XML file in res/xml/serviceconfig.xml, the new tag would look like this:

<service android:name=”.MyAccessibilityService”>
<intent-filter>
<action android:name=”android.accessibilityservice.AccessibilityService” />
</intent-filter>
<meta-data android:name=”android.accessibilityservice”
android:resource=”@xml/serviceconfig” />
</service>

Respond to AccessibilityEvents

Now that your service is set up to run and listen for events, write some code so it knows what to do when an AccessibilityEvent actually arrives! Start by overriding the onAccessibilityEvent(AccessibilityEvent) method. In that method, use getEventType() to determine the type of event, and getContentDescription() to extract any label text associated with the fiew that fired the event.

@Override
public void onAccessibilityEvent(AccessibilityEvent event) {
final int eventType = event.getEventType();
String eventText = null;
switch(eventType) {
case AccessibilityEvent.TYPE_VIEW_CLICKED:
eventText = “Focused: “;
break;
case AccessibilityEvent.TYPE_VIEW_FOCUSED:
eventText = “Focused: “;
break;
}

eventText = eventText + event.getContentDescription();

// Do something nifty with this text, like speak the composed string
// back to the user.
speakToUser(eventText);

}

Query the View Heirarchy for More Context

This step is optional, but highly useful. One of the new features in Android 4.0 (API Level 14) is the ability for an AccessibilityService to query the view hierarchy, collecting information about the the UI component that generated an event, and its parent and children. In order to do this, make sure that you set the following line in your XML configuration:

android:canRetrieveWindowContent=”true”

Once that’s done, get an AccessibilityNodeInfo object using getSource(). This call only returns an object if the window where the event originated is still the active window. If not, it will return null, so behave accordingly. The following example is a snippet of code that, when it receives an event, does the following:

  1. Immediately grab the parent of the view where the event originated
  2. In that view, look for a label and a check box as children views
  3. If it finds them, create a string to report to the user, indicating the label and whether it was checked or not.
  4. If at any point a null value is returned while traversing the view hierarchy, the method quietly gives up.

// Alternative onAccessibilityEvent, that uses AccessibilityNodeInfo

@Override
public void onAccessibilityEvent(AccessibilityEvent event) {

AccessibilityNodeInfo source = event.getSource();
if (source == null) {
return;
}

// Grab the parent of the view that fired the event.
AccessibilityNodeInfo rowNode = getListItemNodeInfo(source);
if (rowNode == null) {
return;
}

// Using this parent, get references to both child nodes, the label and the checkbox.
AccessibilityNodeInfo labelNode = rowNode.getChild(0);
if (labelNode == null) {
rowNode.recycle();
return;
}

AccessibilityNodeInfo completeNode = rowNode.getChild(1);
if (completeNode == null) {
rowNode.recycle();
return;
}

// Determine what the task is and whether or not it’s complete, based on
// the text inside the label, and the state of the check-box.
if (rowNode.getChildCount() < 2 || !rowNode.getChild(1).isCheckable()) {
rowNode.recycle();
return;
}

CharSequence taskLabel = labelNode.getText();
final boolean isComplete = completeNode.isChecked();
String completeStr = null;

if (isComplete) {
completeStr = getString(R.string.checked);
} else {
completeStr = getString(R.string.not_checked);
}
String reportStr = taskLabel + completeStr;
speakToUser(reportStr);
}

Now you have a complete, functioning accessibility service. Try configuring how it interacts with the user, by adding Android’s text-to-speech engine, or using a Vibrator to provide haptic feedback!

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