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@Override
public int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags) {
//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
if (!mSystemBooted) {
// If we have not yet booted, don't let key events do anything.
return 0;
}
...
// Handle special keys.
switch (keyCode) {
//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,即截屏
case KeyEvent.KEYCODE_VOLUME_DOWN:
case KeyEvent.KEYCODE_VOLUME_UP:
case KeyEvent.KEYCODE_VOLUME_MUTE: {
if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) {
if (down) {
//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，则执行下面
if (interactive && !mScreenshotChordVolumeDownKeyTriggered
&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {
mScreenshotChordVolumeDownKeyTriggered = true;
mScreenshotChordVolumeDownKeyTime = event.getDownTime();//按下开始时间
mScreenshotChordVolumeDownKeyConsumed = false;
cancelPendingPowerKeyAction();
interceptScreenshotChord();//系统准备开始执行截屏操作的开始
}
} else {
mScreenshotChordVolumeDownKeyTriggered = false;
cancelPendingScreenshotChordAction();
}
}
...
case KeyEvent.KEYCODE_POWER: {
result &= ~ACTION_PASS_TO_USER;
isWakeKey = false; // wake-up will be handled separately
if (down) {
interceptPowerKeyDown(event, interactive);
} else {
interceptPowerKeyUp(event, interactive, canceled);
}
break;
}
...
return result;
}

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private void interceptScreenshotChord() {
//用两个布尔变量判断是否同时按了音量下键和电源键后，再计算两个按键响应Down事件之间的时间差不超过150毫秒，
//也就认为是同时按了这两个键后，算是真正的捕获到屏幕截屏的组合键。
if (mScreenshotChordEnabled
&& mScreenshotChordVolumeDownKeyTriggered && mScreenshotChordPowerKeyTriggered
&& !mScreenshotChordVolumeUpKeyTriggered) {
final long now = SystemClock.uptimeMillis();// 从开机到现在的毫秒数（手机睡眠的时间不包括在内）
if (now <= mScreenshotChordVolumeDownKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS
&& now <= mScreenshotChordPowerKeyTime
+ SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS) {
mScreenshotChordVolumeDownKeyConsumed = true;
cancelPendingPowerKeyAction();
mHandler.postDelayed(mScreenshotRunnable, getScreenshotChordLongPressDelay());
}
}
}

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

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private void takeScreenshot() {
synchronized (mScreenshotLock) {
if (mScreenshotConnection != null) {
return;
}
//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
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.bindServiceAsUser(
intent, conn, Context.BIND_AUTO_CREATE, UserHandle.CURRENT)) {
mScreenshotConnection = conn;
mHandler.postDelayed(mScreenshotTimeout, 10000);
}
}
}

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public class TakeScreenshotService extends Service {
private static final String TAG = "TakeScreenshotService";
private static GlobalScreenshot mScreenshot;
private Handler mHandler = new Handler() {
@Override
public 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);
}
}
};

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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 orientation
mDisplayMatrix.reset();
mDisplayMatrix.preRotate(-degrees);
mDisplayMatrix.mapPoints(dims);
dims[0] = Math.abs(dims[0]);
dims[1] = Math.abs(dims[1]);
}
// Take the screenshot
//执行截屏事件的具体操作，然后SurfaceControl.screenshot方法截屏之后返回的是一个Bitmap对象
mScreenBitmap = SurfaceControl.screenshot((int) dims[0], (int) dims[1]);
if (mScreenBitmap == null) {//判断是否截屏成功
notifyScreenshotError(mContext, mNotificationManager);//发送截屏失败的notification通知
finisher.run();
return;
}
//判断截屏的图像是否需要旋转
if (requiresRotation) {
// Rotate the screenshot to the current orientation
Bitmap 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 bitmap
mScreenBitmap.recycle();
mScreenBitmap = ss;
}
// Optimizations
mScreenBitmap.setHasAlpha(false);
mScreenBitmap.prepareToDraw();
// Start the post-screenshot animation
//开始截屏的动画
startAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels,
statusBarVisible, navBarVisible);
}

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/**
* Like {@link #screenshot(int, int, int, int)} but includes all
* Surfaces in the screenshot.
*
* @hide
*/
public static native Bitmap screenshot(int width, int height);

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private MediaActionSound mCameraSound;

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// Setup the Camera shutter sound
mCameraSound = new MediaActionSound();
mCameraSound.load(MediaActionSound.SHUTTER_CLICK); //设置截屏的声音

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// Play the shutter sound to notify that we've taken a screenshot.播放快门音以通知我们拍摄了屏幕截图
mCameraSound.play(MediaActionSound.SHUTTER_CLICK);