Android 后置摄像头悬浮窗

要在 Android 应用中实现后置摄像头悬浮窗，你需要进行以下步骤：

1. 添加权限：在 AndroidManifest.xml 文件中添加摄像头权限。

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

2. 创建后置摄像头的预览界面：在你的 Activity 或 Fragment 中创建一个 SurfaceView，并在 `surfaceCreated` 回调中打开相机并设置预览界面。

public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
    private SurfaceHolder holder;
    private Camera camera;

    public CameraPreview(Context context) {
        super(context);

        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        holder = getHolder();
        holder.addCallback(this);
    }

    public void surfaceCreated(SurfaceHolder holder) {
        // Open the camera and set the preview display
        camera = Camera.open();
        try {
            camera.setPreviewDisplay(holder);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
        // Release the camera when the surface is destroyed
        camera.stopPreview();
        camera.release();
        camera = null;
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        // Start the camera preview after the surface is changed
        camera.startPreview();
    }
}

3. 创建悬浮窗：在你的 Activity 或 Service 中创建一个悬浮窗，并将预览界面添加到悬浮窗中。

public class FloatingCameraService extends Service {
    private WindowManager windowManager;
    private CameraPreview cameraPreview;

    public void onCreate() {
        super.onCreate();

        // Create a new SurfaceView and add it to the window manager
        cameraPreview = new CameraPreview(this);
        WindowManager.LayoutParams layoutParams = new WindowManager.LayoutParams(
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.WRAP_CONTENT,
                WindowManager.LayoutParams.TYPE_PHONE,
                WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,
                PixelFormat.TRANSLUCENT);
        windowManager = (WindowManager) getSystemService(WINDOW_SERVICE);
        windowManager.addView(cameraPreview, layoutParams);
    }

    public void onDestroy() {
        super.onDestroy();

        // Remove the SurfaceView from the window manager when the service is destroyed
        windowManager.removeView(cameraPreview);
    }

    public IBinder onBind(Intent intent) {
        // Not used
        return null;
    }
}

4. 启动悬浮窗：在你的 Activity 中启动悬浮窗服务。

Intent intent = new Intent(this, FloatingCameraService.class);
startService(intent);

注意：在 Android 6.0 及以上版本中，你需要动态请求摄像头权限。