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：
- AccessibilityService这个Service跟一般的Service有什么区别？
- AccessibilityService是如何做到监控并捕捉用户行为的？
- 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
1 | public class MyAccessibilityService extends AccessibilityService { |
AccessibilityService配置
res/xml/accessibility_service_config.xml1
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<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
1 | <service |
创建一个text为Test!的button控件，设置监听方法
1 | public class MainActivity extends AppCompatActivity { |
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：
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
1 | public abstract class AccessibilityService extends Service { |
分析：
- AccessibilityService是一个抽象类，继承于Service，提供两个抽象方法 onAccessibilityEvent() 和 onInterrupt()；
- 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
1 | // 以分析onAccessibilityEvent为例，省略部分代码 |
分析：
- 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；
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：
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12public 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);
}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
- 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：1
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14public 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：1
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12public 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1
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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()：1
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9void 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：1
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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)1
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68public 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()：1
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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.
}
}
- 在方法中，最后会调用notifyAccessibilityServicesDelayedLocked()方法，然后将event进行回收；
- 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：1
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25public 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()：1
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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;
}
- 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；
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()：1
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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 */
}
}
}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；
- 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
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()：
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92// 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);
}
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
}HandlerCaller#sendMessageAndWait()：
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19public 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;
}