zxing源码分析（一）camera部分

public static Camera open() {
    // 获取摄像头的数量
    int numCameras = Camera.getNumberOfCameras();
    //如果没有找到相机则退出
    if (numCameras == 0) {
      Log.w(TAG, "No cameras!");
      return null;
    }

    
    int index = 0;
    while (index < numCameras) {
      //初始化相机信息类
      Camera.CameraInfo cameraInfo = new Camera.CameraInfo();
      //获取相机的信息
      Camera.getCameraInfo(index, cameraInfo);
      //判断是否是后置摄像头
      if (cameraInfo.facing == Camera.CameraInfo.CAMERA_FACING_BACK) {
        break;
      }
      index++;
    }
    
    //打开相机
    Camera camera;
    if (index < numCameras) {
      Log.i(TAG, "Opening camera #" + index);
      camera = Camera.open(index);
    } else {
      Log.i(TAG, "No camera facing back; returning camera #0");
      camera = Camera.open(0);
    }

    return camera;
  }

  void initFromCameraParameters(Camera camera) {
    //获取相机的参数
    Camera.Parameters parameters = camera.getParameters();
    WindowManager manager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
    Display display = manager.getDefaultDisplay();
    //构造一个点
    Point theScreenResolution = new Point();
     //给点进行赋值，屏幕的宽和高 Resolution分辨率的意思
    display.getSize(theScreenResolution);
    screenResolution = theScreenResolution;
    Log.i(TAG, "Screen resolution: " + screenResolution);
    cameraResolution = findBestPreviewSizeValue(parameters, screenResolution);
    Log.i(TAG, "Camera resolution: " + cameraResolution);
  }

public synchronized void openDriver(SurfaceHolder holder) throws IOException {
    Camera theCamera = camera;
    if (theCamera == null) {
      theCamera = OpenCameraInterface.open();
      if (theCamera == null) {
        throw new IOException();
      }
      camera = theCamera;
    }
    //设置摄像头预览功能
    theCamera.setPreviewDisplay(holder);

    //初始化执行的操作
    if (!initialized) {
      initialized = true;
      //初始化相机的参数，选择最佳的预览分辨率
      configManager.initFromCameraParameters(theCamera);
      if (requestedFramingRectWidth > 0 && requestedFramingRectHeight > 0) {
        setManualFramingRect(requestedFramingRectWidth, requestedFramingRectHeight);
        requestedFramingRectWidth = 0;
        requestedFramingRectHeight = 0;
      }
    }

    Camera.Parameters parameters = theCamera.getParameters();
    String parametersFlattened = parameters == null ? null : parameters.flatten(); // Save these, temporarily
    try {
      //设置必要的参数，包括焦点，闪光灯等
      configManager.setDesiredCameraParameters(theCamera, false);
    } catch (RuntimeException re) {
      // Driver failed
      Log.w(TAG, "Camera rejected parameters. Setting only minimal safe-mode parameters");
      Log.i(TAG, "Resetting to saved camera params: " + parametersFlattened);
      // Reset:
      if (parametersFlattened != null) {
        parameters = theCamera.getParameters();
        parameters.unflatten(parametersFlattened);
        try {
          theCamera.setParameters(parameters);
          configManager.setDesiredCameraParameters(theCamera, true);
        } catch (RuntimeException re2) {
          // Well, darn. Give up
          Log.w(TAG, "Camera rejected even safe-mode parameters! No configuration");
        }
      }
    }

  }

<SurfaceView android:id="@+id/preview_view"
               android:layout_width="fill_parent"
               android:layout_height="fill_parent"/>
 
  <com.google.zxing.client.android.ViewfinderView
      android:id="@+id/viewfinder_view"
      android:layout_width="fill_parent"
      android:layout_height="fill_parent"
      android:visibility="gone" />