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Android AccessibilityService机制源码解析

96
Xu朝旭
2018.06.20 18:36* 字数 2093

一、本文需要解决的问题

之前本人做了一个项目,需要用到AccessibilityService这个系统提供的拓展服务。这个服务本意是作为Android系统的一个辅助功能,去帮助残疾人更好地使用手机。但是由于它的一些特性,给很多项目的实现提供了一个新的思路,例如之前大名鼎鼎的微信抢红包插件,本质上就是使用了这个服务。我研究AccessibilityService的目的是解决以下几个我在使用过程中所思考的问题:

  1. AccessibilityService这个Service跟一般的Service有什么区别?
  2. AccessibilityService是如何做到监控并捕捉用户行为的?
  3. AccessibilityService是如何做到查找控件,执行点击等操作的?

二、初步分析

本文基于Android 7.1的源码对AccessibilityService进行分析。
为了更好地理解和分析代码,我写了一个demo,如果想学习具体的使用方法,可以参考Google官方文档AccessibilityService。本文不做AccessibilityService的具体使用教程。

创建AccessibilityService
public class MyAccessibilityService extends AccessibilityService {

    private static final String TAG = "MyAccessibilityService";

    @Override
    public void onCreate() {
        super.onCreate();
        Log.i(TAG, "onCreate");
    }

    @Override
    public void onAccessibilityEvent(AccessibilityEvent event) {
        int eventType = event.getEventType();
        switch (eventType) {
            case AccessibilityEvent.TYPE_VIEW_CLICKED:
                // 捕获到点击事件
                Log.i(TAG, "capture click event!");
                AccessibilityNodeInfo nodeInfo = getRootInActiveWindow();
                if (nodeInfo != null) {
                    // 查找text为Test!的控件
                    List<AccessibilityNodeInfo> button = nodeInfo.findAccessibilityNodeInfosByText("Test!");
                    nodeInfo.recycle();
                    for (AccessibilityNodeInfo item : button) {
                        Log.i(TAG, "long-click button!");
                        // 执行长按操作
                        item.performAction(AccessibilityNodeInfo.ACTION_LONG_CLICK);
                    }
                }
                break;
            default:
                break;
        }
    }

    @Override
    public void onInterrupt() {
        Log.i(TAG, "onInterrupt");
    }
}
AccessibilityService配置

res/xml/accessibility_service_config.xml

<?xml version="1.0" encoding="utf-8"?>
<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
    android:accessibilityEventTypes="typeAllMask"
    android:accessibilityFeedbackType="feedbackSpoken"
    android:accessibilityFlags="flagRetrieveInteractiveWindows|flagRequestFilterKeyEvents"
    android:canRequestFilterKeyEvents="true"
    android:canRetrieveWindowContent="true"
    android:description="@string/app_name"
    android:notificationTimeout="100"
    android:packageNames="com.xu.accessibilitydemo" />
在manifest中进行注册
<service
    android:name=".MyAccessibilityService"
    android:enabled="true"
    android:exported="true"
    android:permission="android.permission.BIND_ACCESSIBILITY_SERVICE">
    <intent-filter>
        <action android:name="android.accessibilityservice.AccessibilityService"/>
    </intent-filter>

     <meta-data
         android:name="android.accessibilityservice"
         android:resource="@xml/accessibility_service_config"/>
</service>
创建一个text为Test!的button控件,设置监听方法
public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    private Button button;

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

        button = findViewById(R.id.button);

        button.setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View v) {
                Log.i(TAG, "onLongClick");
                return false;
            }
        });

    }
}
开启AccessibilityService

AccessibilityService服务具体开启位置在设置--无障碍中。

运行应用,点击text为Test!的按钮

会出现以下的日志:


log.png

具体解释:
点击按钮即产生TYPE_VIEW_CLICKED事件 --> 被AcceesibilityService捕获 --> 捕获后执行长按按钮操作 --> 执行长按回调方法。

为什么AcceesibilityService能捕获并执行其他操作呢,接下来我将对源码进行解析~

三、源码解析

AccessibilityService内部逻辑
AccessibilityService.java
public abstract class AccessibilityService extends Service {
      // 省略代码
      public abstract void onAccessibilityEvent(AccessibilityEvent event);
      
      public abstract void onInterrupt();

      @Override
      public final IBinder onBind(Intent intent) {
          return new IAccessibilityServiceClientWrapper(this, getMainLooper(), new Callbacks() {
              @Override
              public void onServiceConnected() {
                  AccessibilityService.this.dispatchServiceConnected();
              }

              @Override
              public void onInterrupt() {
                  AccessibilityService.this.onInterrupt();
              }

              @Override
              public void onAccessibilityEvent(AccessibilityEvent event) {
                  AccessibilityService.this.onAccessibilityEvent(event);
              }

              @Override
              public void init(int connectionId, IBinder windowToken) {
                  mConnectionId = connectionId;
                  mWindowToken = windowToken;

                  // The client may have already obtained the window manager, so
                  // update the default token on whatever manager we gave them.
                  final WindowManagerImpl wm = (WindowManagerImpl) getSystemService(WINDOW_SERVICE);
                  wm.setDefaultToken(windowToken);
              }

              @Override
              public boolean onGesture(int gestureId) {
                  return AccessibilityService.this.onGesture(gestureId);
              }

              @Override
              public boolean onKeyEvent(KeyEvent event) {
                  return AccessibilityService.this.onKeyEvent(event);
              }

              @Override
              public void onMagnificationChanged(@NonNull Region region,
                      float scale, float centerX, float centerY) {
                  AccessibilityService.this.onMagnificationChanged(region, scale, centerX, centerY);
              }

              @Override
              public void onSoftKeyboardShowModeChanged(int showMode) {
                  AccessibilityService.this.onSoftKeyboardShowModeChanged(showMode);
              }

              @Override
              public void onPerformGestureResult(int sequence, boolean completedSuccessfully) {
                  AccessibilityService.this.onPerformGestureResult(sequence, completedSuccessfully);
              }
          });
      }
}

分析:

  1. AccessibilityService是一个抽象类,继承于Service,提供两个抽象方法 onAccessibilityEvent() 和 onInterrupt();
  2. 虽然是抽象类,但是实现了最重要的 onBind() 方法,在其中创建了一个IAccessibilityServiceClientWrapper对象,实现Callbacks接口中的抽象方法。
IAccessibilityServiceClientWrapper
// 以分析onAccessibilityEvent为例,省略部分代码
public static class IAccessibilityServiceClientWrapper extends IAccessibilityServiceClient.Stub
            implements HandlerCaller.Callback {
    private final HandlerCaller mCaller;
    private final Callbacks mCallback;
    private int mConnectionId;
    
    public IAccessibilityServiceClientWrapper(Context context, Looper looper,
                Callbacks callback) {
        mCallback = callback;
        mCaller = new HandlerCaller(context, looper, this, true /*asyncHandler*/);
    }
    
    public void init(IAccessibilityServiceConnection connection, int connectionId,
                IBinder windowToken) {
        Message message = mCaller.obtainMessageIOO(DO_INIT, connectionId,
                    connection, windowToken);
        mCaller.sendMessage(message);
    }
    
    // 省略部分代码 

    public void onAccessibilityEvent(AccessibilityEvent event) {
        Message message = mCaller.obtainMessageO(DO_ON_ACCESSIBILITY_EVENT, event);
        mCaller.sendMessage(message);
    }

    @Override
    public void executeMessage(Message message) {
        switch (message.what) {
            case DO_ON_ACCESSIBILITY_EVENT: {
                AccessibilityEvent event = (AccessibilityEvent) message.obj;
                if (event != null) {
                    AccessibilityInteractionClient.getInstance().onAccessibilityEvent(event);
                    mCallback.onAccessibilityEvent(event);
                    // Make sure the event is recycled.
                    try {
                        event.recycle();
                    } catch (IllegalStateException ise) {
                        /* ignore - best effort */
                    }
                }
            } return;
            // ...         
        }
     }
}

分析:

  1. IAccessibilityServiceClientWrapper继承于IAccessibilityServiceClient类,它是一个aidl接口,同时注意到它是继承于IAccessibilityServiceClient.Stub类,可以大概猜测到,AccessibilityService为一个远程Service,使用到跨进程通信技术,后面我还会继续分析这个;
  2. IAccessibilityServiceClientWrapper的类构造方法中,有两个比较重要的参数,一个是looper,另一个是Callbacks callback。Looper不用说,而Callbacks接口定义了很多方法,代码如下:
public interface Callbacks {
    public void onAccessibilityEvent(AccessibilityEvent event);
    public void onInterrupt();
    public void onServiceConnected();
    public void init(int connectionId, IBinder windowToken);
    public boolean onGesture(int gestureId);
    public boolean onKeyEvent(KeyEvent event);
    public void onMagnificationChanged(@NonNull Region region,
                float scale, float centerX, float centerY);
    public void onSoftKeyboardShowModeChanged(int showMode);
    public void onPerformGestureResult(int sequence, boolean completedSuccessfully);
}
  1. IAccessibilityServiceClientWrapper同时也实现了HandlerCaller.Callback接口,HandlerCaller类通过命名也可以知道,它内部含有一个Handler实例,所以可以把它当做一个Handler,而处理信息的方法就是HandlerCaller.Callback#executeMessage(msg)方法
  2. 代码有点绕,故简单总结一下流程:
    AccessibilityEvent产生
      -> Binder驱动
       -> IAccessibilityServiceClientWrapper#onAccessibilityEvent(AccessibilityEvent)
        -> HandlerCaller#sendMessage(message); // message中包括AccessibilityEvent
         -> IAccessibilityServiceClientWrapper#executeMessage();
          -> Callbacks#onAccessibilityEvent(event);
           -> AccessibilityService.this.onAccessibilityEvent(event);

到这里解决了我们的第一个问题:AccessibilityService同样继承于Service类,它属于远程服务类,是Android系统提供的一种服务,可以绑定此服务,用于捕捉界面的一些特定事件。

AccessibilityService外部逻辑

前面分析了接收到AccessibilityEvent之后的代码逻辑,那么,这些AccessibilityEvent是怎样产生的呢,而且,在回调执行之后是怎么做到点击等操作的(如demo所示)?我们接下来继续分析相关的源码~

我们从demo作为例子开始入手,首先我们知道,一个点击事件的产生,实际代码逻辑是在View#onTouchEvent() -> View#performClick()中:

public boolean performClick() {
    final boolean result;
    final ListenerInfo li = mListenerInfo;
    if (li != null && li.mOnClickListener != null) {
        playSoundEffect(SoundEffectConstants.CLICK);
        li.mOnClickListener.onClick(this);
        result = true;
    } else {
        result = false;
    }
    // !!!
    sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_CLICKED);
    return result;
}

这里找到一个重点方法sendAccessibilityEvent(),继续跟进去,最后走到View#sendAccessibilityEventUncheckedInternal()方法:

public void sendAccessibilityEventUncheckedInternal(AccessibilityEvent event) {
    if (!isShown()) {
        return;
    }
    onInitializeAccessibilityEvent(event);
    // Only a subset of accessibility events populates text content.
    if ((event.getEventType() & POPULATING_ACCESSIBILITY_EVENT_TYPES) != 0) {
        dispatchPopulateAccessibilityEvent(event);
    }
    // In the beginning we called #isShown(), so we know that getParent() is not null.
    getParent().requestSendAccessibilityEvent(this, event);
}

这里的getParent()会返回一个实现ViewParent接口的对象。
我们可以简单理解为,它会让View的父类执行requestSendAccessibilityEvent()方法,而View的父类一般为ViewGroup,我们查看ViewGroup#requestSendAccessibilityEvent()方法

@Override
public boolean requestSendAccessibilityEvent(View child, AccessibilityEvent event) {
    ViewParent parent = mParent;
    if (parent == null) {
        return false;
    }
    final boolean propagate = onRequestSendAccessibilityEvent(child, event);
    if (!propagate) {
        return false;
    }
    return parent.requestSendAccessibilityEvent(this, event);
}

这里涉及到一个变量mParent,我们要找到这个mParent变量是在哪里被赋值的。
首先我们在View类中找到一个相关的方法View#assignParent():

void assignParent(ViewParent parent) {
    if (mParent == null) {
        mParent = parent;
    } else if (parent == null) {
        mParent = null;
    } else {
        throw new RuntimeException("view " + this + " being added, but" + " it already has a parent");
    }
}

但是View类中并没有调用此方法,猜测是View的父类进行调用。
通过对源码进行搜索,发现最后是在ViewRootImpl#setView()中进行调用,赋值的是this即ViewRootImpl本身。
直接跳到ViewRootImpl#requestSendAccessibilityEvent()方法:

@Override
public boolean requestSendAccessibilityEvent(View child, AccessibilityEvent event) {
    if (mView == null || mStopped || mPausedForTransition) {
        return false;
    }
    // Intercept accessibility focus events fired by virtual nodes to keep
    // track of accessibility focus position in such nodes.
    final int eventType = event.getEventType();
    switch (eventType) {
        case AccessibilityEvent.TYPE_VIEW_ACCESSIBILITY_FOCUSED:
            {
                final long sourceNodeId = event.getSourceNodeId();
                final int accessibilityViewId = AccessibilityNodeInfo.getAccessibilityViewId(sourceNodeId);
                View source = mView.findViewByAccessibilityId(accessibilityViewId);
                if (source != null) {
                    AccessibilityNodeProvider provider = source.getAccessibilityNodeProvider();
                    if (provider != null) {
                        final int virtualNodeId = AccessibilityNodeInfo.getVirtualDescendantId(sourceNodeId);
                        final AccessibilityNodeInfo node;
                        if (virtualNodeId == AccessibilityNodeInfo.UNDEFINED_ITEM_ID) {
                            node = provider.createAccessibilityNodeInfo(AccessibilityNodeProvider.HOST_VIEW_ID);
                        } else {
                            node = provider.createAccessibilityNodeInfo(virtualNodeId);
                        }
                        setAccessibilityFocus(source, node);
                    }
                }
            }
            break;
            // 省略部分代码
    }
    // !!!
    mAccessibilityManager.sendAccessibilityEvent(event);
    return true;
}

重点:AccessibilityManager#sendAccessibilityEvent(event)

public void sendAccessibilityEvent(AccessibilityEvent event) {
    final IAccessibilityManager service;
    final int userId;
    synchronized(mLock) {
        service = getServiceLocked();
        if (service == null) {
            return;
        }
        if (!mIsEnabled) {
            Looper myLooper = Looper.myLooper();
            if (myLooper == Looper.getMainLooper()) {
                throw new IllegalStateException("Accessibility off. Did you forget to check that?");
            } else {
                // If we're not running on the thread with the main looper, it's possible for
                // the state of accessibility to change between checking isEnabled and
                // calling this method. So just log the error rather than throwing the
                // exception.
                Log.e(LOG_TAG, "AccessibilityEvent sent with accessibility disabled");
                return;
            }
        }
        userId = mUserId;
    }
    boolean doRecycle = false;
    try {
        event.setEventTime(SystemClock.uptimeMillis());
        // it is possible that this manager is in the same process as the service but
        // client using it is called through Binder from another process. Example: MMS
        // app adds a SMS notification and the NotificationManagerService calls this method
        long identityToken = Binder.clearCallingIdentity();
        // !!!
        doRecycle = service.sendAccessibilityEvent(event, userId);
        Binder.restoreCallingIdentity(identityToken);
        if (DEBUG) {
            Log.i(LOG_TAG, event + " sent");
        }
    } catch (RemoteException re) {
        Log.e(LOG_TAG, "Error during sending " + event + " ", re);
    } finally {
        if (doRecycle) {
            event.recycle();
        }
    }
}

private IAccessibilityManager getServiceLocked() {
    if (mService == null) {
        tryConnectToServiceLocked(null);
    }
    return mService;
}

private void tryConnectToServiceLocked(IAccessibilityManager service) {
    if (service == null) {
        IBinder iBinder = ServiceManager.getService(Context.ACCESSIBILITY_SERVICE);
        if (iBinder == null) {
            return;
        }
        service = IAccessibilityManager.Stub.asInterface(iBinder);
    }
    try {
        final int stateFlags = service.addClient(mClient, mUserId);
        setStateLocked(stateFlags);
        mService = service;
    } catch (RemoteException re) {
        Log.e(LOG_TAG, "AccessibilityManagerService is dead", re);
    }
}

这里有使用到Android Binder机制,重点为IAccessibilityManager#sendAccessibilityEvent()方法,这里调用的是代理方法,实际代码逻辑在AccessibilityManagerService#sendAccessibilityEvent():

@Override
public boolean sendAccessibilityEvent(AccessibilityEvent event, int userId) {
    synchronized(mLock) {
        // We treat calls from a profile as if made by its parent as profiles
        // share the accessibility state of the parent. The call below
        // performs the current profile parent resolution..
        final int resolvedUserId = mSecurityPolicy.resolveCallingUserIdEnforcingPermissionsLocked(userId);
        // This method does nothing for a background user.
        if (resolvedUserId != mCurrentUserId) {
            return true; // yes, recycle the event
        }
        if (mSecurityPolicy.canDispatchAccessibilityEventLocked(event)) {
            mSecurityPolicy.updateActiveAndAccessibilityFocusedWindowLocked(event.getWindowId(), event.getSourceNodeId(), event.getEventType(), event.getAction());
            mSecurityPolicy.updateEventSourceLocked(event);
            // !!!
            notifyAccessibilityServicesDelayedLocked(event, false);
            notifyAccessibilityServicesDelayedLocked(event, true);
        }
        if (mHasInputFilter && mInputFilter != null) {
            mMainHandler.obtainMessage(MainHandler.MSG_SEND_ACCESSIBILITY_EVENT_TO_INPUT_FILTER, AccessibilityEvent.obtain(event)).sendToTarget();
        }
        event.recycle();
    }
    return (OWN_PROCESS_ID != Binder.getCallingPid());
}

private void notifyAccessibilityServicesDelayedLocked(AccessibilityEvent event, boolean isDefault) {
    try {
        UserState state = getCurrentUserStateLocked();
        for (int i = 0, count = state.mBoundServices.size(); i < count; i++) {
            Service service = state.mBoundServices.get(i);
            if (service.mIsDefault == isDefault) {
                if (canDispatchEventToServiceLocked(service, event)) {
                    service.notifyAccessibilityEvent(event);
                }
            }
        }
    } catch (IndexOutOfBoundsException oobe) {
        // An out of bounds exception can happen if services are going away
        // as the for loop is running. If that happens, just bail because
        // there are no more services to notify.
    }
}
  1. 在方法中,最后会调用notifyAccessibilityServicesDelayedLocked()方法,然后将event进行回收;
  2. 在notifyAccessibilityServicesDelayedLocked()方法中,会获得所有Bound即绑定的Service,执行notifyAccessibilityEvent()方法,通过跟踪代码逻辑,最后会调用绑定Service的onAccessibilityEvent()方法。绑定的Service是指我们自己实现的继承于AccessibilityService的Service类,当你在设置-无障碍中开启服务之后即将服务绑定到AccessibilityManagerService中。

这样我们解决了第二个问题:
AccessibilityService是如何做到监控捕捉用户行为的:(以点击事件为例)
AccessibilityEvent产生:
View#performClick()
  -> View#sendAccessibilityEventUncheckedInternal()
   -> ViewGroup#requestSendAccessibilityEvent()
    -> ViewRootImpl#requestSendAccessibilityEvent()
     -> AccessibilityManager#sendAccessibilityEvent(event)
      -> AccessibilityManagerService#sendAccessibilityEvent()
       -> AccessibilityManagerService#notifyAccessibilityServicesDelayedLocked()
        -> Service#notifyAccessibilityEvent(event)

AccessibilityEvent处理:
AccessibilityEvent
  -> Binder驱动
   -> IAccessibilityServiceClientWrapper#onAccessibilityEvent(AccessibilityEvent)
    -> HandlerCaller#sendMessage(message); // message中包括AccessibilityEvent
     -> IAccessibilityServiceClientWrapper#executeMessage();
      -> Callbacks#onAccessibilityEvent(event);
       -> AccessibilityService.this.onAccessibilityEvent(event);

AccessibilityService交互之查找控件

在demo中,我们在MyAccessibilityService中调用了getRootInActiveWindow()方法获取被监控的View的所有结点,这些结点都封装成一个AccessibilityNodeInfo对象中。同时也调用AccessibilityNodeInfo#findAccessibilityNodeInfosByText()方法查找相应的控件。
这些方法的本质是调用了AccessibilityInteractionClient类的对应方法。
以AccessibilityInteractionClient#findAccessibilityNodeInfosByText()为例:

public List<AccessibilityNodeInfo> findAccessibilityNodeInfosByText(int connectionId, int accessibilityWindowId, long accessibilityNodeId, String text) {
    try {
        IAccessibilityServiceConnection connection = getConnection(connectionId);
        if (connection != null) {
            final int interactionId = mInteractionIdCounter.getAndIncrement();
            final long identityToken = Binder.clearCallingIdentity();
            final boolean success = connection.findAccessibilityNodeInfosByText(accessibilityWindowId, accessibilityNodeId, text, interactionId, this, Thread.currentThread().getId());
            Binder.restoreCallingIdentity(identityToken);
            if (success) {
                List<AccessibilityNodeInfo> infos = getFindAccessibilityNodeInfosResultAndClear(interactionId);
                if (infos != null) {
                    finalizeAndCacheAccessibilityNodeInfos(infos, connectionId);
                    return infos;
                }
            }
        } else {
            if (DEBUG) {
                Log.w(LOG_TAG, "No connection for connection id: " + connectionId);
            }
        }
    } catch (RemoteException re) {
        Log.w(LOG_TAG, "Error while calling remote" + " findAccessibilityNodeInfosByViewText", re);
    }
    return Collections.emptyList();
}

代码逻辑比较简单,就是直接调用IAccessibilityServiceConnection#findAccessibilityNodeInfosByText()方法。
IAccessibilityServiceConnection是一个aidl接口,从注释看,它是AccessibilitySerivce和AccessibilityManagerService之间沟通的桥梁。
猜想代码真正的实现在AccessibilityManagerService中。
AccessibilityManagerService.Service#findAccessibilityNodeInfosByText():

@Override
public boolean findAccessibilityNodeInfosByText(int accessibilityWindowId, long accessibilityNodeId, String text, int interactionId, IAccessibilityInteractionConnectionCallback callback, long interrogatingTid) throws RemoteException {
    final int resolvedWindowId;
    IAccessibilityInteractionConnection connection = null;
    Region partialInteractiveRegion = Region.obtain();
    synchronized(mLock) {
        if (!isCalledForCurrentUserLocked()) {
            return false;
        }
        resolvedWindowId = resolveAccessibilityWindowIdLocked(accessibilityWindowId);
        final boolean permissionGranted = mSecurityPolicy.canGetAccessibilityNodeInfoLocked(this, resolvedWindowId);
        if (!permissionGranted) {
            return false;
        } else {
            connection = getConnectionLocked(resolvedWindowId);
            if (connection == null) {
                return false;
            }
        }
        if (!mSecurityPolicy.computePartialInteractiveRegionForWindowLocked(resolvedWindowId, partialInteractiveRegion)) {
            partialInteractiveRegion.recycle();
            partialInteractiveRegion = null;
        }
    }
    final int interrogatingPid = Binder.getCallingPid();
    final long identityToken = Binder.clearCallingIdentity();
    MagnificationSpec spec = getCompatibleMagnificationSpecLocked(resolvedWindowId);
    try {
        connection.findAccessibilityNodeInfosByText(accessibilityNodeId, text, partialInteractiveRegion, interactionId, callback, mFetchFlags, interrogatingPid, interrogatingTid, spec);
        return true;
    } catch (RemoteException re) {
        if (DEBUG) {
            Slog.e(LOG_TAG, "Error calling findAccessibilityNodeInfosByText()");
        }
    } finally {
        Binder.restoreCallingIdentity(identityToken);
        // Recycle if passed to another process.
        if (partialInteractiveRegion != null && Binder.isProxy(connection)) {
            partialInteractiveRegion.recycle();
        }
    }
    return false;
}
  1. 此方法在AccessibilityManagerService的内部类Service中实现,这个Service继承于IAccessibilityServiceConnection.Stub,验证了我上面的猜想是正确的;
  2. 代码重点是调用connection.findAccessibilityNodeInfosByText(),这里的connection实例与上面不同,它隶属于IAccessibilityInteractionConnection类。这个类同样是一个aidl接口,从注释上看,它又是AccessibilityManagerService与指定窗口的ViewRoot之间沟通的桥梁。
    再次猜想,真正的代码逻辑在ViewRootImpl中。
    查看ViewRootImpl.AccessibilityInteractionConnection#findAccessibilityNodeInfosByText():
@Override
public void findAccessibilityNodeInfosByText(long accessibilityNodeId, String text, Region interactiveRegion, int interactionId, IAccessibilityInteractionConnectionCallback callback, int flags, int interrogatingPid, long interrogatingTid, MagnificationSpec spec) {
    ViewRootImpl viewRootImpl = mViewRootImpl.get();
    if (viewRootImpl != null && viewRootImpl.mView != null) {
        viewRootImpl.getAccessibilityInteractionController().findAccessibilityNodeInfosByTextClientThread(accessibilityNodeId, text, interactiveRegion, interactionId, callback, flags, interrogatingPid, interrogatingTid, spec);
    } else {
        // We cannot make the call and notify the caller so it does not wait.
        try {
            callback.setFindAccessibilityNodeInfosResult(null, interactionId);
        } catch (RemoteException re) {
            /* best effort - ignore */
        }
    }
}
  1. 同样的,此方法在ViewRootImpl的内部类AccessibilityInteractionConnection中实现,这个内部类继承于IAccessibilityServiceConnection.Stub,验证了我的猜想;
  2. 查找控件等操作,ViewRootImpl并不是直接处理,而是交给AccessibilityInteractionController类去查找,查找到的结果会保存到一个callback中,这个callback为IAccessibilityInteractionConnectionCallback类型,它也是一个aidl接口,而AccessibilityInteractionClient类继承了IAccessibilityInteractionConnectionCallback.Stub,即最后查询后的结果会回调到AccessibilityInteractionClient类中,如上面AccessibilityInteractionClient#findAccessibilityNodeInfosByText()方法,最后会调用getFindAccessibilityNodeInfosResultAndClear()方法获取结果。具体如何寻找指定控件则不再分析代码。
AccessibilityService交互之执行控件操作

类似的,与上面的流程基本相同,只是回调的时候,返回的是执行操作的返回值(True or False)。

到这里,我们解决了最后一个问题:
AccessibilityService是如何做到查找控件,执行点击等操作的?
总结:
寻找指定控件/执行操作
  -> 交给AccessibilityInteractionClient类处理
    -> Binder
      -> AccessibilityManagerService类进行查找/执行操作
        -> Binder
          -> 指定窗口的ViewRoot(ViewRootImpl)进行查找/执行操作
        <- Binder
    <- 结果回调到AccessibilityInteractionClient类

四、有用代码记录

  1. HandlerCaller类:结合Handler类和自定义的接口类(Caller.java),利用Handler的消息循环机制来分发消息,将最终的处理函数交给Caller#executeMessage():
// HandlerCaller.java
public class HandlerCaller {
    final Looper mMainLooper;
    final Handler mH;

    final Callback mCallback;

    class MyHandler extends Handler {
        MyHandler(Looper looper, boolean async) {
            super(looper, null, async);
        }

        @Override
        public void handleMessage(Message msg) {
            mCallback.executeMessage(msg);
        }
    }

    public interface Callback {
        public void executeMessage(Message msg);
    }

    public HandlerCaller(Context context, Looper looper, Callback callback,
            boolean asyncHandler) {
        mMainLooper = looper != null ? looper : context.getMainLooper();
        mH = new MyHandler(mMainLooper, asyncHandler);
        mCallback = callback;
    }

    public Handler getHandler() {
        return mH;
    }

    public void executeOrSendMessage(Message msg) {
        // If we are calling this from the main thread, then we can call
        // right through.  Otherwise, we need to send the message to the
        // main thread.
        if (Looper.myLooper() == mMainLooper) {
            mCallback.executeMessage(msg);
            msg.recycle();
            return;
        }
        
        mH.sendMessage(msg);
    }

    public void sendMessageDelayed(Message msg, long delayMillis) {
        mH.sendMessageDelayed(msg, delayMillis);
    }

    public boolean hasMessages(int what) {
        return mH.hasMessages(what);
    }
    
    public void removeMessages(int what) {
        mH.removeMessages(what);
    }
    
    public void removeMessages(int what, Object obj) {
        mH.removeMessages(what, obj);
    }
    
    public void sendMessage(Message msg) {
        mH.sendMessage(msg);
    }

    public SomeArgs sendMessageAndWait(Message msg) {
        if (Looper.myLooper() == mH.getLooper()) {
            throw new IllegalStateException("Can't wait on same thread as looper");
        }
        SomeArgs args = (SomeArgs)msg.obj;
        args.mWaitState = SomeArgs.WAIT_WAITING;
        mH.sendMessage(msg);
        synchronized (args) {
            while (args.mWaitState == SomeArgs.WAIT_WAITING) {
                try {
                    args.wait();
                } catch (InterruptedException e) {
                    return null;
                }
            }
        }
        args.mWaitState = SomeArgs.WAIT_NONE;
        return args;
    }

    public Message obtainMessage(int what) {
        return mH.obtainMessage(what);
    }
     
    // 省略部分代码
}
  1. HandlerCaller#sendMessageAndWait():
public SomeArgs sendMessageAndWait(Message msg) {
    if (Looper.myLooper() == mH.getLooper()) {
        throw new IllegalStateException("Can't wait on same thread as looper");
    }
    SomeArgs args = (SomeArgs) msg.obj;
    args.mWaitState = SomeArgs.WAIT_WAITING;
    mH.sendMessage(msg);
    synchronized(args) {
        while (args.mWaitState == SomeArgs.WAIT_WAITING) {
            try {
                args.wait();
            } catch (InterruptedException e) {
                return null;
            }
        }
    }
    args.mWaitState = SomeArgs.WAIT_NONE;
    return args;
}

这篇文章会同步到我的个人日志,如有问题,请大家踊跃提出,谢谢大家!

Android源码解析
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