Android 固定屏幕功能
可能大家看到這個標題不知道是什么東西，我先說明下，android6.0在設置->安全->屏幕固定開啟后，然后再長按home鍵出現最近的幾個Activity可以選擇一個圖釘按鈕就開啟了屏幕固定功能。
屏幕固定開啟后，屏幕只能固定在設定的Task上的Activity切換。
一、設置固定屏幕
我們先來看SystemUI/src/com/Android/systemui/recents/ScreenPinningRequest.Java的代碼，這段代碼就是長按home鍵出現幾個Activity，然后按了圖釘的那個按鈕。在這里直接調用了AMS的startLockTaskModeOnCurrent函數。
@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(); } 我們來看AMS的startLockTaskModeOnCurrent函數，先調用ActivityStackSupervisor的topRunningActivityLocked獲取最前面的Activity，然后調用startLockTaskModeLocked函數，參數是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); } } 我們再來看topRunningActivityLocked函數，先從mFocusedStack中獲取最前面的Activity。如果沒有再遍歷所有的mStacks獲取。
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函數中主要是調用了ActivityStackSupervisor的setLockTaskModeLocked函數，下面我們來看這個函數，我們的task不為null，第一次mLockTaskModeTasks為空，會發送一個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;//發送消息 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();//顯示新的Activity } } 我們再來看消息處理，在消息處理中主要調用了WMS的disableKeyguard函數。
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啟動流程
在固定屏幕后，如果我們啟動其他TaskRecord的Activity是不能啟動的，我們來看下這個原理。在startActivityUncheckedLocked函數中會調用isLockTaskModeViolation函數來判斷是否進一步的Activity的啟動流程，我們來看下這個函數，調用getLockedTaskLocked來看mLockTaskModeTasks（就是鎖定屏幕的那些Task），如果當前的task就是當前正在固定屏幕的task，直接return false就是可以繼續啟動Activity的流程，而如果不是，我們需要看task的mLockTaskAuth變量。
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; } } 我們再來看TaskRecord的setLockedTaskAuth函數，在新建一個TaskRecord的時候會調用setIntent函數，而setIntent函數又是在TaskRecord的構造函數中調用的。我們來看這個函數mLockTaskAuth的值是根據mLockTaskMode來定的，而mLockTaskMode又是ActivityInfo傳入的，這個值是在PKMS解析AndroidManifest.xml的時候構造的，默認就是LOCK_TASK_LAUNCH_MODE_DEFAULT，而當沒有白名單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()); } 我們再來看isLockTaskModeViolation函數如下代碼，現在是task的mLockTaskAuth 是LOCK_TASK_AUTH_PINNABLE，而當前處于固定屏幕，所以mLockTaskModeTasks不為null，最后返回true。那Activity啟動流程就不能走下去了，那就是代表啟動普通的Activity會被阻止。
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
最后我們再來看看取消固定屏幕，取消屏幕會在PhoneStatusBar中取消，但是一定是要有虛擬鍵，原生就是這么設定的。最后調用了AMS的stopLockTaskModeOnCurrent函數。這個函數主要是調用了stopLockTaskMode函數，這個函數中主要是調用了ActivityStackSupervisor的setLockTaskModeLocked函數，之前在固定屏幕時也是調用了這個函數，但是這里我們仔細看，其第一個參數為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); } } 我們來看下這個函數，如果為空，現在調用getLockedTaskLocked獲取當前固定屏幕的TaskRecord，然后調用removeLockedTaskLocked去除這個TaskRecord，如果還不為null，調用resumeTopActivitiesLocked啟動下個Activity（一般也就是下個屏幕鎖定的TaskRecord的Activity）。
如果為空了，直接返回。但是在我們下次啟動普通的Activity的時候就恢復正常了，因為mLockTaskModeTasks已經為空了。
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、沒有虛擬鍵如何取消屏幕固定
前面說過如果沒有虛擬鍵就不能取消屏幕固定了，我們說下幾種方式
1.使用am命令 am task lock stop可以調用am的stopLockTaskMode函數
2.另一種我們可以在Activity.java中修改代碼，比較長按返回鍵調用AMS的stopLockTaskMode方法，下面就是實現，Activity本身提供了stopLockTask就是調用了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中對這項進行置灰處理
在SecuritySettings會讀取security_settings_misc.xml文件然后加入相關perference，這其中就會有如下是屏幕固定相關的
<PreferenceScreen android:key="screen_pinning_settings" android:title="@string/screen_pinning_title" android:summary="@string/switch_off_text" android:fragment="com.android.settings.ScreenPinningSettings"/>
我們可以在SecuritySettings讀取該文件之后，調用WMS的hasNavigationBar來看有沒有虛擬鍵（沒有虛擬按鍵到時候不能取消屏幕固定），如果沒有直接把Settings中這項置灰。
// 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."); } 感謝閱讀，希望能幫助到大家，謝謝大家對本站的支持！
新聞熱點
疑難解答
