Android 固定屏幕功能
可能大家看到这个标题不知道是什么东西，我先说明下，android6.0在设置->安全->屏幕固定开启后，然后再长按home键出现最近的几个Activity可以选择一个图钉按钮就开启了屏幕固定功能。
屏幕固定开启后，屏幕只能固定在设定的Task上的Activity切换。
一、设置固定屏幕
我们先来看SystemUI/src/com/Android/systemui/recents/ScreenPinningRequest.Java的代码，这段代码就是长按home键出现几个Activity，然后按了图钉的那个按钮。在这里直接调用了AMS的startLockTaskModeOnCurrent函数。
@Override public void onClick(View v) { if (v.getId() == R.id.screen_pinning_ok_button || mRequestWindow == v) { try { ActivityManagerNative.getDefault().startLockTaskModeOnCurrent(); } catch (RemoteException e) {} } clearPrompt(); } 我们来看AMS的startLockTaskModeOnCurrent函数，先调用ActivityStackSupervisor的topRunningActivityLocked获取最前面的Activity，然后调用startLockTaskModeLocked函数，参数是TaskRecord。
public void startLockTaskModeOnCurrent() throws RemoteException { enforceCallingPermission(android.Manifest.permission.MANAGE_ACTIVITY_STACKS, "startLockTaskModeOnCurrent"); long ident = Binder.clearCallingIdentity(); try { synchronized (this) { ActivityRecord r = mStackSupervisor.topRunningActivityLocked(); if (r != null) { startLockTaskModeLocked(r.task); } } } finally { Binder.restoreCallingIdentity(ident); } } 我们再来看topRunningActivityLocked函数，先从mFocusedStack中获取最前面的Activity。如果没有再遍历所有的mStacks获取。
ActivityRecord topRunningActivityLocked() { final ActivityStack focusedStack = mFocusedStack; ActivityRecord r = focusedStack.topRunningActivityLocked(null); if (r != null) { return r; } // Return to the home stack. final ArrayList<ActivityStack> stacks = mHomeStack.mStacks; for (int stackNdx = stacks.size() - 1; stackNdx >= 0; --stackNdx) { final ActivityStack stack = stacks.get(stackNdx); if (stack != focusedStack && isFrontStack(stack)) { r = stack.topRunningActivityLocked(null); if (r != null) { return r; } } } return null; } 在startLockTaskModeLocked函数中主要是调用了ActivityStackSupervisor的setLockTaskModeLocked函数，下面我们来看这个函数，我们的task不为null，第一次mLockTaskModeTasks为空，会发送一个LOCK_TASK_START_MSG消息
void setLockTaskModeLocked(TaskRecord task, int lockTaskModeState, String reason, boolean andResume) { if (task == null) { // Take out of lock task mode if necessary final TaskRecord lockedTask = getLockedTaskLocked(); if (lockedTask != null) { removeLockedTaskLocked(lockedTask); if (!mLockTaskModeTasks.isEmpty()) { // There are locked tasks remaining, can only finish this task, not unlock it. if (DEBUG_LOCKTASK) Slog.w(TAG_LOCKTASK, "setLockTaskModeLocked: Tasks remaining, can't unlock"); lockedTask.performClearTaskLocked(); resumeTopActivitiesLocked(); return; } } if (DEBUG_LOCKTASK) Slog.w(TAG_LOCKTASK, "setLockTaskModeLocked: No tasks to unlock. Callers=" + Debug.getCallers(4)); return; } // Should have already been checked, but do it again. if (task.mLockTaskAuth == LOCK_TASK_AUTH_DONT_LOCK) { if (DEBUG_LOCKTASK) Slog.w(TAG_LOCKTASK, "setLockTaskModeLocked: Can't lock due to auth"); return; } if (isLockTaskModeViolation(task)) { Slog.e(TAG_LOCKTASK, "setLockTaskMode: Attempt to start an unauthorized lock task."); return; } if (mLockTaskModeTasks.isEmpty()) { // First locktask. final Message lockTaskMsg = Message.obtain(); lockTaskMsg.obj = task.intent.getComponent().getPackageName(); lockTaskMsg.arg1 = task.userId; lockTaskMsg.what = LOCK_TASK_START_MSG;//发送消息 lockTaskMsg.arg2 = lockTaskModeState; mHandler.sendMessage(lockTaskMsg); } // Add it or move it to the top. if (DEBUG_LOCKTASK) Slog.w(TAG_LOCKTASK, "setLockTaskModeLocked: Locking to " + task + " Callers=" + Debug.getCallers(4)); mLockTaskModeTasks.remove(task); mLockTaskModeTasks.add(task);//加入到mLockModeTasks中 if (task.mLockTaskUid == -1) { task.mLockTaskUid = task.effectiveUid; } if (andResume) { findTaskToMoveToFrontLocked(task, 0, null, reason);//把task放最前面 resumeTopActivitiesLocked();//显示新的Activity } } 我们再来看消息处理，在消息处理中主要调用了WMS的disableKeyguard函数。
case LOCK_TASK_START_MSG: { // When lock task starts, we disable the status bars. try { if (mLockTaskNotify == null) { mLockTaskNotify = new LockTaskNotify(mService.mContext); } mLockTaskNotify.show(true); mLockTaskModeState = msg.arg2; if (getStatusBarService() != null) { int flags = 0; if (mLockTaskModeState == LOCK_TASK_MODE_LOCKED) { flags = StatusBarManager.DISABLE_MASK & (~StatusBarManager.DISABLE_BACK); } else if (mLockTaskModeState == LOCK_TASK_MODE_PINNED) { flags = StatusBarManager.DISABLE_MASK & (~StatusBarManager.DISABLE_BACK) & (~StatusBarManager.DISABLE_HOME) & (~StatusBarManager.DISABLE_RECENT); } getStatusBarService().disable(flags, mToken, mService.mContext.getPackageName()); } mWindowManager.disableKeyguard(mToken, LOCK_TASK_TAG); if (getDevicePolicyManager() != null) { getDevicePolicyManager().notifyLockTaskModeChanged(true, (String)msg.obj, msg.arg1); } } catch (RemoteException ex) { throw new RuntimeException(ex); } } break; 二、固定屏幕后Activity启动流程
在固定屏幕后，如果我们启动其他TaskRecord的Activity是不能启动的，我们来看下这个原理。在startActivityUncheckedLocked函数中会调用isLockTaskModeViolation函数来判断是否进一步的Activity的启动流程，我们来看下这个函数，调用getLockedTaskLocked来看mLockTaskModeTasks（就是锁定屏幕的那些Task），如果当前的task就是当前正在固定屏幕的task，直接return false就是可以继续启动Activity的流程，而如果不是，我们需要看task的mLockTaskAuth变量。
boolean isLockTaskModeViolation(TaskRecord task, boolean isNewClearTask) { if (getLockedTaskLocked() == task && !isNewClearTask) { return false; } final int lockTaskAuth = task.mLockTaskAuth; switch (lockTaskAuth) { case LOCK_TASK_AUTH_DONT_LOCK: return !mLockTaskModeTasks.isEmpty(); case LOCK_TASK_AUTH_LAUNCHABLE_PRIV: case LOCK_TASK_AUTH_LAUNCHABLE: case LOCK_TASK_AUTH_WHITELISTED: return false; case LOCK_TASK_AUTH_PINNABLE: // Pinnable tasks can't be launched on top of locktask tasks. return !mLockTaskModeTasks.isEmpty(); default: Slog.w(TAG, "isLockTaskModeViolation: invalid lockTaskAuth value=" + lockTaskAuth); return true; } } 我们再来看TaskRecord的setLockedTaskAuth函数，在新建一个TaskRecord的时候会调用setIntent函数，而setIntent函数又是在TaskRecord的构造函数中调用的。我们来看这个函数mLockTaskAuth的值是根据mLockTaskMode来定的，而mLockTaskMode又是ActivityInfo传入的，这个值是在PKMS解析AndroidManifest.xml的时候构造的，默认就是LOCK_TASK_LAUNCH_MODE_DEFAULT，而当没有白名单mLockTaskAuth最后就是LOCK_TASK_AUTH_PINNABLE。
void setLockTaskAuth() { if (!mPrivileged && (mLockTaskMode == LOCK_TASK_LAUNCH_MODE_ALWAYS || mLockTaskMode == LOCK_TASK_LAUNCH_MODE_NEVER)) { // Non-priv apps are not allowed to use always or never, fall back to default mLockTaskMode = LOCK_TASK_LAUNCH_MODE_DEFAULT; } switch (mLockTaskMode) { case LOCK_TASK_LAUNCH_MODE_DEFAULT: mLockTaskAuth = isLockTaskWhitelistedLocked() ? LOCK_TASK_AUTH_WHITELISTED : LOCK_TASK_AUTH_PINNABLE; break; case LOCK_TASK_LAUNCH_MODE_NEVER: mLockTaskAuth = LOCK_TASK_AUTH_DONT_LOCK; break; case LOCK_TASK_LAUNCH_MODE_ALWAYS: mLockTaskAuth = LOCK_TASK_AUTH_LAUNCHABLE_PRIV; break; case LOCK_TASK_LAUNCH_MODE_IF_WHITELISTED: mLockTaskAuth = isLockTaskWhitelistedLocked() ? LOCK_TASK_AUTH_LAUNCHABLE : LOCK_TASK_AUTH_PINNABLE; break; } if (DEBUG_LOCKTASK) Slog.d(TAG_LOCKTASK, "setLockTaskAuth: task=" + this + " mLockTaskAuth=" + lockTaskAuthToString()); } 我们再来看isLockTaskModeViolation函数如下代码，现在是task的mLockTaskAuth 是LOCK_TASK_AUTH_PINNABLE，而当前处于固定屏幕，所以mLockTaskModeTasks不为null，最后返回true。那Activity启动流程就不能走下去了，那就是代表启动普通的Activity会被阻止。
case LOCK_TASK_AUTH_PINNABLE: // Pinnable tasks can't be launched on top of locktask tasks. return !mLockTaskModeTasks.isEmpty();
三、取消固定屏幕
最后我们再来看看取消固定屏幕，取消屏幕会在PhoneStatusBar中取消，但是一定是要有虚拟键，原生就是这么设定的。最后调用了AMS的stopLockTaskModeOnCurrent函数。这个函数主要是调用了stopLockTaskMode函数，这个函数中主要是调用了ActivityStackSupervisor的setLockTaskModeLocked函数，之前在固定屏幕时也是调用了这个函数，但是这里我们仔细看，其第一个参数为null。
public void stopLockTaskMode() { final TaskRecord lockTask = mStackSupervisor.getLockedTaskLocked(); if (lockTask == null) { // Our work here is done. return; } final int callingUid = Binder.getCallingUid(); final int lockTaskUid = lockTask.mLockTaskUid; // Ensure the same caller for startLockTaskMode and stopLockTaskMode. // It is possible lockTaskMode was started by the system process because // android:lockTaskMode is set to a locking value in the application manifest instead of // the app calling startLockTaskMode. In this case {@link TaskRecord.mLockTaskUid} will // be 0, so we compare the callingUid to the {@link TaskRecord.effectiveUid} instead. if (getLockTaskModeState() == ActivityManager.LOCK_TASK_MODE_LOCKED && callingUid != lockTaskUid && (lockTaskUid != 0 || (lockTaskUid == 0 && callingUid != lockTask.effectiveUid))) { throw new SecurityException("Invalid uid, expected " + lockTaskUid + " callingUid=" + callingUid + " effectiveUid=" + lockTask.effectiveUid); } long ident = Binder.clearCallingIdentity(); try { Log.d(TAG, "stopLockTaskMode"); // Stop lock task synchronized (this) { mStackSupervisor.setLockTaskModeLocked(null, ActivityManager.LOCK_TASK_MODE_NONE, "stopLockTask", true); } } finally { Binder.restoreCallingIdentity(ident); } } 我们来看下这个函数，如果为空，现在调用getLockedTaskLocked获取当前固定屏幕的TaskRecord，然后调用removeLockedTaskLocked去除这个TaskRecord，如果还不为null，调用resumeTopActivitiesLocked启动下个Activity（一般也就是下个屏幕锁定的TaskRecord的Activity）。
如果为空了，直接返回。但是在我们下次启动普通的Activity的时候就恢复正常了，因为mLockTaskModeTasks已经为空了。
void setLockTaskModeLocked(TaskRecord task, int lockTaskModeState, String reason, boolean andResume) { if (task == null) { // Take out of lock task mode if necessary final TaskRecord lockedTask = getLockedTaskLocked(); if (lockedTask != null) { removeLockedTaskLocked(lockedTask); if (!mLockTaskModeTasks.isEmpty()) { // There are locked tasks remaining, can only finish this task, not unlock it. if (DEBUG_LOCKTASK) Slog.w(TAG_LOCKTASK, "setLockTaskModeLocked: Tasks remaining, can't unlock"); lockedTask.performClearTaskLocked(); resumeTopActivitiesLocked(); return; } } if (DEBUG_LOCKTASK) Slog.w(TAG_LOCKTASK, "setLockTaskModeLocked: No tasks to unlock. Callers=" + Debug.getCallers(4)); return; } 四、没有虚拟键如何取消屏幕固定
前面说过如果没有虚拟键就不能取消屏幕固定了，我们说下几种方式
1.使用am命令 am task lock stop可以调用am的stopLockTaskMode函数
2.另一种我们可以在Activity.java中修改代码，比较长按返回键调用AMS的stopLockTaskMode方法，下面就是实现，Activity本身提供了stopLockTask就是调用了AMS的stopLockTaskMode方法
public boolean onKeyLongPress(int keyCode, KeyEvent event) { if (keyCode == KeyEvent.KEYCODE_BACK) { stopLockTask(); } return false; } 3.直接在Settings中对这项进行置灰处理
在SecuritySettings会读取security_settings_misc.xml文件然后加入相关perference，这其中就会有如下是屏幕固定相关的
<PreferenceScreen android:key="screen_pinning_settings" android:title="@string/screen_pinning_title" android:summary="@string/switch_off_text" android:fragment="com.android.settings.ScreenPinningSettings"/>
我们可以在SecuritySettings读取该文件之后，调用WMS的hasNavigationBar来看有没有虚拟键（没有虚拟按键到时候不能取消屏幕固定），如果没有直接把Settings中这项置灰。
// Append the rest of the settings addPreferencesFromResource(R.xml.security_settings_misc); IWindowManager windowManager = WindowManagerGlobal.getWindowManagerService(); try { boolean is_screen_pining = windowManager.hasNavigationBar(); root.findPreference(KEY_SCREEN_PINNING).setEnabled(is_screen_pining); } catch(RemoteException e) { Log.e("SecuritySettings", "get window service remoteException."); } 感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！
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