无障碍服务可以模拟一些用户操作，无障碍可以处理的对象，通过类 AccessibilityNodeInfo 表示，通过无障碍服务，可以通过它的 performAction 方法来触发一些 action ，包括：
ACTION_FOCUS // 获取焦点
ACTION_CLEAR_FOCUS // 清除焦点
ACTION_SELECT // 选中
ACTION_CLEAR_SELECTION // 清除选中状态
ACTION_ACCESSIBILITY_FOCUS // 无障碍焦点
ACTION_CLEAR_ACCESSIBILITY_FOCUS // 清除无障碍焦点
ACTION_CLICK // 点击
ACTION_LONG_CLICK // 长按
ACTION_NEXT_AT_MOVEMENT_GRANULARITY // 下一步移动
ACTION_PREVIOUS_AT_MOVEMENT_GRANULARITY // 上一步移动
ACTION_NEXT_HTML_ELEMENT // 下一个 html 元素
ACTION_PREVIOUS_HTML_ELEMENT // 上一个 html 元素
ACTION_SCROLL_FORWARD // 向前滑动
ACTION_SCROLL_BACKWARD // 向后滑动他们都可以通过performAction方法进行处理：
// in AccessibilityNodeInfo
public boolean performAction(int action) {
enforceSealed();
if (!canPerformRequestOverConnection(mConnectionId, mWindowId, mSourceNodeId)) {
return false;
}
AccessibilityInteractionClient client = AccessibilityInteractionClient.getInstance();
return client.performAccessibilityAction(mConnectionId, mWindowId, mSourceNodeId,
action, null);
}在这个方法中，第一步是检查 perform 是否可以通过 connection 请求，这里 connection 检查是根据通过 binder 通信传递过来的 id 检查连接是否正常。 然后通过AccessibilityInteractionClient对象，调用它的performAccessibilityAction方法去进行实际操作的。
AccessibilityInteractionClient
这个类是一个执行可访问性交互的单例，它可以根据 View 的快照查询远程的 View 层次结构，以及通过 View 层次结构，来请求对 View 执行某项操作。
基本原理：内容检索 API 从客户端的角度来看是同步的，但在内部它们是异步的。客户端线程调用系统请求操作并提供回调以接收结果，然后等待该结果的超时。系统强制执行安全性并将请求委托给给定的视图层次结构， 在该视图层次结构中发布消息（来自 Binder 线程），描述 UI 线程要执行的内容，其结果是通过上述回调传递的。但是，被阻塞的客户端线程和目标视图层次结构的主 UI 线程可以是同一个线程，例如无障碍服务和 Activity 在同一个进程中运行，因此它们在同一个主线程上执行。 在这种情况下，检索将会失败，因为 UI 线程在等待检索结果，会导致阻塞。 为了避免在进行调用时出现这种情况，客户端还会传递其进程和线程 ID，以便访问的视图层次结构可以检测发出请求的客户端是否正在其主 UI 线程中运行。 在这种情况下，视图层次结构，特别是对它执行 IPC 的绑定线程，不会发布要在 UI 线程上运行的消息，而是将其传递给单例交互客户端，通过该客户端发生所有交互，后者负责执行开始等待通过回调传递的异步结果之前的消息。在这种情况下，已经收到预期的结果，因此不执行等待。
上面是官方备注的描述，大概意思最好不要在主线程执行检索操作。
继续跟进它的performAccessibilityAction方法：
public boolean performAccessibilityAction(int connectionId, int accessibilityWindowId,
long accessibilityNodeId, int action, Bundle arguments) {
try {
IAccessibilityServiceConnection connection = getConnection(connectionId);
if (connection != null) {
final int interactionId = mInteractionIdCounter.getAndIncrement();
final long identityToken = Binder.clearCallingIdentity();
final boolean success;
try {
success = connection.performAccessibilityAction(
accessibilityWindowId, accessibilityNodeId, action, arguments,
interactionId, this, Thread.currentThread().getId()); // 【*】
} finally {
Binder.restoreCallingIdentity(identityToken);
}
if (success) {
return getPerformAccessibilityActionResultAndClear(interactionId);
}
}
} catch (RemoteException re) {
Log.w(LOG_TAG, "Error while calling remote performAccessibilityAction", re);
}
return false;
}这里通过 getConnection(connectionId) 获取了一个 IAccessibilityServiceConnection 。
public static IAccessibilityServiceConnection getConnection(int connectionId) {
synchronized (sConnectionCache) {
return sConnectionCache.get(connectionId);
}
}这里的 sConnectionCache 通过 AccessibilityInteractionClient 的addConnection添加数据的，addConnection在 AccessbilityService 创建初始化时调用的：
case DO_INIT: {
mConnectionId = message.arg1;
SomeArgs args = (SomeArgs) message.obj;
IAccessibilityServiceConnection connection = (IAccessibilityServiceConnection) args.arg1;
IBinder windowToken = (IBinder) args.arg2;
args.recycle();
if (connection != null) {
AccessibilityInteractionClient.getInstance(mContext).addConnection(mConnectionId, connection);
mCallback.init(mConnectionId, windowToken);
mCallback.onServiceConnected();
} else {
AccessibilityInteractionClient.getInstance(mContext).removeConnection(mConnectionId);
mConnectionId = AccessibilityInteractionClient.NO_ID;
AccessibilityInteractionClient.getInstance(mContext).clearCache();
mCallback.init(AccessibilityInteractionClient.NO_ID, null);
}
return;
}也就是说，在 AccessbilityService 创建时，会将一个表示连接的对象存到 AccessibilityInteractionClient 的连接缓存中。
IAccessibilityServiceConnection
它是 AccessibilityManagerService 向 AccessbilityService 暴露的 AIDL 接口，提供给 AccessbilityService 调用AccessibilityManagerService 的能力。 上面的 performAction 流程中，调用到了 connection 的performAccessibilityAction方法。 而 IAccessibilityServiceConnection 有两个实现类，AccessibilityServiceConnectionImpl和AbstractAccessibilityServiceConnection，前者都是空实现，显然不是我们要调用到的地方，后者的performAccessibilityAction：
@Override
public boolean performAccessibilityAction(int accessibilityWindowId,
long accessibilityNodeId, int action, Bundle arguments, int interactionId,
IAccessibilityInteractionConnectionCallback callback, long interrogatingTid)
throws RemoteException {
final int resolvedWindowId;
synchronized (mLock) {
if (!hasRightsToCurrentUserLocked()) {
return false;
}
resolvedWindowId = resolveAccessibilityWindowIdLocked(accessibilityWindowId);
if (!mSecurityPolicy.canGetAccessibilityNodeInfoLocked(
mSystemSupport.getCurrentUserIdLocked(), this, resolvedWindowId)) {
return false;
}
}
if (!mSecurityPolicy.checkAccessibilityAccess(this)) {
return false;
}
return performAccessibilityActionInternal(
mSystemSupport.getCurrentUserIdLocked(), resolvedWindowId, accessibilityNodeId,
action, arguments, interactionId, callback, mFetchFlags, interrogatingTid);
}最后的一行调用：
private boolean performAccessibilityActionInternal(int userId, int resolvedWindowId, long accessibilityNodeId, int action, Bundle arguments, int interactionId, IAccessibilityInteractionConnectionCallback callback, int fetchFlags, long interrogatingTid) {
RemoteAccessibilityConnection connection;
IBinder activityToken = null;
// 同步获取 connection
synchronized (mLock) {
connection = mA11yWindowManager.getConnectionLocked(userId, resolvedWindowId);
if (connection == null) {
return false;
}
final boolean isA11yFocusAction = (action == ACTION_ACCESSIBILITY_FOCUS) || (action == ACTION_CLEAR_ACCESSIBILITY_FOCUS);
if (!isA11yFocusAction) {
final WindowInfo windowInfo = mA11yWindowManager.findWindowInfoByIdLocked(resolvedWindowId);
if (windowInfo != null) activityToken = windowInfo.activityToken;
}
final AccessibilityWindowInfo a11yWindowInfo = mA11yWindowManager.findA11yWindowInfoByIdLocked(resolvedWindowId);
if (a11yWindowInfo != null && a11yWindowInfo.isInPictureInPictureMode() && mA11yWindowManager.getPictureInPictureActionReplacingConnection() != null && !isA11yFocusAction) {
connection = mA11yWindowManager.getPictureInPictureActionReplacingConnection();
}
}
// 通过 connection 调用到远程服务的performAccessibilityAction
final int interrogatingPid = Binder.getCallingPid();
final long identityToken = Binder.clearCallingIdentity();
try {
// 无论操作是否成功，它都是由用户操作的无障碍服务生成的，因此请注意用户Activity。
mPowerManager.userActivity(SystemClock.uptimeMillis(), PowerManager.USER_ACTIVITY_EVENT_ACCESSIBILITY, 0);
if (action == ACTION_CLICK || action == ACTION_LONG_CLICK) {
mA11yWindowManager.notifyOutsideTouch(userId, resolvedWindowId);
}
if (activityToken != null) {
LocalServices.getService(ActivityTaskManagerInternal.class).setFocusedActivity(activityToken);
}
connection.getRemote().performAccessibilityAction(accessibilityNodeId, action, arguments, interactionId, callback, fetchFlags, interrogatingPid, interrogatingTid);
} catch (RemoteException re) {
if (DEBUG) {
Slog.e(LOG_TAG, "Error calling performAccessibilityAction: " + re);
}
return false;
} finally {
Binder.restoreCallingIdentity(identityToken);
}
return true;
}在这个方法中，通过 connection 调用到了远端的 performAccessibilityAction 方法。
关键的一行是：
connection.getRemote().performAccessibilityAction(accessibilityNodeId, action, arguments, interactionId, callback, fetchFlags, interrogatingPid, interrogatingTid);这里的 connection 类型定义成了RemoteAccessibilityConnection。
RemoteAccessibilityConnection
RemoteAccessibilityConnection 是AccessibilityWindowManager的内部类，它的getRemote()返回类型是IAccessibilityInteractionConnection。
AccessibilityWindowManager
此类为 AccessibilityManagerService 提供 API 来管理 AccessibilityWindowInfo 和 WindowInfos。
IAccessibilityInteractionConnection
这是一个 AIDL 中定义的接口，用来进行 给定 window 中 AccessibilityManagerService 和 ViewRoot 之间交互的接口。
也就是说getRemote(). performAccessibilityAction(...)最终来到了 ViewRootImpl 中。
AccessibilityInteractionConnection
ViewRootImpl 中存在一个内部类AccessibilityInteractionConnection，它是这个 ViewAncestor 提供给 AccessibilityManagerService 的一个接口，后者可以与这个 ViewAncestor 中的视图层次结构进行交互。
它的performAccessibilityAction实现是：
@Override
public void performAccessibilityAction(long accessibilityNodeId, int action, Bundle arguments, int interactionId, IAccessibilityInteractionConnectionCallback callback, int flags, int interrogatingPid, long interrogatingTid) {
ViewRootImpl viewRootImpl = mViewRootImpl.get();
if (viewRootImpl != null && viewRootImpl.mView != null) {
viewRootImpl.getAccessibilityInteractionController().performAccessibilityActionClientThread(accessibilityNodeId, action, arguments,
interactionId, callback, flags, interrogatingPid, interrogatingTid);
} else {
// We cannot make the call and notify the caller so it does not wait.
try {
callback.setPerformAccessibilityActionResult(false, interactionId);
} catch (RemoteException re) {
}
}
}内部又是通过代理调用 ，ViewRootImpl 的 getAccessibilityInteractionController() 返回了一个 AccessibilityInteractionController 对象。
AccessibilityInteractionController
它的 performAccessibilityActionClientThread :
public void performAccessibilityActionClientThread(long accessibilityNodeId, int action,
Bundle arguments, int interactionId,
IAccessibilityInteractionConnectionCallback callback, int flags, int interrogatingPid,
long interrogatingTid) {
Message message = mHandler.obtainMessage();
message.what = PrivateHandler.MSG_PERFORM_ACCESSIBILITY_ACTION;
message.arg1 = flags;
message.arg2 = AccessibilityNodeInfo.getAccessibilityViewId(accessibilityNodeId);
SomeArgs args = SomeArgs.obtain();
args.argi1 = AccessibilityNodeInfo.getVirtualDescendantId(accessibilityNodeId);
args.argi2 = action;
args.argi3 = interactionId;
args.arg1 = callback;
args.arg2 = arguments;
message.obj = args;
scheduleMessage(message, interrogatingPid, interrogatingTid, CONSIDER_REQUEST_PREPARERS);
}组装了一个 message ，并通过 scheduleMessage 方法去执行：
private void scheduleMessage(Message message, int interrogatingPid, long interrogatingTid, boolean ignoreRequestPreparers) {
if (ignoreRequestPreparers || !holdOffMessageIfNeeded(message, interrogatingPid, interrogatingTid)) {
if (interrogatingPid == mMyProcessId && interrogatingTid == mMyLooperThreadId
&& mHandler.hasAccessibilityCallback(message)) {
AccessibilityInteractionClient.getInstanceForThread(
interrogatingTid).setSameThreadMessage(message);
} else {
if (!mHandler.hasAccessibilityCallback(message) && Thread.currentThread().getId() == mMyLooperThreadId) {
mHandler.handleMessage(message);
} else {
mHandler.sendMessage(message);
}
}
}
}这里实际上，如果是在主线程，则处理消息，如果不是，则发送消息到主线程处理。handler 的类型是 PrivateHandler ，在 AccessibilityInteractionController 内部定义。
它的处理消息方法的实现是：
@Override
public void handleMessage(Message message) {
final int type = message.what;
switch (type) {
// ...
case MSG_PERFORM_ACCESSIBILITY_ACTION: {
performAccessibilityActionUiThread(message);
} break;
// ...
default:
throw new IllegalArgumentException("Unknown message type: " + type);
}
}执行到了 performAccessibilityActionUiThread(message); :
private void performAccessibilityActionUiThread(Message message) {
// ...
boolean succeeded = false;
try {
// ...
final View target = findViewByAccessibilityId(accessibilityViewId);
if (target != null && isShown(target)) {
mA11yManager.notifyPerformingAction(action);
if (action == R.id.accessibilityActionClickOnClickableSpan) {
// 单独处理这个 hidden action
succeeded = handleClickableSpanActionUiThread(target, virtualDescendantId, arguments);
} else {
AccessibilityNodeProvider provider = target.getAccessibilityNodeProvider();
if (provider != null) {
succeeded = provider.performAction(virtualDescendantId, action, arguments);
} else if (virtualDescendantId == AccessibilityNodeProvider.HOST_VIEW_ID) {
succeeded = target.performAccessibilityAction(action, arguments);
}
}
mA11yManager.notifyPerformingAction(0);
}
} finally {
try {
mViewRootImpl.mAttachInfo.mAccessibilityFetchFlags = 0;
callback.setPerformAccessibilityActionResult(succeeded, interactionId);
} catch (RemoteException re) {
}
}
}在这个流程中，分为三种情况去真正执行 performAction ：
1. action == R.id.accessibilityActionClickOnClickableSpan
private boolean handleClickableSpanActionUiThread(
View view, int virtualDescendantId, Bundle arguments) {
Parcelable span = arguments.getParcelable(ACTION_ARGUMENT_ACCESSIBLE_CLICKABLE_SPAN);
if (!(span instanceof AccessibilityClickableSpan)) {
return false;
}
// Find the original ClickableSpan if it's still on the screen
AccessibilityNodeInfo infoWithSpan = null;
AccessibilityNodeProvider provider = view.getAccessibilityNodeProvider();
if (provider != null) {
infoWithSpan = provider.createAccessibilityNodeInfo(virtualDescendantId);
} else if (virtualDescendantId == AccessibilityNodeProvider.HOST_VIEW_ID) {
infoWithSpan = view.createAccessibilityNodeInfo();
}
if (infoWithSpan == null) {
return false;
}
// Click on the corresponding span
ClickableSpan clickableSpan = ((AccessibilityClickableSpan) span).findClickableSpan(
infoWithSpan.getOriginalText());
if (clickableSpan != null) {
clickableSpan.onClick(view);
return true;
}
return false;
}2. View. AccessibilityNodeProvider != null
当能够通过 View 获取到 AccessibilityNodeProvider 对象是，通过它的 performAction 方法，去执行真正的调用，它的真正调用在 AccessibilityNodeProviderCompat中，这个 Compat 的实现在ExploreByTouchHelper中的内部类MyNodeProvider中：
@Override
public boolean performAction(int virtualViewId, int action, Bundle arguments) {
return ExploreByTouchHelper.this.performAction(virtualViewId, action, arguments);
}在 ExploreByTouchHelper 中继续查看：
boolean performAction(int virtualViewId, int action, Bundle arguments) {
switch (virtualViewId) {
case HOST_ID:
return performActionForHost(action, arguments);
default:
return performActionForChild(virtualViewId, action, arguments);
}
}private boolean performActionForHost(int action, Bundle arguments) {
return ViewCompat.performAccessibilityAction(mHost, action, arguments);
}
private boolean performActionForChild(int virtualViewId, int action, Bundle arguments) {
switch (action) {
case AccessibilityNodeInfoCompat.ACTION_ACCESSIBILITY_FOCUS:
return requestAccessibilityFocus(virtualViewId);
case AccessibilityNodeInfoCompat.ACTION_CLEAR_ACCESSIBILITY_FOCUS:
return clearAccessibilityFocus(virtualViewId);
case AccessibilityNodeInfoCompat.ACTION_FOCUS:
return requestKeyboardFocusForVirtualView(virtualViewId);
case AccessibilityNodeInfoCompat.ACTION_CLEAR_FOCUS:
return clearKeyboardFocusForVirtualView(virtualViewId);
default:
return onPerformActionForVirtualView(virtualViewId, action, arguments);
}
}前者调用到了 ViewCompat :
public static boolean performAccessibilityAction(@NonNull View view, int action,
Bundle arguments) {
if (Build.VERSION.SDK_INT >= 16) {
return view.performAccessibilityAction(action, arguments);
}
return false;
}然后是 View 的 ：
public boolean performAccessibilityAction(int action, Bundle arguments) {
if (mAccessibilityDelegate != null) {
return mAccessibilityDelegate.performAccessibilityAction(this, action, arguments);
} else {
return performAccessibilityActionInternal(action, arguments);
}
}mAccessibilityDelegate.performAccessibilityAction的实现是：
public boolean performAccessibilityAction(View host, int action, Bundle args) {
return host.performAccessibilityActionInternal(action, args);
}也是调用到了 View 的performAccessibilityActionInternal 。 performAccessibilityActionInternal 的实现是：
// in View.java
public boolean performAccessibilityActionInternal(int action, Bundle arguments) {
if (isNestedScrollingEnabled()
&& (action == AccessibilityNodeInfo.ACTION_SCROLL_BACKWARD
|| action == AccessibilityNodeInfo.ACTION_SCROLL_FORWARD
|| action == R.id.accessibilityActionScrollUp
|| action == R.id.accessibilityActionScrollLeft
|| action == R.id.accessibilityActionScrollDown
|| action == R.id.accessibilityActionScrollRight)) {
if (dispatchNestedPrePerformAccessibilityAction(action, arguments)) {
return true;
}
}
switch (action) {
case AccessibilityNodeInfo.ACTION_CLICK: {
if (isClickable()) {
performClickInternal();
return true;
}
} break;
case AccessibilityNodeInfo.ACTION_LONG_CLICK: {
if (isLongClickable()) {
performLongClick();
return true;
}
} break;
// ...
}
return false;
}以 AccessibilityNodeInfo.ACTION_CLICK 为例，内部调用是：
private boolean performClickInternal() {
// Must notify autofill manager before performing the click actions to avoid scenarios where
// the app has a click listener that changes the state of views the autofill service might
// be interested on.
notifyAutofillManagerOnClick();
return performClick();
}这样就调用到了 View 的点击事件。
3. View. AccessibilityNodeProvider == null && virtualDescendantId == AccessibilityNodeProvider.HOST_VIEW_ID
target.performAccessibilityAction(action, arguments);这里 target 是个 View， 也是走的 View 的 performAccessibilityAction ，和上面流程一样。
View 的 performClick 方法是同步的还是异步的？
public boolean performClick() {
// We still need to call this method to handle the cases where performClick() was called
// externally, instead of through performClickInternal()
notifyAutofillManagerOnClick();
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);
notifyEnterOrExitForAutoFillIfNeeded(true);
return result;
}同步的。
总结
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