在 Android 平台预览相机画面主要用到的是 SurfaceView、TextureView 这两个类。那么它们的区别是什么呢？
SurfaceView 可以在独立的线程中进行绘制，不会影响主线程。它使用双缓冲机制，播放视频时画面更流畅。不足之处是，SurfaceView 的内容不再应用窗口上，它的显示不受 View 的属性控制，所以不能进行平移、缩放等变换，也不能嵌套在其它 ViewGroup 中使用。
TextureView 支持移动、旋转、缩放等变换，可以使用 View 控件的一些特性，比如设置透明度。然而它必须在硬件加速的窗口中使用，占用内存比 SurfaceView 高，还会有 1-3 s 的延迟。在 5.0 以前在主线程渲染，5.0 以后有单独的渲染线程。
接下来，我们使用 SurfaceView 和 TextureView 实现相机预览的功能。
使用 SurfaceView
添加相机权限
<uses-permission android:name="android.permission.CAMERA" />
实现 SurfaceHolder.Callback，在 surfaceCreated 方法中打开相机预览，在 surfaceDestroy 方法中关闭相机预览就可以了。Camera 的 open 方法有些耗时，为了避免阻塞 UI 线程，可以创建子线程打开相机。
public class CameraSurfacePreview extends SurfaceView implements SurfaceHolder.Callback {
private SurfaceHolder mSurfaceHolder;
private Camera mCamera;
private Activity mActivity;
public CameraSurfacePreview(Context context) {
super(context);
init();
}
private void init() {
mSurfaceHolder = getHolder();
mSurfaceHolder.addCallback(this);
mActivity = (Activity) getContext();
}
@Override
public void surfaceCreated(SurfaceHolder holder) {
logger.debug("surfaceCreated");
openCamera();
startPreviewDisplay();
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
logger.debug("surfaceChanged: format:{}, width:{}, height:{}", format, width, height);
}
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
logger.debug("surfaceDestroyed");
releaseCamera();
}
// 打开相机
private void openCamera() {
Camera.CameraInfo cameraInfo = new Camera.CameraInfo();
int number = Camera.getNumberOfCameras();
for (int i = 0; i < number; i++) {
Camera.getCameraInfo(i, cameraInfo);
if (cameraInfo.facing == Camera.CameraInfo.CAMERA_FACING_FRONT) {
try {
mCamera = Camera.open(i);
CameraUtils.setCameraDisplayOrientation(mActivity, i, mCamera);
Camera.Parameters params = mCamera.getParameters();
params.setFocusMode(Camera.Parameters.FOCUS_MODE_AUTO);
} catch (Exception e) {
logger.error("openCamera error", e);
mActivity.onBackPressed();
}
break;
}
}
}
// 开启预览
private void startPreviewDisplay() {
if (mCamera != null) {
try {
// 设置预览回调，取得 NV21 数据，进一步处理
mCamera.setPreviewCallback(new Camera.PreviewCallback() {
@Override
public void onPreviewFrame(byte[] data, Camera camera) {
}
});
mCamera.setPreviewDisplay(mSurfaceHolder);
mCamera.startPreview();
} catch (IOException e) {
logger.error("startPreviewDisplay error", e);
}
}
}
// 关闭相机
private void releaseCamera() {
if (mCamera != null) {
try {
mCamera.stopPreview();
mCamera.setPreviewDisplay(null);
mCamera.release();
} catch (IOException e) {
logger.error("releaseCamera error", e);
}
mCamera = null;
}
}
}
使用 TextureView
实现 TextureView.SurfaceTextureListener 接口，在 SurfaceTexture 可用时，打开相机预览，使用 SurfaceTexture 呈现画面。在 SurfaceTexture 销毁时，释放相机资源。Textureview 必须在硬件加速开启的窗口中使用，应用默认开启。
public class CameraTexturePreview extends TextureView implements TextureView.SurfaceTextureListener {
private Camera mCamera;
private Activity mActivity;
public CameraTexturePreview(Context context) {
super(context);
init();
}
private void init() {
setSurfaceTextureListener(this);
mActivity = (Activity) getContext();
}
@Override
public void onSurfaceTextureAvailable(final SurfaceTexture surface, int width, int height) {
logger.debug("onSurfaceTextureAvailable. width:{}, height:{}", width, height);
openCamera();
startPreviewDisplay(surface);
}
@Override
public void onSurfaceTextureSizeChanged(SurfaceTexture surface, int width, int height) {
logger.debug("onSurfaceTextureSizeChanged. width:{}, height:{}", width, height);
// Ignored, Camera does all the work for us
}
@Override
public boolean onSurfaceTextureDestroyed(SurfaceTexture surface) {
logger.debug("onSurfaceTextureDestroyed");
releaseCamera();
return true;
}
@Override
public void onSurfaceTextureUpdated(SurfaceTexture surface) {
// Invoked every time there's a new Camera preview frame
}
// 开始预览
private void startPreviewDisplay(SurfaceTexture surfaceTexture) {
if (mCamera != null) {
try {
mCamera.setPreviewTexture(surfaceTexture);
mCamera.startPreview();
} catch (IOException e) {
logger.error("startPreviewDisplay error", e);
}
}
}
// 关闭相机
private void releaseCamera() {
if (mCamera != null) {
try {
mCamera.stopPreview();
mCamera.setPreviewTexture(null);
mCamera.release();
} catch (IOException e) {
logger.error("releaseCamera error", e);
}
mCamera = null;
}
}
}
// 设置相机预览方向
public static void setCameraDisplayOrientation(Activity activity, int cameraId, Camera camera) {
Camera.CameraInfo info = new Camera.CameraInfo();
Camera.getCameraInfo(cameraId, info);
int rotation = activity.getWindowManager().getDefaultDisplay().getRotation();
int degrees = 0;
switch (rotation) {
case Surface.ROTATION_0:
degrees = 0;
break;
case Surface.ROTATION_90:
degrees = 90;
break;
case Surface.ROTATION_180:
degrees = 180;
break;
case Surface.ROTATION_270:
degrees = 270;
break;
default:
}
int result;
if (info.facing == Camera.CameraInfo.CAMERA_FACING_FRONT) {
result = (info.orientation + degrees) % 360;
result = (360 - result) % 360; // compensate the mirror
} else { // back-facing
result = (info.orientation - degrees + 360) % 360;
}
camera.setDisplayOrientation(result);
}
详细的代码在 GitHub 上，欢迎提出意见和评论。
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