Android源码中屏幕截图的实现

时间:2022-09-05 15:23:13

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(跟之前的函数重新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