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 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

在manifest中进行注册

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：


文章图片
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); }

3. 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
4. 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 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 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 }

2. 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; }

【Android|Android AccessibilityService机制源码解析】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！

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