Android无障碍浅析

前言

Android无障碍,我们平常接触时,比较熟悉的有“绿色守护”以及“抢红包”这些,其便利性便是在没有“root权限”的情况下,可以“触摸”其他应用来做一些操作。然而,无障碍的初衷却是为视觉障碍的人提供操控手机的可能。

Android中的实现

Android包含几个支持视觉障碍用户访问的特性;他们不需要应用做出巨大的视觉改变。
TalkBack是由Google公司提供的一个预安装屏幕阅读服务。它使用语音反馈描述操作的结果(如启动一个app)和事件(如通知)。
Explore by Touch(触摸浏览)是与TalkBack协作的系统特性,允许用户触摸设备屏幕并通过语音反馈听取手指触摸的内容。该特性对低视力用户有帮助。
无障碍设置允许用户修改设备的展示和声音选择,例如放大文本字体,改变文本阅读的速度等等。
一些用户使用硬件或软件定向控制(例如,D-pad,轨迹球,键盘)从屏幕上的一个选择跳转到另一个选择。他们以线性顺序与应用的结构进行交互,这种线性顺序类似于电视的四个方向远程控制导航。

基本组件的无障碍开发

对于Android的基础组件,只需要简单的在xml或代码中设置contentDescription属性。

  • AccessibilityEvent : 在用户和ui交互时,由系统发送的无障碍事件,例如按钮被点击,或者一个view被focus,参见AccessibilityService,一个无障碍事件的最主要作用就是暴露给AccessibilityService足够多的信息,以提供给用户界面良好的反馈。

  • AccessibilityNodeInfo:一个view状态的快照,代表了窗口中包含的节点的信息。

  • View.AccessibilityDelegate:View 的内部类,通过组合而非继承的方式来控制处理无障碍事件。包括发送,初始化事件以及节点属性。

辣么我们来看下view里面是如何初始化无障碍事件的:

/**
     * Initializes an {@link AccessibilityEvent} with information about
     * this View which is the event source. In other words, the source of
     * an accessibility event is the view whose state change triggered firing
     * the event.
     * 

* Example: Setting the password property of an event in addition * to properties set by the super implementation: *

 public void onInitializeAccessibilityEvent(AccessibilityEvent event) {
     *     super.onInitializeAccessibilityEvent(event);
     *     event.setPassword(true);
     * }
*

* If an {@link AccessibilityDelegate} has been specified via calling * {@link #setAccessibilityDelegate(AccessibilityDelegate)} its * {@link AccessibilityDelegate#onInitializeAccessibilityEvent(View, AccessibilityEvent)} * is responsible for handling this call. *

*

Note: Always call the super implementation before adding * information to the event, in case the default implementation has basic information to add. *

* @param event The event to initialize. * * @see #sendAccessibilityEvent(int) * @see #dispatchPopulateAccessibilityEvent(AccessibilityEvent) */ @CallSuper public void onInitializeAccessibilityEvent(AccessibilityEvent event) { if (mAccessibilityDelegate != null) { mAccessibilityDelegate.onInitializeAccessibilityEvent(this, event); } else { onInitializeAccessibilityEventInternal(event); } } /** * @see #onInitializeAccessibilityEvent(AccessibilityEvent) * * Note: Called from the default {@link AccessibilityDelegate}. * * @hide */ public void onInitializeAccessibilityEventInternal(AccessibilityEvent event) { event.setSource(this); event.setClassName(getAccessibilityClassName()); event.setPackageName(getContext().getPackageName()); event.setEnabled(isEnabled()); event.setContentDescription(mContentDescription); switch (event.getEventType()) { case AccessibilityEvent.TYPE_VIEW_FOCUSED: { ArrayList focusablesTempList = (mAttachInfo != null) ? mAttachInfo.mTempArrayList : new ArrayList(); getRootView().addFocusables(focusablesTempList, View.FOCUS_FORWARD, FOCUSABLES_ALL); event.setItemCount(focusablesTempList.size()); event.setCurrentItemIndex(focusablesTempList.indexOf(this)); if (mAttachInfo != null) { focusablesTempList.clear(); } } break; case AccessibilityEvent.TYPE_VIEW_TEXT_SELECTION_CHANGED: { CharSequence text = getIterableTextForAccessibility(); if (text != null && text.length() > 0) { event.setFromIndex(getAccessibilitySelectionStart()); event.setToIndex(getAccessibilitySelectionEnd()); event.setItemCount(text.length()); } } break; } }

在初始化调用onInitializeAccessibilityEvent时,会将设置到view中的mContentDescription等属性放进AccessibilityEvent中去。
无障碍事件如何发送呢?

 /**
     * Sends an accessibility event of the given type. If accessibility is
     * not enabled this method has no effect. The default implementation calls
     * {@link #onInitializeAccessibilityEvent(AccessibilityEvent)} first
     * to populate information about the event source (this View), then calls
     * {@link #dispatchPopulateAccessibilityEvent(AccessibilityEvent)} to
     * populate the text content of the event source including its descendants,
     * and last calls
     * {@link ViewParent#requestSendAccessibilityEvent(View, AccessibilityEvent)}
     * on its parent to request sending of the event to interested parties.
     * 

* If an {@link AccessibilityDelegate} has been specified via calling * {@link #setAccessibilityDelegate(AccessibilityDelegate)} its * {@link AccessibilityDelegate#sendAccessibilityEvent(View, int)} is * responsible for handling this call. *

* * @param eventType The type of the event to send, as defined by several types from * {@link android.view.accessibility.AccessibilityEvent}, such as * {@link android.view.accessibility.AccessibilityEvent#TYPE_VIEW_CLICKED} or * {@link android.view.accessibility.AccessibilityEvent#TYPE_VIEW_HOVER_ENTER}. * * @see #onInitializeAccessibilityEvent(AccessibilityEvent) * @see #dispatchPopulateAccessibilityEvent(AccessibilityEvent) * @see ViewParent#requestSendAccessibilityEvent(View, AccessibilityEvent) * @see AccessibilityDelegate */ public void sendAccessibilityEvent(int eventType) { if (mAccessibilityDelegate != null) { mAccessibilityDelegate.sendAccessibilityEvent(this, eventType); } else { sendAccessibilityEventInternal(eventType); } }

在AccessibilityService中调用view 的sendAccessibilityEvent,由view中的内部类对象AccessibilityDelegate来处理。
这样就完成了一个完整的处理流程,从初始化->用户接触产生并发送事件->接受事件->talkback和 Explore by Touch 反馈给用户。
这里有个问题,我们没有设置过TextView中的contentDesciption属性,为什么开启无障碍后,居然能够读出上面的文字呢?

自定义view的无障碍开发

在做自定义view的开发时,会遇到一个问题,我们知道继承View时,此时一个单独的contentDescription是不能够描述当前的布局属性的,来给无障碍很好的反馈支持,最典型的栗子便是月历这个自定义view,单独设置contentDescription时,我们只能知道它是一个月历显示,不能知道里面的每一个节点的具体信息,星期几?几号?
Android 便提供了一种解决方案:** 既然不是真实存在的,就虚拟出节点来。**

  • AccessibilityNodeProvider: This class is the contract a client should implement to enable support of a virtual view hierarchy rooted at a given view for accessibility purposes. A virtual view hierarchy is a tree of imaginary Views that is reported as a part of the view hierarchy when an AccessibilityService
    explores the window content. Since the virtual View tree does not exist this class is responsible for managing the AccessibilityNodeInfo
    s describing that tree to accessibility services.

进一步的,为了降低开发成本,google为开发者提供了ExploreByTouchHelper来降低开发成本。
整个过程大致分三个步骤:
1 . 初始化

 mAccessHelper = new MyExploreByTouchHelper(someView);
 ViewCompat.setAccessibilityDelegate(someView, mAccessHelpe

2 . 处理以及发送事件

    @Override
     public boolean dispatchHoverEvent(MotionEvent event) {
       return mHelper.dispatchHoverEvent(this, event)
           || super.dispatchHoverEvent(event);
     }
sendEventForVirtualView(int, int))(int virtualViewId, int eventType)
Populates an event of the specified type with information about an item and attempts to send it up through the view hierarchy.

3 . 生成虚拟节点以及描述信息初始化

 class MyExploreByTouchHelper extends ExploreByTouchHelper{
        public MyExploreByTouchHelper(View forView) {
            super(forView);
        }

        @Override
        protected int getVirtualViewAt(float x, float y) {
            //根据想x,y坐标点返回虚拟节点的id
            return 0;
        }

        @Override
        protected void getVisibleVirtualViews(List virtualViewIds) {
            //虚拟节点的id list
        }

        @Override
        protected void onPopulateEventForVirtualView(int virtualViewId, AccessibilityEvent accessibilityEvent) {
            //填充无障碍事件的属性,例如contentDescription
            accessibilityEvent.setContentDescription(getItemDescription(virtualViewId));

        }

        @Override
        protected void onPopulateNodeForVirtualView(int virtualViewId, AccessibilityNodeInfoCompat accessibilityNodeInfoCompat) {
            //初始化虚拟节点的位置等其他属性
            accessibilityNodeInfoCompat.setBoundsInParent(mTempRect);
            accessibilityNodeInfoCompat.addAction(AccessibilityNodeInfoCompat.ACTION_CLICK);

        }

        @Override
        protected boolean onPerformActionForVirtualView(int i, int i1, Bundle bundle) {
            return false;
        }
    }

查看

虚拟的节点如何能 查看到呢?

Android Studio ->Tools -> Android ->Android Device Monitor -> touch Process (选中某个界面进程) -> Dump View Hierarhchy for UI Automator。

这里dump 出来的不止真实的ui组件,还包括我们自建的虚拟的veiw树,这样更具象化,可以很好的定位问题。

总结

我们在做日常开发的过程中,很少会关注到无障碍这部分的内容,但是仍有一部分人也希望能够和我们一样方便的使用移动设备,所以也希望开发者们能够关注到这一点,虽然很小的一点改变,却能让这个世界更美好一点。
也有部分开发者通过创新,希望能够利用到contentDescription字段做一些协议层面的东西,毕竟Android框架提供了这个不需要root权限变可以通过一个app来做跨应用服务。但是这里可能也需要权衡,毕竟有一部分人是需要【初衷】的无障碍。

参考

  • 构建无障碍服务|Android开发
  • PickerTouchHelper
  • MonthView
  • 无障碍测试清单

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