感谢网友cjd6568358的帮助，新版的Android系统截屏功能已经实现，需要的朋友请移步项目主页：https://github.com/Android-ScreenShot/AndroidScreenShotService（别忘点个star哦）

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（转载请注明出处：http://blog.csdn.net/buptgshengod）

1.背景

       我们知道android提供了一个系统截屏功能，就是按住电源键和音量减的按键0.5秒，系统将执行截屏功能。所以要实现系统截屏的功能，就是要捕获系统的这两个组合键下面的函数，然后一层一层的向下挖掘。现在网上找到的版本是在Surface.java文件下存在ScreenShot()函数，是@hide的。但是这是之前版本的办法，在android4.3之后已经是不适用的，因为在/frameworks/base/core/java/android/view/的Surface.java下并没有ScreenShot()函数，我猜google不会这么绝情，一定会在framework层给开发者留了接口，只不过写到了别的地方。所以博主按照前任的思路自行挖掘，最后找到了新版本的ScreenShot函数，在这与大家分享。

2.挖掘过程

  （1）Android源码中对按键的捕获位于文件PhoneWindowManager.java（\frameworks\base\policy\src\com\android\internal\policy\impl）中    

ase 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();
                    }

我们看到了，如果同时按下电源键与音量减会启动函数，

interceptScreenshotChord();

我们来看下着个函数

private void interceptScreenshotChord() {
        if (mVolumeDownKeyTriggered && mPowerKeyTriggered && !mVolumeUpKeyTriggered) {
            final long now = SystemClock.uptimeMillis();
            if (now <= mVolumeDownKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS
                    && now <= mPowerKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS) {
                mVolumeDownKeyConsumedByScreenshotChord = true;
                cancelPendingPowerKeyAction();

                mHandler.postDelayed(mScreenshotChordLongPress,
                        ViewConfiguration.getGlobalActionKeyTimeout());
            }
        }
    }

我们看到handle中的这个函数，应该就是我们要执行的截屏功能

mScreenshotChordLongPress

我们进入这个函数

private final Runnable mScreenshotChordLongPress = new Runnable() {
        public void run() {
            takeScreenshot();
        }
    };

终于让我们找到了takeScreenShot，不过别急，我们转到这个函数

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)) {
                mScreenshotConnection = conn;
                mHandler.postDelayed(mScreenshotTimeout, 10000);
            }
        }
    }

我们看到它启动了一个截图的service（ "com.android.systemui.screenshot.TakeScreenshotService")。（这也印证了我一个猜想，android的一切功能都是handle通过sendmessage然后通过service实现的）

（2）找到那个service

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);
            }
        }
    };

    @Override
    public IBinder onBind(Intent intent) {
        return new Messenger(mHandler).getBinder();
    }
}

我们看到类GlobalScreenshot的对象执行了截图的功能。

（3）打开GlobalScreenshot

 /**
     * 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 orientation
            mDisplayMatrix.reset();
            mDisplayMatrix.preRotate(-degrees);
            mDisplayMatrix.mapPoints(dims);
            dims[0] = Math.abs(dims[0]);
            dims[1] = Math.abs(dims[1]);
        }

        // Take the screenshot
        mScreenBitmap = 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 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
        startAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels,
                statusBarVisible, navBarVisible);
    }

我们主要看这句，我们终于找到了screenshot()，这个地方跟低版本的android源码是有改动的，之前的surface操作是写到surface类里，现在增加了这个surfacecontrol类来控制surface。

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

（4）现在我们打开surfacecontrol类，位于/frameworks/base/core/java/android/view

代码开头我们可以看到，这个类是被google隐藏了，所以不能直接调用，若是想用截屏功能要在源码中编译才行，至于如何调用这个功能以后会讲，have fun！

/**
 * SurfaceControl
 *  @hide
 */