Android6.0 固定屏幕功能实现方法及实例

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."); 
} 

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