安卓截图

1,基于Android SDK的截屏方法

(1)主要就是利用SDK提供的View.getDrawingCache()方法。网上已经有很多的实例了。首先创建一个android project，然后进行Layout，画一个按键（res/layout/main.xml）：

<?xmlversion="1.0"encoding="utf-8"?>
<LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello"
    />
<Button
  android:text="NiceButton"
  android:id="@+id/my_button"
  android:layout_width="fill_parent"
  android:layout_height="wrap_content"
  android:layout_alignParentBottom="true"></Button>
</LinearLayout>

HelloAndroid.java实现代码为：

packagecom.example.helloandroid;
 
importjava.io.FileOutputStream;
importjava.text.SimpleDateFormat;
importjava.util.Date;
importjava.util.Locale;
 
importandroid.app.Activity;
importandroid.graphics.Bitmap;
importandroid.os.Bundle;
importandroid.view.View;
importandroid.view.View.OnClickListener;
importandroid.widget.Button;
 
publicclassHelloAndroidextendsActivity {
 
  privateButton button;
 
  /** Called when the activity is first created. */
  @Override
  publicvoidonCreate(Bundle savedInstanceState) {
 
    super.onCreate(savedInstanceState);
    this.setContentView(R.layout.main);
    this.button = (Button) this.findViewById(R.id.my_button);
    this.button.setOnClickListener(newOnClickListener() {
 
      publicvoidonClick(View v) {
        SimpleDateFormat sdf = newSimpleDateFormat(
            "yyyy-MM-dd_HH-mm-ss", Locale.US);
        String fname = "/sdcard/"+ sdf.format(newDate()) + ".png";
        View view = v.getRootView();
        view.setDrawingCacheEnabled(true);
        view.buildDrawingCache();
        Bitmap bitmap = view.getDrawingCache();
        if(bitmap != null) {
          System.out.println("bitmap got!");
          try{
            FileOutputStream out = newFileOutputStream(fname);
            bitmap.compress(Bitmap.CompressFormat.PNG,100, out);
            System.out.println("file "  + fname + "output done.");
          }catch(Exception e) {
            e.printStackTrace();
          }
        }else{
          System.out.println("bitmap is NULL!");
        }
      }
 
    });
 
  }
}

这个代码会在按下app中按键的时候自动在手机的/sdcard/目录下生成一个时间戳命名的png截屏文件。

这种截屏有一个问题，就是只能截到一部分，比如电池指示部分就截不出来了。

(2)在APK中调用“adb shell screencap -pfilepath” 命令

该命令读取系统的framebuffer，需要获得系统权限：

（1）. 在AndroidManifest.xml文件中添加

<uses-permissionandroid:name="android.permission.READ_FRAME_BUFFER"/>

（2）. 修改APK为系统权限，将APK放到源码中编译，修改Android.mk

LOCAL_CERTIFICATE := platform

publicvoid takeScreenShot(){
String mSavedPath = Environment.getExternalStorageDirectory()+File. separator + "screenshot.png" ;
try {
Runtime. getRuntime().exec("screencap -p " + mSavedPath);
} catch (Exception e) {
e.printStackTrace();
}

(3).利用系统的API，实现Screenshot,这部分代码是系统隐藏的，需要在源码下编译，

1).修改Android.mk，添加系统权限

LOCAL_CERTIFICATE := platform

2).修改AndroidManifest.xml 文件，添加

权限

<uses-permissionandroid:name="android.permission.READ_FRAME_BUFFER"/>

      public boolean takeScreenShot(String imagePath){
                     
                    
                     
             if(imagePath.equals("" )){
                      imagePath = Environment.getExternalStorageDirectory()+File. separator+"Screenshot.png" ;
             }
                     
          Bitmap mScreenBitmap;
          WindowManager mWindowManager;
          DisplayMetrics mDisplayMetrics;
          Display mDisplay;
                  
          mWindowManager = (WindowManager) mcontext.getSystemService(Context.WINDOW_SERVICE);
          mDisplay = mWindowManager.getDefaultDisplay();
          mDisplayMetrics = new DisplayMetrics();
          mDisplay.getRealMetrics(mDisplayMetrics);
                                 
          float[] dims = {mDisplayMetrics.widthPixels , mDisplayMetrics.heightPixels };
          mScreenBitmap = Surface. screenshot((int) dims[0], ( int) dims[1]);
                     
          if (mScreenBitmap == null) {  
                 return false ;
          }
                  
       try {
          FileOutputStream out = new FileOutputStream(imagePath);
          mScreenBitmap.compress(Bitmap.CompressFormat. PNG, 100, out);
             
        } catch (Exception e) {
                
                
          return false ;
        }       
                            
       return true ;
}
 

2 基于Android ddmlib进行截屏

public class ScreenShot {
 private BufferedImage image = null;
 /**
  * @param args
  */
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  AndroidDebugBridge.init(false); //
  ScreenShot screenshot = new ScreenShot();
  IDevice device = screenshot.getDevice();
  
  for (int i = 0; i < 10; i++) {
   Date date=new Date();
   SimpleDateFormat df=new SimpleDateFormat("MM-dd-HH-mm-ss"); 
   String nowTime = df.format(date);
   screenshot.getScreenShot(device, "Robotium" + nowTime);
   try {
    Thread.sleep(1000);
   } catch (InterruptedException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
  }
 }

 
 public void getScreenShot(IDevice device,String filename) {
  RawImage rawScreen = null;
  try {
   rawScreen = device.getScreenshot();
  } catch (TimeoutException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } catch (AdbCommandRejectedException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
  if (rawScreen != null) {
   Boolean landscape = false;
   int width2 = landscape ? rawScreen.height : rawScreen.width;
   int height2 = landscape ? rawScreen.width : rawScreen.height;
   if (image == null) {
    image = new BufferedImage(width2, height2,
      BufferedImage.TYPE_INT_RGB);
   } else {
    if (image.getHeight() != height2 || image.getWidth() != width2) {
     image = new BufferedImage(width2, height2,
       BufferedImage.TYPE_INT_RGB);
    }
   }
   int index = 0;
   int indexInc = rawScreen.bpp >> 3;
   for (int y = 0; y < rawScreen.height; y++) {
    for (int x = 0; x < rawScreen.width; x++, index += indexInc) {
     int value = rawScreen.getARGB(index);
     if (landscape)
      image.setRGB(y, rawScreen.width - x - 1, value);
     else
      image.setRGB(x, y, value);
    }
   }
   try {
    ImageIO.write((RenderedImage) image, "PNG", new File("D:/"
      + filename + ".jpg"));
   } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
  }
 }

 /**
  * 获取得到device对象
  * @return
  */
 private IDevice getDevice(){
  IDevice device;
  AndroidDebugBridge bridge = AndroidDebugBridge
    .createBridge("adb", true);//如果代码有问题请查看API，修改此处的参数值试一下
  waitDevicesList(bridge);
  IDevice devices[] = bridge.getDevices();
  device = devices[0];
  return device;
 }
 
 /**
  * 等待查找device
  * @param bridge
  */
 private void waitDevicesList(AndroidDebugBridge bridge) {
  int count = 0;
  while (bridge.hasInitialDeviceList() == false) {
   try {
    Thread.sleep(500); 
    count++;
   } catch (InterruptedException e) {
   }
   if (count > 240) {
    System.err.print("等待获取设备超时");
    break;
   }
  }
 }

3 Android本地编程（Native Programming）读取framebuffer

(1)命令行，框架的截屏功能是通过framebuffer来实现的，所以我们先来介绍一下framebuffer。

framebuffer介绍 帧缓冲（framebuffer）是Linux为显示设备提供的一个接口，把显存抽象后的一种设备，他允许上层应用程序在图形模式下直接对显示缓冲区进行读写操作。这种操作是抽象的，统一的。用户不必关心物理显存的位置、换页机制等等具体细节。这些都是由Framebuffer设备驱动来完成的。 Linux FrameBuffer 本质上只是提供了对图形设备的硬件抽象，在开发者看来，FrameBuffer 是一块显示缓存，往显示缓存中写入特定格式的数据就意味着向屏幕输出内容。所以说FrameBuffer就是一块白板。例如对于初始化为16 位色的FrameBuffer 来说， FrameBuffer中的两个字节代表屏幕上一个点，从上到下，从左至右，屏幕位置与内存地址是顺序的线性关系。帧缓存有个地址，是在内存里。我们通过不停的向frame buffer中写入数据，显示控制器就自动的从frame buffer中取数据并显示出来。全部的图形都共享内存中同一个帧缓存。

Android截屏实现思路 Android系统是基于Linux内核的，所以也存在framebuffer这个设备，我们要实现截屏的话只要能获取到framebuffer中的数据，然后把数据转换成图片就可以了，android中的framebuffer数据是存放在 /dev/graphics/fb0 文件中的，所以我们只需要来获取这个文件的数据就可以得到当前屏幕的内容。现在我们的测试代码运行时候是通过RC(remote controller)方式来运行被测应用的，那就需要在PC机上来访问模拟器或者真机上的framebuffer数据，这个的话可以通过android的ADB命令来实现。

具体实现
/*********************************************************************** * * ScreenShot.java ***********************************************************************/ import java.awt.image.BufferedImage; import java.io.ByteArrayOutputStream; import java.io.DataInput; import java.io.EOFException; import java.io.File; import java.io.FileInputStream; import java.io.IOException; import java.io.InputStream; import javax.imageio.ImageIO; import org.openqa.selenium.OutputType; import org.openqa.selenium.internal.Base64Encoder; import com.google.common.io.Closeables; import com.google.common.io.LittleEndianDataInputStream;
/** */ public class ScreenShot {
/** * @param args * @throws InterruptedException */ public static void main(String[] args) throws InterruptedException { try { //分辨率大小，后续可以通过代码来获取到当前的分辨率 int xResolution = 320; int yResolution = 480; //执行adb命令，把framebuffer中内容保存到fb1文件中 Runtime.getRuntime().exec("adb pull /dev/graphics/fb0 C:/fb1"); //等待几秒保证framebuffer中的数据都被保存下来，如果没有保存完成进行读取操作会有IO异常 Thread.sleep(15000); //读取文件中的数据 InputStream in = (InputStream)new FileInputStream("C:/fb1"); DataInput frameBuffer = new LittleEndianDataInputStream(in); BufferedImage screenImage = new BufferedImage( xResolution, yResolution, BufferedImage.TYPE_INT_ARGB); int[] oneLine = new int[xResolution]; for (int y = 0; y < yResolution; y++) { //从frameBuffer中计算出rgb值 convertToRgba32(frameBuffer, oneLine); //把rgb值设置到image对象中 screenImage.setRGB(0, y, xResolution, 1, oneLine, 0, xResolution); } Closeables.closeQuietly(in); ByteArrayOutputStream rawPngStream = new ByteArrayOutputStream(); try { if (!ImageIO.write(screenImage, "png", rawPngStream)) { throw new RuntimeException( "This Java environment does not support converting to PNG."); } } catch (IOException exception) { // This should never happen because rawPngStream is an in-memory stream. System.out.println("IOException=" + exception); } byte[] rawPngBytes = rawPngStream.toByteArray(); String base64Png = new Base64Encoder().encode(rawPngBytes); File screenshot = OutputType.FILE.convertFromBase64Png(base64Png); System.out.println("screenshot==" + screenshot.toString()); screenshot.renameTo(new File("C:\screenshottemp.png")); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); System.out.println(e); } } public static void convertToRgba32(DataInput frameBuffer, int[] into) { try { for (int x = 0; x < into.length; x++) { try{ int rgb = frameBuffer.readShort() & 0xffff; int red = rgb >> 11; red = (red << 3) | (red >> 2); int green = (rgb >> 5) & 63; green = (green << 2) | (green >> 4); int blue = rgb & 31; blue = (blue << 3) | (blue >> 2); into[x] = 0xff000000 | (red << 16) | (green << 8) | blue; }catch (EOFException e){ System.out.println("EOFException=" + e); } } } catch (IOException exception) { System.out.println("convertToRgba32Exception=" + exception); } } }

（2）

首先是直接移植SystemUI的代码，实现截图效果，这部分的代码就不贴出来了，直接去下载代码吧， 关键的代码没有几句，最最主要的是：Surface.screenshot()，请看代码吧。

[java]
<SPAN style="FONT-SIZE: 16px">package org.winplus.ss; 
 
import java.io.File; 
import java.io.FileNotFoundException; 
import java.io.FileOutputStream; 
import java.io.IOException; 
import java.text.SimpleDateFormat; 
import java.util.Date; 
 
import android.app.Activity; 
import android.content.Context; 
import android.graphics.Bitmap; 
import android.graphics.Canvas; 
import android.graphics.Matrix; 
import android.os.Bundle; 
import android.util.DisplayMetrics; 
import android.util.Log; 
import android.view.Display; 
import android.view.Surface; 
import android.view.WindowManager; 
import android.os.SystemProperties; 
 
public class SimpleScreenshotActivity extends Activity { 
 
    private Display mDisplay; 
    private WindowManager mWindowManager; 
    private DisplayMetrics mDisplayMetrics; 
    private Bitmap mScreenBitmap; 
    private Matrix mDisplayMatrix; 
 
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 
        setContentView(R.layout.main); 
 
        new Thread(new Runnable() { 
 
            @Override 
            public void run() { 
                takeScreenshot(); 
 
            } 
        }).start(); 
    } 
 
    private float getDegreesForRotation(int value) { 
        switch (value) { 
        case Surface.ROTATION_90: 
            return 360f - 90f; 
        case Surface.ROTATION_180: 
            return 360f - 180f; 
        case Surface.ROTATION_270: 
            return 360f - 270f; 
        } 
        return 0f; 
    } 
 
    private void takeScreenshot() { 
        mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE); 
        mDisplay = mWindowManager.getDefaultDisplay(); 
        mDisplayMetrics = new DisplayMetrics(); 
        mDisplay.getRealMetrics(mDisplayMetrics); 
        mDisplayMatrix = new Matrix(); 
        float[] dims = { mDisplayMetrics.widthPixels, 
                mDisplayMetrics.heightPixels }; 
 
        int value = mDisplay.getRotation(); 
        String hwRotation = SystemProperties.get("ro.sf.hwrotation", "0"); 
        if (hwRotation.equals("270") || hwRotation.equals("90")) { 
            value = (value + 3) % 4; 
        } 
        float degrees = getDegreesForRotation(value); 
 
        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]); 
        } 
 
        mScreenBitmap = Surface.screenshot((int) dims[0], (int) dims[1]); 
 
        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; 
        } 
 
        // If we couldn't take the screenshot, notify the user  
        if (mScreenBitmap == null) { 
            return; 
        } 
 
        // Optimizations  
        mScreenBitmap.setHasAlpha(false); 
        mScreenBitmap.prepareToDraw(); 
         
        try { 
            saveBitmap(mScreenBitmap); 
        } catch (IOException e) { 
            System.out.println(e.getMessage()); 
        } 
    } 
 
    public void saveBitmap(Bitmap bitmap) throws IOException { 
        String imageDate = new SimpleDateFormat("yyyy-MM-dd-HH-mm-ss") 
                .format(new Date(System.currentTimeMillis())); 
        File file = new File("/mnt/sdcard/Pictures/"+imageDate+".png"); 
        if(!file.exists()){ 
            file.createNewFile(); 
        } 
        FileOutputStream out; 
        try { 
            out = new FileOutputStream(file); 
            if (bitmap.compress(Bitmap.CompressFormat.PNG, 70, out)) { 
                out.flush(); 
                out.close(); 
            } 
        } catch (FileNotFoundException e) { 
            e.printStackTrace(); 
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
    } 
} 
</SPAN> 

package org.winplus.ss;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;

import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Matrix;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.util.Log;
import android.view.Display;
import android.view.Surface;
import android.view.WindowManager;
import android.os.SystemProperties;

public class SimpleScreenshotActivity extends Activity {

 private Display mDisplay;
 private WindowManager mWindowManager;
 private DisplayMetrics mDisplayMetrics;
 private Bitmap mScreenBitmap;
 private Matrix mDisplayMatrix;

 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);

  new Thread(new Runnable() {

   @Override
   public void run() {
    takeScreenshot();

   }
  }).start();
 }

 private float getDegreesForRotation(int value) {
  switch (value) {
  case Surface.ROTATION_90:
   return 360f - 90f;
  case Surface.ROTATION_180:
   return 360f - 180f;
  case Surface.ROTATION_270:
   return 360f - 270f;
  }
  return 0f;
 }

 private void takeScreenshot() {
  mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
  mDisplay = mWindowManager.getDefaultDisplay();
  mDisplayMetrics = new DisplayMetrics();
  mDisplay.getRealMetrics(mDisplayMetrics);
  mDisplayMatrix = new Matrix();
  float[] dims = { mDisplayMetrics.widthPixels,
    mDisplayMetrics.heightPixels };

  int value = mDisplay.getRotation();
  String hwRotation = SystemProperties.get("ro.sf.hwrotation", "0");
  if (hwRotation.equals("270") || hwRotation.equals("90")) {
   value = (value + 3) % 4;
  }
  float degrees = getDegreesForRotation(value);

  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]);
  }

  mScreenBitmap = Surface.screenshot((int) dims[0], (int) dims[1]);

  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;
        }

        // If we couldn't take the screenshot, notify the user
        if (mScreenBitmap == null) {
            return;
        }

        // Optimizations
        mScreenBitmap.setHasAlpha(false);
        mScreenBitmap.prepareToDraw();
       
  try {
   saveBitmap(mScreenBitmap);
  } catch (IOException e) {
   System.out.println(e.getMessage());
  }
 }

 public void saveBitmap(Bitmap bitmap) throws IOException {
  String imageDate = new SimpleDateFormat("yyyy-MM-dd-HH-mm-ss")
    .format(new Date(System.currentTimeMillis()));
  File file = new File("/mnt/sdcard/Pictures/"+imageDate+".png");
  if(!file.exists()){
   file.createNewFile();
  }
  FileOutputStream out;
  try {
   out = new FileOutputStream(file);
   if (bitmap.compress(Bitmap.CompressFormat.PNG, 70, out)) {
    out.flush();
    out.close();
   }
  } catch (FileNotFoundException e) {
   e.printStackTrace();
  } catch (IOException e) {
   e.printStackTrace();
  }
 }
}

PS：1、需要在AndroidManifest.xml中加入代码：android:sharedUserId="android.uid.system"

         2、由于调用了@hide的API，所以编译得时候请使用makefile编译。或者通过在Eclipse中添加Jar文件通过编译。

         3、此代码只在Android4.0中使用过，2.3的就没去做测试了。

4 利用TakeScreenShotService截图

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（alpsframeworksasepolicysrccomandroidinternalpolicyimpl）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：

.......
 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：

    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：

    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():

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：

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）：

 /**
     * 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：

    /**
     * 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：

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。