开发高体验应用程序

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.