可能大家看到这个标题不知道是甚么东西，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1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。
1、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获得最前面的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中获得最前面的Activity。如果没有再遍历所有的mStacks获得。
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1次mLockTaskModeTasks为空，会发送1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 == ⑴) {
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;
2、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函数来判断是不是进1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1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，默许就是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函数以下代码，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会被禁止。
case LOCK_TASK_AUTH_PINNABLE:
// Pinnable tasks can't be launched on top of locktask tasks.
return !mLockTaskModeTasks.isEmpty();
3、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1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1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获得当前固定屏幕的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（1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，由于mLockTaskModeTasks已为空了。
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;
}
4、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.另外一种我们可以在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文件然后加入相干perference，这其中就会有以下是屏幕固定相干的
<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读取该文件以后，调用WMS的hasNavigationBar来看有无虚拟键（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.");
}