android Camera 怎么判断当前使用的摄像头是前置还是后置

在你打开camera的时候，你会传入一个cameraid：
public static camera open (int cameraid)
当你设置了cameraid，打开camera之后，可以通过下面的代码获取camera信息：

android.hardware.camera.camerainfo info =
new android.hardware.camera.camerainfo();
android.hardware.camera.getcamerainfo(cameraid, info);

if (info.facing == camerainfo.camera_facing_front) {
//前置摄像头
} else { // 后置摄像头

}

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，

[java] view plaincopy
/**
* 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

[java] view plaincopy
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。

[java] view plaincopy
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。