Android源码中屏幕截图的实现

Android手机一般都自带有手机屏幕截图的功能：在手机任何界面（当然手机要是开机点亮状态），通过按组合键，屏幕闪一下，然后咔嚓一声，截图的照片会保存到当前手机的图库中，真是一个不错的功能！

以我手头的测试手机为例，是同时按电源键+音量下键来实现截屏，苹果手机则是电源键 + HOME键，小米手机是菜单键+音量下键，而HTC一般是按住电源键再按左下角的“主页”键。那么Android源码中使用组合键是如何实现屏幕截图功能呢？前段时间由于工作的原因仔细看了一下，这两天不忙，便把相关的知识点串联起来整理一下，分下面两部分简单分析下实现流程：

Android源码中对组合键的捕获。

Android源码中对按键的捕获位于文件PhoneWindowManager.java（alps\frameworks\base\policy\src\com\android\internal\policy\impl）中，这个类处理所有的键盘输入事件，其中函数interceptKeyBeforeQueueing（）会对常用的按键做特殊处理。以我手头的测试机为例，是同时按电源键和音量下键来截屏，那么在这个函数中我们会看到这么两段代码：

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....... 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();                    }......            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();                    }......

可以看到正是在这里（响应Down事件）捕获是否按了音量下键和电源键的，而且两个地方都会进入函数interceptScreenshotChord()中，那么接下来看看这个函数干了什么工作：

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    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());            }        }    }

在这个函数中，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。

附言：文件PhoneWindowManager.java类是拦截键盘消息的处理类，在此类中还有对home键、返回键等好多按键的处理。

Android源码中调用屏幕截图的接口。

捕获到组合键后，我们再看看android源码中是如何调用屏幕截图的函数接口。在上面的函数interceptScreenshotChord中我们看到用handler判断长按组合键500毫秒之后，会进入如下函数：

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

在这里启动了一个线程来完成截屏的功能，接着看函数takeScreenshot():

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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);            }        }    }

可以看到这个函数使用AIDL绑定了service服务到"com.android.systemui.screenshot.TakeScreenshotService"，注意在service连接成功时，对message的msg.arg1和msg.arg2两个参数的赋值。其中在mScreenshotTimeout中对服务service做了超时处理。接着我们找到实现这个服务service的类TakeScreenshotService，看看其实现的流程：

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

在这个类中，我们主要看调用接口，用到了mScreenshot.takeScreenshot（）传递了三个参数，第一个是个runnable，第二和第三个是之前message传递的两个参数msg.arg1和msg.arg2。最后我们看看这个函数takeScreenshot()，位于文件GlobalScreenshot.java中（跟之前的函数重名但是文件路径不一样）：

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 /**     * 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 = Surface.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);            mScreenBitmap = ss;        }        // Optimizations        mScreenBitmap.setHasAlpha(false);        mScreenBitmap.prepareToDraw();        // Start the post-screenshot animation        startAnimation(finisher, mDisplayMetrics.widthPixels, mDisplayMetrics.heightPixels,                statusBarVisible, navBarVisible);    }

这段代码的注释比较详细，其实看到这里，我们算是真正看到截屏的操作了，具体的工作包括对屏幕大小、旋转角度的获取，然后调用Surface类的screenshot方法截屏保存到bitmap中，之后把这部分位图填充到一个画布上，最后再启动一个延迟的拍照动画效果。如果再往下探究screenshot方法，发现已经是一个native方法了：

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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);

使用JNI技术调用底层的代码，如果再往下走，会发现映射这这个jni函数在文件android_view_Surface.cpp中，这个真的已经是底层c++语言了，统一调用的底层函数是：

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static jobject doScreenshot(JNIEnv* env, jobject clazz, jint width, jint height,        jint minLayer, jint maxLayer, bool allLayers){    ScreenshotPixelRef* pixels = new ScreenshotPixelRef(NULL);    if (pixels->update(width, height, minLayer, maxLayer, allLayers) != NO_ERROR) {        delete pixels;        return 0;    }    uint32_t w = pixels->getWidth();    uint32_t h = pixels->getHeight();    uint32_t s = pixels->getStride();    uint32_t f = pixels->getFormat();    ssize_t bpr = s * android::bytesPerPixel(f);    SkBitmap* bitmap = new SkBitmap();    bitmap->setConfig(convertPixelFormat(f), w, h, bpr);    if (f == PIXEL_FORMAT_RGBX_8888) {        bitmap->setIsOpaque(true);    }    if (w > 0 && h > 0) {        bitmap->setPixelRef(pixels)->unref();        bitmap->lockPixels();    } else {        // be safe with an empty bitmap.        delete pixels;        bitmap->setPixels(NULL);    }    return GraphicsJNI::createBitmap(env, bitmap, false, NULL);}

由于对C++不熟，我这里就不敢多言了。其实到这里，算是对手机android源码中通过组合键屏幕截图的整个流程有个大体了解了，一般我们在改动中熟悉按键的捕获原理，并且清楚调用的截屏函数接口即可，如果有兴趣的，可以继续探究更深的底层是如何实现的。

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原文作者： aoratec

原文地址： http://my.eoe.cn/viver120/archive/5448.html