通过源码角度看看AccessibilityService

简介

AccessibilityService的设计初衷是为了辅助有身体缺陷的群体使用Android应用，它的设计贯穿着Android的控件树View, ViewGroup, ViewRootImpl体系。借助于system_server进程的中转，能够注册Accessibility事件的客户端可以具备通过system_server提供的Accessibility服务来实现监听、操作其它应用视图的功能。这个功能十分强大，可以模拟用户的行为去操作其它APP，常常被用在自动化测试、微信抢红包、自动回复等功能实现中。

写这个的初衷有二：

• 之前已经完成了Android View控件树的绘制、事件分发的源码分析，知识储备足够
• 最近接触到了一些自动化方面的项目，并且对使用无障碍服务实现的自动微信抢红包功能原理十分好奇

整体图

类图

• AccessibilityService: APP端直接继承的类，本质上是Service，通过onBind获取匿名Binder对象实现通信
• IAccessibilityServiceClientWrapper: 用于和system_server通信的匿名Binder服务
• AccessibilityInteractionClient: 本质上是个binder服务，用于获取Node信息
• AccessibilityManagerService: 运行在system_server的实名binder服务，是整体的管理类
• Service: AccessibilityManagerService的内部类，用于响应AccessibilityInteractionClient的binder通信请求
• AccessibilityInteractionConnection: 运行在被监测的APP端，提供查找、点击视图等服务
• AccessibilityManager: 运行在各个APP端，用于发送视图变化事件
• AccessibilityInteractionController: 具体视图查找、点击服务的中间控制器
• AccessibilityNodeProvider: 由客户端实现的视图节点内容提供者，最终操作的实现者

整体设计图

实例代码

public class AutoDismissService extends AccessibilityService {
 @Override
 public void onAccessibilityEvent(AccessibilityEvent event) {
 if (event == null) {
 return;
 }

 // 自动将android系统弹出的其它crash dialog取消
 dismissAppErrorDialogIfExists(event);
 }

 private void dismissAppErrorDialogIfExists(AccessibilityEvent event) {
 // WINDOW视图变化才进行对应操作
 if ((event.getEventType() == AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED
 && event.getPackageName().equals("android")) {
 // 查找带有"OK"字符的可点击Node
 AccessibilityNodeInfo nodeInfo = findViewByText("OK", true);
 if (nodeInfo != null) {
 // 查找到后执行点击操作
 performViewClick(nodeInfo);
 }
 }
 public AccessibilityNodeInfo findViewByText(String text, boolean clickable) {
 // 获取当前窗口父节点
 AccessibilityNodeInfo accessibilityNodeInfo = getRootInActiveWindow();
 if (accessibilityNodeInfo == null) {
 return null;
 }
 // 获取到满足字符要求的节点
 List<AccessibilityNodeInfo> nodeInfoList = accessibilityNodeInfo.findAccessibilityNodeInfosByText(text);
 if (nodeInfoList != null && !nodeInfoList.isEmpty()) {
 for (AccessibilityNodeInfo nodeInfo : nodeInfoList) {
 if (nodeInfo != null && (nodeInfo.isClickable() == clickable)) {
  return nodeInfo;
 }
 }
 }
 return null;
 }

 public void performViewClick(AccessibilityNodeInfo nodeInfo) {
 if (nodeInfo == null) {
 return;
 }
 // 由下至上进行查询，直到寻找到可点击的节点
 while (nodeInfo != null) {
 if (nodeInfo.isClickable()) {
 nodeInfo.performAction(AccessibilityNodeInfo.ACTION_CLICK);
 break;
 }
 nodeInfo = nodeInfo.getParent();
 }
 }
}

以上是一个典型的实现Accessibility功能的JAVA代码，主要涉及三点功能：

• 当系统中有应用视图变化后，onAccessibilityEvent 方法会自动被system_server调用
• 通过AccessibilityService的getRootInActiveWindow与findAccessibilityNodeInfosByText方法，可以获取到节点信息
• 通过AccessibilityNodeInfo的performAction方法，最终会在被监听APP中执行对应操作

本篇文章将会围绕着这三点主要功能进行源码分析

源码分析

常见 AccessibilityEvent 事件种类

onAccessibilityEvent 触发流程

这里以TextView.setText触发事件变化流程为例进行分析

TextView.setText

应用组件状态发生变化

frameworks/base/core/java/android/widget/TextView.java

private void setText(CharSequence text, BufferType type,
   boolean notifyBefore, int oldlen) {
 ...
 notifyViewAccessibilityStateChangedIfNeeded(AccessibilityEvent.CONTENT_CHANGE_TYPE_TEXT);
 ...
}
public void notifyViewAccessibilityStateChangedIfNeeded(int changeType) {
 if (!AccessibilityManager.getInstance(mContext).isEnabled() || mAttachInfo == null) {
 return;
 }
 if (mSendViewStateChangedAccessibilityEvent == null) {
 // 本质上是一个Runnable，意味着这里的流程会进入异步处理
 mSendViewStateChangedAccessibilityEvent =
  new SendViewStateChangedAccessibilityEvent();
 }
 mSendViewStateChangedAccessibilityEvent.runOrPost(changeType);
}
private class SendViewStateChangedAccessibilityEvent implements Runnable {
 ...
 @Override
 public void run() {
 mPosted = false;
 mPostedWithDelay = false;
 mLastEventTimeMillis = SystemClock.uptimeMillis();
 if (AccessibilityManager.getInstance(mContext).isEnabled()) {
  final AccessibilityEvent event = AccessibilityEvent.obtain();
  event.setEventType(AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED);
  event.setContentChangeTypes(mChangeTypes);
  // 最终TYPE_WINDOW_CONTENT_CHANGED事件在这里异步发送
  sendAccessibilityEventUnchecked(event);
 }
 mChangeTypes = 0;
 }
 ...
}
public void sendAccessibilityEventUnchecked(AccessibilityEvent event) {
 if (mAccessibilityDelegate != null) {
 mAccessibilityDelegate.sendAccessibilityEventUnchecked(this, event);
 } else {
 sendAccessibilityEventUncheckedInternal(event);
 }
}
public void sendAccessibilityEventUnchecked(View host, AccessibilityEvent event) {
 host.sendAccessibilityEventUncheckedInternal(event);
}
public void sendAccessibilityEventUncheckedInternal(AccessibilityEvent event) {
 if (!isShown()) {
 return;
 }
 ...
 // 此处交由TextView所在父View进行处理，为责任链模式，事件经过层层向上传递，最终交由ViewRootImpl进行处理
 ViewParent parent = getParent();
 if (parent != null) {
 getParent().requestSendAccessibilityEvent(this, event);
 }
}

ViewRootImpl.requestSendAccessibilityEvent

ViewRootImpl将事件派发到system_server

frameworks/base/core/java/android/view/ViewRootImpl.java

@Override
public boolean requestSendAccessibilityEvent(View child, AccessibilityEvent event) {
 ...
 // 本地调用到AccessibilityManager进行事件发送
 mAccessibilityManager.sendAccessibilityEvent(event);
 return true;
}

frameworks/base/core/java/android/view/accessibility/AccessibilityManager.java

public void sendAccessibilityEvent(AccessibilityEvent event) {
 final IAccessibilityManager service;
 final int userId;
 synchronized (mLock) {
 // 获取system_server的Accessibility实名服务
 service = getServiceLocked();
 ...
 }

 try {
 ...
 long identityToken = Binder.clearCallingIdentity();
 // binder call 到服务端，进行事件分发中转
 doRecycle = service.sendAccessibilityEvent(event, userId);
 Binder.restoreCallingIdentity(identityToken);
 ...
 } catch (RemoteException re) {
 Log.e(LOG_TAG, "Error during sending " + event + " ", re);
 } finally {
 ...
 }
}

AccessibilityManagerService.sendAccessibilityEvent

system_server将事件分发到各个监听组件变化的Service

frameworks/base/services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java

// binder call 到服务端，触发事件派发
@Override
public boolean sendAccessibilityEvent(AccessibilityEvent event, int userId) {
 synchronized (mLock) {
 ...
 if (mSecurityPolicy.canDispatchAccessibilityEventLocked(event)) {
  ...
  notifyAccessibilityServicesDelayedLocked(event, false);
  notifyAccessibilityServicesDelayedLocked(event, true);
 }
 ...
 }
 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)) {
   // 调用内部服务，以触发事件派发
   service.notifyAccessibilityEvent(event);
  }
  }
 }
 } catch (IndexOutOfBoundsException oobe) {
 ...
 }
}
class Service extends IAccessibilityServiceConnection.Stub
 implements ServiceConnection, DeathRecipient {
 public void notifyAccessibilityEvent(AccessibilityEvent event) {
 synchronized (mLock) {
  ...
  if ((mNotificationTimeout > 0)
   && (eventType != AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED)) {
  ...
  // 按照惯例，异步分发到客户端进行派发
  message = mEventDispatchHandler.obtainMessage(eventType);
  } else {
  message = mEventDispatchHandler.obtainMessage(eventType, newEvent);
  }
  mEventDispatchHandler.sendMessageDelayed(message, mNotificationTimeout);
 }
 }
}
public Handler mEventDispatchHandler = new Handler(mMainHandler.getLooper()) {
 @Override
 public void handleMessage(Message message) {
 final int eventType = message.what;
 AccessibilityEvent event = (AccessibilityEvent) message.obj;
 notifyAccessibilityEventInternal(eventType, event);
 }
};
private void notifyAccessibilityEventInternal(int eventType, AccessibilityEvent event) {
 IAccessibilityServiceClient listener;
 ...
 // mServiceInterface是通过bind客户端的AccessibilityService，在onServiceConnected连接成功后，获取到binder proxy转化来的，以这种方式实现了system_server与客户端的通信
 listener = mServiceInterface;
 ...
 try {
 listener.onAccessibilityEvent(event);
 if (DEBUG) {
  Slog.i(LOG_TAG, "Event " + event + " sent to " + listener);
 }
 } catch (RemoteException re) {
 Slog.e(LOG_TAG, "Error during sending " + event + " to " + listener, re);
 } finally {
 event.recycle();
 }
}

AccessibilityService.onAccessibilityEvent

APP接收到组件变化的事件，并可以选择做出相应的处理

frameworks/base/core/java/android/accessibilityservice/AccessibilityService.java

// 抽象方法，模板模式，被系统主动调用
public abstract void onAccessibilityEvent(AccessibilityEvent event);
// 该service是被system_server主动绑定的，获取到IAccessibilityServiceClientWrapper的proxy来实现系统的主动调用
@Override
public final IBinder onBind(Intent intent) {
 return new IAccessibilityServiceClientWrapper(this, getMainLooper(), new Callbacks() {
 ...
 @Override
 public void onAccessibilityEvent(AccessibilityEvent event) {
  AccessibilityService.this.onAccessibilityEvent(event);
 }
 ...
 }
}
// 收到binder调用后，使用handler异步进行事件的处理
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);
  // 通过回调调用以触发事件
  mCallback.onAccessibilityEvent(event);
  ...
  }
 } return;
 }
}

getRootInActiveWindow 父节点获取流程

在调用findAccessibilityNodeInfosByText之前，需要通过getRootInActiveWindow方法获取到父节点，才能通过调用父AccessibilityNodeInfo的方法进行其子节点信息查询

AccessibilityService.getRootInActiveWindow

frameworks/base/core/java/android/accessibilityservice/AccessibilityService.java

public AccessibilityNodeInfo getRootInActiveWindow() {
 // 查找父节点的操作没有在自己的类中实现，而是交由了同一进程的Client管理类进行处理
 return AccessibilityInteractionClient.getInstance().getRootInActiveWindow(mConnectionId);
}

frameworks/base/core/java/android/view/accessibility/AccessibilityInteractionClient.java

public AccessibilityNodeInfo getRootInActiveWindow(int connectionId) {
 return findAccessibilityNodeInfoByAccessibilityId(connectionId,
  AccessibilityNodeInfo.ACTIVE_WINDOW_ID, AccessibilityNodeInfo.ROOT_NODE_ID,
  false, AccessibilityNodeInfo.FLAG_PREFETCH_DESCENDANTS);
}
public AccessibilityNodeInfo findAccessibilityNodeInfoByAccessibilityId(int connectionId,
  int accessibilityWindowId, long accessibilityNodeId, boolean bypassCache,
  int prefetchFlags) {
 ...
 // 尝试binder call到system_server，请求中转到其它APP进程中查询父节点信息，注意的是这里AccessibilityInteractionClient本身是个binder服务端，把this传到system_server后，其它进程可以通过这个引用拿到binder proxy，以实现通信
 final boolean success = connection.findAccessibilityNodeInfoByAccessibilityId(
   accessibilityWindowId, accessibilityNodeId, interactionId, this,
   prefetchFlags, Thread.currentThread().getId());
 Binder.restoreCallingIdentity(identityToken);
 // If the scale is zero the call has failed.
 if (success) {
 // 调用成功后，这里会尝试同步获取结果
 List<AccessibilityNodeInfo> infos = getFindAccessibilityNodeInfosResultAndClear(
  interactionId);
 finalizeAndCacheAccessibilityNodeInfos(infos, connectionId);
 if (infos != null && !infos.isEmpty()) {
  return infos.get(0);
 }
 }
 ...
}

Service.findAccessibilityNodeInfoByAccessibilityId

注意一下，这里的Service不是Android中的四大组件的Service，取名叫AccessiblitManagerServiceInternal其实更合适

frameworks/base/services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java

@Override
public boolean findAccessibilityNodeInfoByAccessibilityId(
 int accessibilityWindowId, long accessibilityNodeId, int interactionId,
 IAccessibilityInteractionConnectionCallback callback, int flags,
 long interrogatingTid) throws RemoteException {
 ...
 // 获取到其他APP的节点获取服务
 IAccessibilityInteractionConnection connection = null;
 ...
 resolvedWindowId = resolveAccessibilityWindowIdLocked(accessibilityWindowId);
 ...
 if (!permissionGranted) {
 return false;
 } else {
 connection = getConnectionLocked(resolvedWindowId);
 if (connection == null) {
  return false;
 }
 }
 ...
 // 这里的callback为之前应用的服务proxy句柄，将它传入是为了之后的信息通信不再需要经过system_server中转，而是直接可以APP对APP的进行通信
 connection.findAccessibilityNodeInfoByAccessibilityId(accessibilityNodeId,
  partialInteractiveRegion, interactionId, callback, mFetchFlags | flags,
  interrogatingPid, interrogatingTid, spec);
 ...
}

AccessibilityInteractionConnection.findAccessibilityNodeInfoByAccessibilityId

这里调用到了APP端，其实同onAccessibilityEvent调用流程一样，是APP->SYSTEM->APP的调用顺序

frameworks/base/core/java/android/view/ViewRootImpl.java

@Override
public void findAccessibilityNodeInfoByAccessibilityId(long accessibilityNodeId,
 Region interactiveRegion, int interactionId,
 IAccessibilityInteractionConnectionCallback callback, int flags,
 int interrogatingPid, long interrogatingTid, MagnificationSpec spec) {
 ViewRootImpl viewRootImpl = mViewRootImpl.get();
 if (viewRootImpl != null && viewRootImpl.mView != null) {
 // 这里也只是委托给控制类进行细节操作的处理
 viewRootImpl.getAccessibilityInteractionController()
  .findAccessibilityNodeInfoByAccessibilityIdClientThread(accessibilityNodeId,
   interactiveRegion, interactionId, callback, flags, interrogatingPid,
   interrogatingTid, spec);
 } else {
 ...
 }
}

frameworks/base/core/java/android/view/AccessibilityInteractionController.java

private void findAccessibilityNodeInfoByAccessibilityIdUiThread(Message message) {
 ...
 // 初始化将会返回的节点
 List<AccessibilityNodeInfo> infos = mTempAccessibilityNodeInfoList;
 infos.clear();
 try {
 if (mViewRootImpl.mView == null || mViewRootImpl.mAttachInfo == null) {
  return;
 }
 mViewRootImpl.mAttachInfo.mAccessibilityFetchFlags = flags;
 View root = null;
 if (accessibilityViewId == AccessibilityNodeInfo.UNDEFINED_ITEM_ID) {
  root = mViewRootImpl.mView;
 } else {
  root = findViewByAccessibilityId(accessibilityViewId);
 }
 ...
 } finally {
 try {
  ...
  adjustIsVisibleToUserIfNeeded(infos, interactiveRegion);
  // 通过callback binder proxy句柄，将节点信息binder回应用
  callback.setFindAccessibilityNodeInfosResult(infos, interactionId);
  infos.clear();
 } catch (RemoteException re) {
  /* ignore - the other side will time out */
 }
 ...
 }
}

AccessibilityInteractionClient.setFindAccessibilityNodeInfosResult

frameworks/base/core/java/android/view/accessibility/AccessibilityInteractionClient.java

public void setFindAccessibilityNodeInfosResult(List<AccessibilityNodeInfo> infos,
  int interactionId) {
 synchronized (mInstanceLock) {
 if (interactionId > mInteractionId) {
  if (infos != null) {
  ...
  // 设置应用的返回节点信息
  if (!isIpcCall) {
   mFindAccessibilityNodeInfosResult = new ArrayList<>(infos);
  } else {
   mFindAccessibilityNodeInfosResult = infos;
  }
  } else {
  mFindAccessibilityNodeInfosResult = Collections.emptyList();
  }
  mInteractionId = interactionId;
 }
 // 释放锁，停止等待，节点信息已经取回
 mInstanceLock.notifyAll();
 }
}

findAccessibilityNodeInfosByText与performAction 对目标节点进行操作

AccessibilityNodeInfo.findAccessibilityNodeInfosByText

找到父节点信息后，就可以通过父节点获取对应的子节点信息了

frameworks/base/core/java/android/view/accessibility/AccessibilityNodeInfo.java

public List<AccessibilityNodeInfo> findAccessibilityNodeInfosByText(String text) {
 ...
 // 一样的流程，通过AccessibilityInteractionClient去获取信息
 AccessibilityInteractionClient client = AccessibilityInteractionClient.getInstance();
 return client.findAccessibilityNodeInfosByText(mConnectionId, mWindowId, mSourceNodeId,
  text);
}
``` 

以下的代码流程同getRootInActiveWindow大概一致，就不详细分析了

#### AccessibilityNodeInfo.performAction

获取到对应子节点后，通过performAction可以执行对应的操作了，如常用的点击

最终回调用到AccessibilityInteractionController,获取到AccessibilityProvier后就可以执行performAction的最终操作了

frameworks/base/core/java/android/view/AccessibilityInteractionController.java

```java
private void performAccessibilityActionUiThread(Message message) {
 View target = null;
 if (accessibilityViewId != AccessibilityNodeInfo.UNDEFINED_ITEM_ID) {
 target = findViewByAccessibilityId(accessibilityViewId);
 } else {
 target = mViewRootImpl.mView;
 }
 if (target != null && isShown(target)) {
 AccessibilityNodeProvider provider = target.getAccessibilityNodeProvider();
 if (provider != null) {
  if (virtualDescendantId != AccessibilityNodeInfo.UNDEFINED_ITEM_ID) {
  // 在客户端执行performAction操作
  succeeded = provider.performAction(virtualDescendantId, action,
   arguments);
  } else {
  succeeded = provider.performAction(AccessibilityNodeProvider.HOST_VIEW_ID,
   action, arguments);
  }
 } else if (virtualDescendantId == AccessibilityNodeInfo.UNDEFINED_ITEM_ID) {
  succeeded = target.performAccessibilityAction(action, arguments);
 }
 }
}

frameworks/base/core/java/android/view/View.java

public boolean performAccessibilityActionInternal(int action, Bundle arguments) {
 ...
 switch (action) {
 case AccessibilityNodeInfo.ACTION_CLICK: {
  if (isClickable()) {
  // 最终调用到我们熟悉的View.performClick方法
  performClick();
  return true;
  }
 } break;
 ...
}

分析到这里可以看到，Accessibility服务框架类似于hook在Android View组件树中的一套实现，它并不是独立的一套机制，而是”寄生”在View的显示、事件分发的流程中。

总结

功能实现依赖于ViewRootImpl, ViewGroup, View视图层级管理的基本架构。在视图变化时发出事件、当收到视图操作请求时也能够作出响应。

system_server在实现该功能的过程中扮演着中间人的角色。当被监听APP视图变化时，APP首先会发出事件到system_server，随后再中转到监听者APP端。当监听者APP想要执行视图操作时，也是首先在system_server中找到对应的客户端binder proxy，再调用相应接口调用到被监听APP中。完成相关操作后，通过已经获取到的监听APP binder proxy句柄，直接binder call到对应的监听客户端。

无障碍权限十分重要，切记不可滥用，APP自身也需要有足够的安全意识，防止恶意应用通过该服务获取用户隐私信息

好了，以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对我们的支持。