Android源码解析之截屏事件流程

今天这篇文章我们主要讲一下Android系统中的截屏事件处理流程。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）。那么这里的截屏事件是如何触发的呢？触发之后android系统是如何实现截屏操作的呢？带着这两个问题，开始我们的源码阅读流程。

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，该方法主要用于执行当前App处理按键之前的操作，我们具体看一下该方法的实现。

/** {@inheritDoc} */@Overridepublic KeyEvent dispatchUnhandledKey(WindowState win, KeyEvent event, int 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) {...final int flags = event.getFlags() | KeyEvent.FLAG_FALLBACK;fallbackEvent = KeyEvent.obtain(event.getDownTime(), event.getEventTime(),event.getAction(), fallbackAction.keyCode,event.getRepeatCount(), fallbackAction.metaState,event.getDeviceId(), event.getScanCode(),flags, event.getSource(), null);if (!interceptFallback(win, fallbackEvent, policyFlags)) {fallbackEvent.recycle();fallbackEvent = null;}if (initialDown) {mFallbackActions.put(keyCode, fallbackAction);} else if (event.getAction() == KeyEvent.ACTION_UP) {mFallbackActions.remove(keyCode);fallbackAction.recycle();}}}...return fallbackEvent;}

这里我们关注一下方法体中调用的：interceptFallback方法，通过调用该方法将处理按键的操作下发到该方法中，我们继续看一下该方法的实现逻辑。

private boolean interceptFallback(WindowState win, KeyEvent fallbackEvent, int policyFlags) {int actions = interceptKeyBeforeQueueing(fallbackEvent, policyFlags);if ((actions & ACTION_PASS_TO_USER) != 0) {long delayMillis = interceptKeyBeforeDispatching(win, fallbackEvent, policyFlags);if (delayMillis == 0) {return true;}}return false;}

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：

@Overridepublic int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags) {if (!mSystemBooted) {// If we have not yet booted, don't let key events do anything.return 0;}...// Handle special keys.switch (keyCode) {case KeyEvent.KEYCODE_VOLUME_DOWN:case KeyEvent.KEYCODE_VOLUME_UP:case KeyEvent.KEYCODE_VOLUME_MUTE: {if (mUseTvRouting) {// On TVs volume keys never go to the foreground appresult &= ~ACTION_PASS_TO_USER;}if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) {if (down) {if (interactive && !mScreenshotChordVolumeDownKeyTriggered&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {mScreenshotChordVolumeDownKeyTriggered = true;mScreenshotChordVolumeDownKeyTime = event.getDownTime();mScreenshotChordVolumeDownKeyConsumed = false;cancelPendingPowerKeyAction();interceptScreenshotChord();}} else {mScreenshotChordVolumeDownKeyTriggered = false;cancelPendingScreenshotChordAction();}}...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。另外这里多说一句想安卓系统的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，后续中我们会陆续介绍这方面的内容。

回到我们的interceptKeyBeforeQueueing方法，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：

if (interactive && !mScreenshotChordVolumeDownKeyTriggered&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {mScreenshotChordVolumeDownKeyTriggered = true;mScreenshotChordVolumeDownKeyTime = event.getDownTime();mScreenshotChordVolumeDownKeyConsumed = false;cancelPendingPowerKeyAction();interceptScreenshotChord();}

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。

private void interceptScreenshotChord() {if (mScreenshotChordEnabled&& mScreenshotChordVolumeDownKeyTriggered && mScreenshotChordPowerKeyTriggered&& !mScreenshotChordVolumeUpKeyTriggered) {final long now = SystemClock.uptimeMillis();if (now <= mScreenshotChordVolumeDownKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS&& now <= mScreenshotChordPowerKeyTime+ SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS) {mScreenshotChordVolumeDownKeyConsumed = true;cancelPendingPowerKeyAction();mHandler.postDelayed(mScreenshotRunnable, getScreenshotChordLongPressDelay());}}}

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，可看一下getScreenshotChordLongPressDelay方法的具体实现。

private long getScreenshotChordLongPressDelay() {if (mKeyguardDelegate.isShowing()) {// Double the time it takes to take a screenshot from the keyguardreturn (long) (KEYGUARD_SCREENSHOT_CHORD_DELAY_MULTIPLIER *ViewConfiguration.get(mContext).getDeviceGlobalActionKeyTimeout());}return ViewConfiguration.get(mContext).getDeviceGlobalActionKeyTimeout();}

回到我们的interceptScreenshotChord方法，发送了异步消息之后系统最终会被我们发送的Runnable对象的run方法执行，这里关于异步消息的逻辑可参考：android源码解析之（二）– 异步消息机制

这样我们看一下Runnable类型的mScreenshotRunnable的run方法的实现:

private final Runnable mScreenshotRunnable = new Runnable() {@Overridepublic void run() {takeScreenshot();}};

好吧，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。

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() {@Overridepublic 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()) {@Overridepublic 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) {}}}@Overridepublic void onServiceDisconnected(ComponentName name) {}};if (mContext.bindServiceAsUser(intent, conn, Context.BIND_AUTO_CREATE, UserHandle.CURRENT)) {mScreenshotConnection = conn;mHandler.postDelayed(mScreenshotTimeout, 10000);}}}

可以发现这里通过反射机制创建了一个TakeScreenshotService对象然后调用了bindServiceAsUser，这样就创建了TakeScreenshotService服务并在服务创建之后发送了一个异步消息。好了，我们看一下TakeScreenshotService的实现逻辑。

public class TakeScreenshotService extends Service {private static final String TAG = "TakeScreenshotService";private static GlobalScreenshot mScreenshot;private Handler mHandler = new Handler() {@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case 1:final Messenger callback = msg.replyTo;if (mScreenshot == null) {mScreenshot = new GlobalScreenshot(TakeScreenshotService.this);}mScreenshot.takeScreenshot(new Runnable() {@Override public void run() {Message reply = Message.obtain(null, 1);try {callback.send(reply);} catch (RemoteException e) {}}}, msg.arg1   0, msg.arg2   0);}}};@Overridepublic IBinder onBind(Intent intent) {return new Messenger(mHandler).getBinder();}
}

可以发现在在TakeScreenshotService类的定义中有一个Handler成员变量，而我们在启动TakeScreentshowService的时候回发送一个异步消息，这样就会执行mHandler的handleMessage方法，然后在handleMessage方法中我们创建了一个GlobalScreenshow对象，然后执行了takeScreenshot方法，好吧，继续看一下takeScreentshot方法的执行逻辑。

/*** Takes a screenshot of the current display and shows an animation.*/void takeScreenshot(Runnable finisher, boolean statusBarVisible, boolean navBarVisible) {// We need to orient the screenshot correctly (and the Surface api seems to take screenshots// only in the natural orientation of the device :!)mDisplay.getRealMetrics(mDisplayMetrics);float[] dims = {mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels};float degrees = getDegreesForRotation(mDisplay.getRotation());boolean requiresRotation = (degrees   0);if (requiresRotation) {// Get the dimensions of the device in its native orientationmDisplayMatrix.reset();mDisplayMatrix.preRotate(-degrees);mDisplayMatrix.mapPoints(dims);dims[0] = Math.abs(dims[0]);dims[1] = Math.abs(dims[1]);}// Take the screenshotmScreenBitmap = SurfaceControl.screenshot((int) dims[0], (int) dims[1]);if (mScreenBitmap == null) {notifyScreenshotError(mContext, mNotificationManager);finisher.run();return;}if (requiresRotation) {// Rotate the screenshot to the current orientationBitmap ss = Bitmap.createBitmap(mDisplayMetrics.widthPixels,mDisplayMetrics.heightPixels, Bitmap.Config.ARGB_8888);Canvas c = new Canvas(ss);c.translate(ss.getWidth() / 2, ss.getHeight() / 2);c.rotate(degrees);c.translate(-dims[0] / 2, -dims[1] / 2);c.drawBitmap(mScreenBitmap, 0, 0, null);c.setBitmap(null);// Recycle the previous bitmapmScreenBitmap.recycle();mScreenBitmap = ss;}// OptimizationsmScreenBitmap.setHasAlpha(false);mScreenBitmap.prepareToDraw();// Start the post-screenshot animationstartAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels,statusBarVisible, navBarVisible);}

可以看到这里后两个参数：statusBarVisible，navBarVisible是否可见，而这两个参数在我们PhoneWindowManager.takeScreenshot方法传递的：

if (mStatusBar != null && mStatusBar.isVisibleLw())msg.arg1 = 1;if (mNavigationBar != null && mNavigationBar.isVisibleLw())msg.arg2 = 1;

可见若果mStatusBar可见，则传递的statusBarVisible为true，若mNavigationBar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。继续回到我们的takeScreenshot方法，然后调用了：

// Take the screenshot
mScreenBitmap = SurfaceControl.screenshot((int) dims[0], (int) dims[1]);

方法，看注释，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绘制机制的童鞋应该知道android中所有显示能够显示的东西，在内存中表现都是Bitmap对象。

public static Bitmap screenshot(int width, int height) {// TODO: should take the display as a parameterIBinder displayToken = SurfaceControl.getBuiltInDisplay(SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN);return nativeScreenshot(displayToken, new Rect(), width, height, 0, 0, true,false, Surface.ROTATION_0);}

好吧，这里调用的是nativeScreenshot方法，它是一个native方法，具体的实现在JNI层，这里就不做过多的介绍了。继续回到我们的takeScreenshot方法，在调用了截屏方法screentshot之后，判断是否截屏成功：

if (mScreenBitmap == null) {notifyScreenshotError(mContext, mNotificationManager);finisher.run();return;}

若截屏之后，截屏的bitmap对象为空，这里判断截屏失败，调用了notifyScreenshotError方法，发送截屏失败的notification通知。

static void notifyScreenshotError(Context context, NotificationManager nManager) {Resources r = context.getResources();// Clear all existing notification, compose the new notification and show itNotification.Builder b = new Notification.Builder(context).setTicker(r.getString(R.string.screenshot_failed_title)).setContentTitle(r.getString(R.string.screenshot_failed_title)).setContentText(r.getString(R.string.screenshot_failed_text)).setSmallIcon(R.drawable.stat_notify_image_error).setWhen(System.currentTimeMillis()).setVisibility(Notification.VISIBILITY_PUBLIC) // ok to show outside lockscreen.setCategory(Notification.CATEGORY_ERROR).setAutoCancel(true).setColor(context.getColor(com.android.internal.R.color.system_notification_accent_color));Notification n =new Notification.BigTextStyle(b).bigText(r.getString(R.string.screenshot_failed_text)).build();nManager.notify(R.id.notification_screenshot, n);}

然后继续看takeScreenshot方法，判断截屏的图像是否需要旋转，若需要的话，则旋转图像：

if (requiresRotation) {// Rotate the screenshot to the current orientationBitmap ss = Bitmap.createBitmap(mDisplayMetrics.widthPixels,mDisplayMetrics.heightPixels, Bitmap.Config.ARGB_8888);Canvas c = new Canvas(ss);c.translate(ss.getWidth() / 2, ss.getHeight() / 2);c.rotate(degrees);c.translate(-dims[0] / 2, -dims[1] / 2);c.drawBitmap(mScreenBitmap, 0, 0, null);c.setBitmap(null);// Recycle the previous bitmapmScreenBitmap.recycle();mScreenBitmap = ss;}

在takeScreenshot方法的最后若截屏成功，我们调用了：

// Start the post-screenshot animationstartAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels,statusBarVisible, navBarVisible);

开始截屏的动画，好吧，看一下动画效果的实现：

/*** Starts the animation after taking the screenshot*/private void startAnimation(final Runnable finisher, int w, int h, boolean statusBarVisible,boolean navBarVisible) {// Add the view for the animationmScreenshotView.setImageBitmap(mScreenBitmap);mScreenshotLayout.requestFocus();// Setup the animation with the screenshot just takenif (mScreenshotAnimation != null) {mScreenshotAnimation.end();mScreenshotAnimation.removeAllListeners();}mWindowManager.addView(mScreenshotLayout, mWindowLayoutParams);ValueAnimator screenshotDropInAnim = createScreenshotDropInAnimation();ValueAnimator screenshotFadeOutAnim = createScreenshotDropOutAnimation(w, h,statusBarVisible, navBarVisible);mScreenshotAnimation = new AnimatorSet();mScreenshotAnimation.playSequentially(screenshotDropInAnim, screenshotFadeOutAnim);mScreenshotAnimation.addListener(new AnimatorListenerAdapter() {@Overridepublic void onAnimationEnd(Animator animation) {// Save the screenshot once we have a bit of time nowsaveScreenshotInWorkerThread(finisher);mWindowManager.removeView(mScreenshotLayout);// Clear any references to the bitmapmScreenBitmap = null;mScreenshotView.setImageBitmap(null);}});mScreenshotLayout.post(new Runnable() {@Overridepublic void run() {// Play the shutter sound to notify that we've taken a screenshotmCameraSound.play(MediaActionSound.SHUTTER_CLICK);mScreenshotView.setLayerType(View.LAYER_TYPE_HARDWARE, null);mScreenshotView.buildLayer();mScreenshotAnimation.start();}});}

好吧，经过着一些列的操作之后我们实现了截屏之后的动画效果了，这里暂时不分析动画效果，我们看一下动画效果之后做了哪些？还记不记的一般情况下我们截屏之后都会收到一个截屏的notification通知？这里应该也是在其AnimatorListenerAdapter的onAnimationEnd方法中实现的，也就是动画执行完成之后，我们看一下其saveScreenshotInWorkerThread方法的实现：

/*** Creates a new worker thread and saves the screenshot to the media store.*/private void saveScreenshotInWorkerThread(Runnable finisher) {SaveImageInBackgroundData data = new SaveImageInBackgroundData();data.context = mContext;data.image = mScreenBitmap;data.iconSize = mNotificationIconSize;data.finisher = finisher;data.previewWidth = mPreviewWidth;data.previewheight = mPreviewHeight;if (mSaveInBgTask != null) {mSaveInBgTask.cancel(false);}mSaveInBgTask = new SaveImageInBackgroundTask(mContext, data, mNotificationManager,R.id.notification_screenshot).execute(data);}

好吧，这里主要逻辑就是构造了一个SaveImageInBackgroundTask对象，看样子发送截屏成功的通知应该是在这里实现的，我们看一下SaveImageInBackgroundTask构造方法的实现逻辑：

SaveImageInBackgroundTask(Context context, SaveImageInBackgroundData data,NotificationManager nManager, int nId) {...// Show the intermediate notificationmTickerAddSpace = !mTickerAddSpace;mNotificationId = nId;mNotificationManager = nManager;final long now = System.currentTimeMillis();mNotificationBuilder = new Notification.Builder(context).setTicker(r.getString(R.string.screenshot_saving_ticker)+ (mTickerAddSpace ? " " : "")).setContentTitle(r.getString(R.string.screenshot_saving_title)).setContentText(r.getString(R.string.screenshot_saving_text)).setSmallIcon(R.drawable.stat_notify_image).setWhen(now).setColor(r.getColor(com.android.internal.R.color.system_notification_accent_color));mNotificationStyle = new Notification.BigPictureStyle().bigPicture(picture.createAshmemBitmap());mNotificationBuilder.setStyle(mNotificationStyle);// For "public" situations we want to show all the same info but// omit the actual screenshot image.mPublicNotificationBuilder = new Notification.Builder(context).setContentTitle(r.getString(R.string.screenshot_saving_title)).setContentText(r.getString(R.string.screenshot_saving_text)).setSmallIcon(R.drawable.stat_notify_image).setCategory(Notification.CATEGORY_PROGRESS).setWhen(now).setColor(r.getColor(com.android.internal.R.color.system_notification_accent_color));mNotificationBuilder.setPublicVersion(mPublicNotificationBuilder.build());Notification n = mNotificationBuilder.build();n.flags |= Notification.FLAG_NO_CLEAR;mNotificationManager.notify(nId, n);// On the tablet, the large icon makes the notification appear as if it is clickable (and// on small devices, the large icon is not shown) so defer showing the large icon until// we compose the final post-save notification below.mNotificationBuilder.setLargeIcon(icon.createAshmemBitmap());// But we still don't set it for the expanded view, allowing the smallIcon to show here.mNotificationStyle.bigLargeIcon((Bitmap) null);}