Android如何在应用层进行截屏及截屏源码分析（上）
最近在看framework层代码时发现其中有一个是测试截屏操作的专门的包，于是潜意识的驱使下就研究了这方面的知识，今天作个总结吧！以及我们在写上层应用时如何做截屏操作的，那么我们先来看看截屏的源码分析，其实截屏操作就java这部分是放在了系统SystemUI那里，用过android系统手机的同学应该都知道，一般的android手机按下音量减少键和电源按键就会触发截屏事件（国内定制机做个修改的这里就不做考虑了）
我们知道这里的截屏事件是通过我们的按键操作触发的，所以这里就需要我们从android系统的按键触发模块开始看起，由于我们在不同的App页面，操作音量减少键和电源键都会触发系统的截屏处理，所以这里的按键触发逻辑应该是Android系统的全局按键处理逻辑。
在android系统中，由于我们的每一个Android界面都是一个Activity，而界面的显示都是通过Window对象实现的，每个Window对象实际上都是PhoneWindow的实例，而每个PhoneWindow对象都一个PhoneWindowManager对象，当我们在Activity界面执行按键操作的时候，在将按键的处理操作分发到App之前，首先会回调PhoneWindowManager中的dispatchUnhandledKey方法，按键分发处理，见名知意该方法主要用于执行当前App处理按键之前的操作，PhoneWindowManager所在包如下图所示：
那么在我们看该方法是怎么实现时先来看看这个方法在哪被调用吧，
ctrl+Shift+G可以发现PhoneWindowManager的dispatchUnhandledKey方法在InputManagerService的dispatchUnhandledKey执行，而InputManagerService.dispatchUnhandledKey是一个Native callback.，学过NDK的人都知道这个方法看来是通过JNI回调了，即是硬件驱动屏检测到按键输入再包装到库层通过C++实现，再C++那边调用了该方法传回一些参数然后传给我们上层操作。
在InputManagerService.dispatchUnhandledKey方法中通过一个mWindowManagerCallbacks实现，那么我们再看看mWindowManagerCallbacks吧，该mWindowManagerCallbacks是InputManagerService的内部接口
该接口的实例是被作为观察者模式传进来的，在SystemServer中传入
而InputManagerService又作为Android的一个服务被添加到SystemServer中，如果对这方面不是很了解的同学，请移步至Android开发如何定制framework层服务 作个具体的了解。
看上图可知mWindowManagerCallbacks最终是通过WindowManagerService.getInputMonitor()得到的，我们去WindowManagerService这个服务里面看看这个getInputMonitor方法：
该方法返回一个InputMonitor，我们再进去InputMonitor看看什么情况：
该InputMonitor实现了InputManagerService.WindowManagerCallbacks这个接口，dispatchUnhandledKey方法如下：
看图可知dispatchUnhandledKey该方法最后还是通过WindowManagerService这个Android服务来实现的，对Android而言所有的UI都是通过WindowManagerService这个服务去操作；
接下来我们再继续看一下具体该方法的实现。
/** {@inheritDoc} */
@Override
public KeyEvent dispatchUnhandledKey(WindowState win, KeyEvent event, int policyFlags) {
// Note: This method is only called if the initial down was unhandled.
if (DEBUG_INPUT) {
Slog.d(TAG, "Unhandled key: win=" + win + ", action=" + event.getAction()
+ ", flags=" + event.getFlags()
+ ", keyCode=" + event.getKeyCode()
+ ", scanCode=" + event.getScanCode()
+ ", metaState=" + event.getMetaState()
+ ", repeatCount=" + event.getRepeatCount()
+ ", policyFlags=" + policyFlags);
}
KeyEvent fallbackEvent = null;
if ((event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
final KeyCharacterMap kcm = event.getKeyCharacterMap();
final int keyCode = event.getKeyCode();
final int metaState = event.getMetaState();
final boolean initialDown = event.getAction() == KeyEvent.ACTION_DOWN
&& event.getRepeatCount() == 0;
// Check for fallback actions specified by the key character map.
final FallbackAction fallbackAction;
if (initialDown) {
fallbackAction = kcm.getFallbackAction(keyCode, metaState);
} else {
fallbackAction = mFallbackActions.get(keyCode);
}
if (fallbackAction != null) {
if (DEBUG_INPUT) {
Slog.d(TAG, "Fallback: keyCode=" + fallbackAction.keyCode
+ " metaState=" + Integer.toHexString(fallbackAction.metaState));
}
final int flags = event.getFlags() | KeyEvent.FLAG_FALLBACK;
fallbackEvent = KeyEvent.obtain(
event.getDownTime(), event.getEventTime(),
event.getAction(), fallbackAction.keyCode,
该方法主要是包装了一下KeyEvent event，关键代码如下：
记住这个红色框框部分，该部分再下面要说到，那么这里将KeyEvent 放到了interceptFallback这个方法中处理了，我们再进去这个interceptFallback里面看看吧，如下：
然后我们看到在interceptFallback方法中我们调用了interceptKeyBeforeQueueing方法，通过阅读我们我们知道该方法主要实现了对截屏按键的处理流程，这样我们继续看一下interceptKeyBeforeWueueing方法的处理，该方法比较长：
/** {@inheritDoc} */
@Override
public int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags, boolean isScreenOn) {
if (!mSystemBooted) {
// If we have not yet booted, don't let key events do anything.
return 0;
}
final boolean down = event.getAction() == KeyEvent.ACTION_DOWN;
final boolean canceled = event.isCanceled();
final int keyCode = event.getKeyCode();
final boolean isInjected = (policyFlags & WindowManagerPolicy.FLAG_INJECTED) != 0;
// If screen is off then we treat the case where the keyguard is open but hidden
// the same as if it were open and in front.
// This will prevent any keys other than the power button from waking the screen
// when the keyguard is hidden by another activity.
final boolean keyguardActive = (mKeyguardMediator == null ? false :
(isScreenOn ?
mKeyguardMediator.isShowingAndNotHidden() :
mKeyguardMediator.isShowing()));
if (keyCode == KeyEvent.KEYCODE_POWER) {
policyFlags |= WindowManagerPolicy.FLAG_WAKE;
}
final boolean isWakeKey = (policyFlags & (WindowManagerPolicy.FLAG_WAKE
| WindowManagerPolicy.FLAG_WAKE_DROPPED)) != 0;
if (DEBUG_INPUT) {
Log.d(TAG, "interceptKeyTq keycode=" + keyCode
+ " screenIsOn=" + isScreenOn + " keyguardActive=" + keyguardActive
+ " policyFlags=" + Integer.toHexString(policyFlags)
+ " isWakeKey=" + isWakeKey);
}
if (down && (policyFlags & WindowManagerPolicy.FLAG_VIRTUAL) != 0
&& event.getRepeatCount() == 0) {
performHapticFeedbackLw(null, HapticFeedbackConstants.VIRTUAL_KEY, false);
}
// Basic policy basedn screen state and keyguard.
// FIXME: This policy isn't quite correct. We shouldn't care whether the screen
// is on or off, really. We should care about whether the device is in an
// interactive state or is in suspend pretending to be "off".
// The primary screen might be turned off due to proximity sensor or
// because we are presenting media on an auxiliary screen or remotely controlling
// the device some other way (which is why we have an exemption here for injected
// events).
int result;
if ((isScreenOn && !mHeadless) || (isInjected && !isWakeKey)) {
// When the screen is on or if the key is injected pass the key to the application.
result = ACTION_PASS_TO_USER;
} else {
// When the screen is off and the key is not injected, determine whether
// to wake the device but don't pass the key to the application.
result = 0;
if (down && isWakeKey && isWakeKeyWhenScreenOff(keyCode)) {
if (keyguardActive) {
// If the keyguard is showing, let it wake the device when ready.
mKeyguardMediator.onWakeKeyWhenKeyguardShowingTq(keyCode);
} else {
// Otherwise, wake the device ourselves.
result |= ACTION_WAKE_UP;
}
}
}
// Handle special keys. switch (keyCode) {
case KeyEvent.KEYCODE_VOLUME_DOWN:
case KeyEvent.KEYCODE_VOLUME_UP:
case KeyEvent.KEYCODE_VOLUME_MUTE: {
if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) {
if (down) {
if (isScreenOn && !mVolumeDownKeyTriggered
&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
mVolumeDownKeyTriggered = true;
mVolumeDownKeyTime = event.getDownTime();
mVolumeDownKeyConsumedByScreenshotChord = false;
cancelPendingPowerKeyAction();
interceptScreenshotChord();
}
} else {
mVolumeDownKeyTriggered = false;
cancelPendingScreenshotChordAction();
}
} else if (keyCode == KeyEvent.KEYCODE_VOLUME_UP) {
if (down) {
if (isScreenOn && !mVolumeUpKeyTriggered
&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
mVolumeUpKeyTriggered = true;
cancelPendingPowerKeyAction();
cancelPendingScreenshotChordAction();
}
} else {
mVolumeUpKeyTriggered = false;
cancelPendingScreenshotChordAction();
}
}
if (down) { ITelephony telephonyService = getTelephonyService();
if (telephonyService != null) {
try {
if (telephonyService.isRinging()) {
// If an incoming call is ringing, either VOLUME key means
// "silence ringer". We handle these keys here, rather than
// in the InCallScreen, to make sure we'll respond to them
// even if the InCallScreen hasn't come to the foreground yet.
// Look for the DOWN event here, to agree with the "fallback"
// behavior in the InCallScreen.
Log.i(TAG, "interceptKeyBeforeQueueing:"
+ " VOLUME key-down while ringing: Silence ringer!");
// Silence the ringer. (It's safe to call this
// even if the ringer has already been silenced.)
telephonyService.silenceRinger();
// And *don't* pass this key thru to the current activity
// (which is probably the InCallScreen.)
result &= ~ACTION_PASS_TO_USER;
break;
}
if (telephonyService.isOffhook()
&& (result & ACTION_PASS_TO_USER) == 0) {
// If we are in call but we decided not to pass the key to
// the application, handle the volume change here.
handleVolumeKey(AudioManager.STREAM_VOICE_CALL, keyCode);
break;
}
} catch (RemoteException ex) {
Log.w(TAG, "ITelephony threw RemoteException", ex);
}
}
if (isMusicActive() && (result & ACTION_PASS_TO_USER) == 0) {
// If music is playing but we decided not to pass the key to the
// application, handle the volume change here.
handleVolumeKey(AudioManager.STREAM_MUSIC, keyCode);
break;
}
}
break;
}
case KeyEvent.KEYCODE_ENDCALL: { result &= ~ACTION_PASS_TO_USER;
if (down) {
ITelephony telephonyService = getTelephonyService();
boolean hungUp = false;
if (telephonyService != null) {
try {
hungUp = telephonyService.endCall();
} catch (RemoteException ex) {
Log.w(TAG, "ITelephony threw RemoteException", ex);
}
}
interceptPowerKeyDown(!isScreenOn || hungUp);
} else {
if (interceptPowerKeyUp(canceled)) {
if ((mEndcallBehavior
& Settings.System.END_BUTTON_BEHAVIOR_HOME) != 0) {
if (goHome()) {
break;
}
}
if ((mEndcallBehavior
& Settings.System.END_BUTTON_BEHAVIOR_SLEEP) != 0) {
result = (result & ~ACTION_WAKE_UP) | ACTION_GO_TO_SLEEP;
}
}
}
break;
}
case KeyEvent.KEYCODE_POWER: { result &= ~ACTION_PASS_TO_USER;
if (down) {
if (isScreenOn && !mPowerKeyTriggered
&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
mPowerKeyTriggered = true;
mPowerKeyTime = event.getDownTime();
interceptScreenshotChord();
}
ITelephony telephonyService = getTelephonyService();
boolean hungUp = false;
if (telephonyService != null) {
try {
if (telephonyService.isRinging()) {
// Pressing Power while there's a ringing incoming
// call should silence the ringer.
telephonyService.silenceRinger();
} else if ((mIncallPowerBehavior
& Settings.Secure.INCALL_POWER_BUTTON_BEHAVIOR_HANGUP) != 0
&& telephonyService.isOffhook()) {
// Otherwise, if "Power button ends call" is enabled,
// the Power button will hang up any current active call.
hungUp = telephonyService.endCall();
}
} catch (RemoteException ex) {
Log.w(TAG, "ITelephony threw RemoteException", ex);
}
}
interceptPowerKeyDown(!isScreenOn || hungUp|| mVolumeDownKeyTriggered || mVolumeUpKeyTriggered);
} else {
mPowerKeyTriggered = false;
cancelPendingScreenshotChordAction();
if (interceptPowerKeyUp(canceled || mPendingPowerKeyUpCanceled)) {
result = (result & ~ACTION_WAKE_UP) | ACTION_GO_TO_SLEEP;
}
mPendingPowerKeyUpCanceled = false;
}
break;
}
case KeyEvent.KEYCODE_MEDIA_PLAY:
case KeyEvent.KEYCODE_MEDIA_PAUSE:
case KeyEvent.KEYCODE_MEDIA_PLAY_PAUSE:if (down) {
ITelephony telephonyService = getTelephonyService();
if (telephonyService != null) {
try {
if (!telephonyService.isIdle()) {
// Suppress PLAY/PAUSE toggle when phone is ringing or in-call
// to avoid music playback.
break;
}
} catch (RemoteException ex) {
Log.w(TAG, "ITelephony threw RemoteException", ex);
}
}
}
case KeyEvent.KEYCODE_HEADSETHOOK:
case KeyEvent.KEYCODE_MUTE:
case KeyEvent.KEYCODE_MEDIA_STOP:
case KeyEvent.KEYCODE_MEDIA_NEXT:
case KeyEvent.KEYCODE_MEDIA_PREVIOUS:
case KeyEvent.KEYCODE_MEDIA_REWIND:
case KeyEvent.KEYCODE_MEDIA_RECORD:
case KeyEvent.KEYCODE_MEDIA_FAST_FORWARD: {
if ((result & ACTION_PASS_TO_USER) == 0) { // Only do this if we would otherwise not pass it to the user. In that
// case, the PhoneWindow class will do the same thing, except it will
// only do it if the showing app doesn't process the key on its own.
// Note that we need to make a copy of the key event here because the
// original key event will be recycled when we return.
mBroadcastWakeLock.acquire();
Message msg = mHandler.obtainMessage(MSG_DISPATCH_MEDIA_KEY_WITH_WAKE_LOCK,
new KeyEvent(event));
msg.setAsynchronous(true);
msg.sendToTarget();
}
break;
}
case KeyEvent.KEYCODE_CALL: {
if (down) {
ITelephony telephonyService = getTelephonyService();
if (telephonyService != null) {
try {
if (telephonyService.isRinging()) {
Log.i(TAG, "interceptKeyBeforeQueueing:"
+ " CALL key-down while ringing: Answer the call!");
telephonyService.answerRingingCall();
// And *don't* pass this key thru to the current activity
// (which is presumably the InCallScreen.)
result &= ~ACTION_PASS_TO_USER;
}
} catch (RemoteException ex) {
Log.w(TAG, "ITelephony threw RemoteException", ex);
}
}
}
break;
}
}
return result;
}可以发现这里首先判断当前系统是否已经boot完毕，若尚未启动完毕，则所有的按键操作都将失效，若启动完成，则执行后续的操作，这里我们只是关注音量减少按键和电源按键组合的处理事件。另外这里多说一句像安卓系统的HOME按键事件，MENU按键事件，进程列表按键事件等等都是在这里实现的
我们关注一下关键代码部分就好了，看看按键捕获部分，如下：
当我用按下音量减少按键的时候回进入到：case KeyEvent.KEYCODE_VOLUME_MUTE分支并执行相应的逻辑，然后同时判断用户是否按下了电源键，若同时按下了电源键，则执行：
见名知意可以发现这里的interceptScreenshotChrod方法就是系统准备开始执行截屏操作的开始，我们继续看一下interceptcreenshotChord方法的实现。
在方法体中我们最终会执行发送一个延迟的异步消息，请求执行截屏的操作而这里的延时时间，若当前输入框是打开状态，则延时时间为输入框关闭时间加上系统配置的按键超时时间，若当前输入框没有打开则直接是系统配置的按键超时处理时间，再看看mScreenshotChordLongPress这个Runnable的具体实现。
方法体中并未执行其他操作，直接就是调用了takeScreenshot方法，这样我们继续看一下takeScreenshot方法的实现。
// Assume this is called from the Handler thread.
private void takeScreenshot() {
synchronized (mScreenshotLock) {
if (mScreenshotConnection != null) {
return;
}
ComponentName cn = new ComponentName("com.android.systemui",
"com.android.systemui.screenshot.TakeScreenshotService");
Intent intent = new Intent();
intent.setComponent(cn);
ServiceConnection conn = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
synchronized (mScreenshotLock) {
if (mScreenshotConnection != this) {
return;
}
Messenger messenger = new Messenger(service);
Message msg = Message.obtain(null, 1);
final ServiceConnection myConn = this;
Handler h = new Handler(mHandler.getLooper()) {
@Override
public void handleMessage(Message msg) {
synchronized (mScreenshotLock) {
if (mScreenshotConnection == myConn) {
mContext.unbindService(mScreenshotConnection);
mScreenshotConnection = null;
mHandler.removeCallbacks(mScreenshotTimeout);
}
}
}
};
msg.replyTo = new Messenger(h); msg.arg1 = msg.arg2 = 0;
if (mStatusBar != null && mStatusBar.isVisibleLw())
msg.arg1 = 1;
if (mNavigationBar != null && mNavigationBar.isVisibleLw())
msg.arg2 = 1;
try {
messenger.send(msg);
} catch (RemoteException e) {
}
}
}
@Override
public void onServiceDisconnected(ComponentName name) {}
};
if (mContext.bindService(
intent, conn, Context.BIND_AUTO_CREATE, UserHandle.USER_CURRENT)) {
mScreenshotConnection = conn;
mHandler.postDelayed(mScreenshotTimeout, 10000);
}
}
}那么看代码可知这里是启动了一个TakeScreenshotService，该service即是最上面的图中systemUI下的，我们再看看TakeScreenshotService这个类里面到底做了什么事吧！
该service在被成功绑定时候回有一个handler回调过来然后拿到一个GlobalScreenshot去执行takeScreenshot方法，好吧，继续看一下takeScreentshot方法的执行逻辑。
该方法后面有两个参数：statusBarVisible，navBarVisible是否可见，而这两个参数在我们PhoneWindowManager.takeScreenshot方法传递的，在我们启动TakeScreenshotService时传入：
可见若果状态条可见，则传递的statusBarVisible为true，若导航条可见，则传递的navBarVisible为true。然后我们在截屏的时候判断nStatusBar是否可见，mNavigationBar是否可见，若可见的时候则截屏同样将其截屏出来。
再来看看GlobalScreenshot.takeScreenshot方法中截屏最关键的代码：
看注释可知，这里就是执行截屏事件的具体操作了，然后我看一下SurfaceControl.screenshot方法的具体实现，另外这里需要注意的是，截屏之后返回的是一个Bitmap对象，其实熟悉android绘制机制的朋友都应该要知道android中所有显示能够显示的东西，在内存中表现都是Bitmap对象。
如图可知，那么这个Surface.screenshot方法被@了一个hide，看来是被Google隐藏掉了，该方法最后是调用了本地方法nativeScreenshot函数，这个是在C++那边操作的，具体的实现在JNI层，由于个人对C++不是很熟练那么这边不做过多的介绍。framework中间层和HAL库函数打交道基本上都是这个模式。
另外在GlobalScreenshot.takeScreenshot这个方法中其它是做了一些动画，通知等操作，比如我们截屏时保存的图片会有一个动画以及截屏成功之后会在通知栏收到一条通知等。