android Camera 如何判断当前使用的摄像头是前置还是后置

现在 android 平台的智能手机一般都标配有两颗摄像头。在 Camera 中都存在摄像头切换的功能。

并且有一些功能前后置摄像头上会有所不同。譬如人脸检测，人脸识别，自动对焦，闪光灯等功能，

如果前置摄像头的像素太低，不支持该功能的话，就需要在前置摄像头上关掉该 feature.

那么是如何判断并切换前后置摄像头的呢？

我们先来看下 CameraInfo 这个类，

 /**
  * Information about a camera
  */
 public static class CameraInfo {
     /**
      * The facing of the camera is opposite to that of the screen.
      */
     public static final int CAMERA_FACING_BACK = 0;  

     /**
      * The facing of the camera is the same as that of the screen.
      */
     public static final int CAMERA_FACING_FRONT = 1;  

     /**
      * The direction that the camera faces. It should be
      * CAMERA_FACING_BACK or CAMERA_FACING_FRONT.
      */
     public int facing;  

     /**
      * <p>The orientation of the camera image. The value is the angle that the
      * camera image needs to be rotated clockwise so it shows correctly on
      * the display in its natural orientation. It should be 0, 90, 180, or 270.</p>
      *
      * <p>For example, suppose a device has a naturally tall screen. The
      * back-facing camera sensor is mounted in landscape. You are looking at
      * the screen. If the top side of the camera sensor is aligned with the
      * right edge of the screen in natural orientation, the value should be
      * 90. If the top side of a front-facing camera sensor is aligned with
      * the right of the screen, the value should be 270.</p>
      *
      * @see #setDisplayOrientation(int)
      * @see Parameters#setRotation(int)
      * @see Parameters#setPreviewSize(int, int)
      * @see Parameters#setPictureSize(int, int)
      * @see Parameters#setJpegThumbnailSize(int, int)
      */
     public int orientation;
 };  

见名知义，它就是一个 Camera 信息类。它是通过与屏幕的方向是否一致来定义前后置摄像头的。

与屏幕方向相反即为 BACK_FACING_CAMERA

与屏幕方向一致即为 FRONT_FACING_CAMERA

那么在代码中我们是如何获取当前使用的 CamerInfo 呢

 Camera.CameraInfo info = new Camera.CameraInfo();
 Camera.getCameraInfo(cameraId, info); 

当然，使用该代码的前提是要 import android.hardware.Camera.CameraInfo;

判断使用是前置还是后置摄像头，可以通过if (info.facing == CameraInfo.CAMERA_FACING_FRONT) 来判断。

当Camera 的实例已经创建了的情况下，则需要通过如下方式来判断。

 CameraInfo info = CameraHolder.instance().getCameraInfo()[mCameraId];
 if (info.facing == CameraInfo.CAMERA_FACING_FRONT) {
     //stopFaceDetection();
 }  

也可以通过 if(mCameraId == CameraInfo.CAMERA_FACING_FRONT) 来判断。

其中 mCameraId 是当前使用的 CameraId, 一般前置为1， 后置为 0。