Android 打开相机预览

要在 Android 中打开相机预览，你需要使用 Camera2 API。以下是一个简单的代码示例，可以在 Android 中打开相机预览：

1. 添加权限到 AndroidManifest.xml 文件中：

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

2. 在布局文件中添加一个 SurfaceView：

<SurfaceView
    android:id="@+id/surfaceView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

3. 在 Activity 中获取相机和 SurfaceView 对象：

private CameraDevice cameraDevice;
private CameraCaptureSession cameraCaptureSession;
private CaptureRequest.Builder captureRequestBuilder;
private SurfaceView surfaceView;

private CameraDevice.StateCallback cameraDeviceStateCallback = new CameraDevice.StateCallback() {
    @Override
    public void onOpened(@NonNull CameraDevice camera) {
        cameraDevice = camera;
    }

    @Override
    public void onDisconnected(@NonNull CameraDevice camera) {
        camera.close();
        cameraDevice = null;
    }

    @Override
    public void onError(@NonNull CameraDevice camera, int error) {
        camera.close();
        cameraDevice = null;
    }
};

private SurfaceHolder.Callback surfaceHolderCallback = new SurfaceHolder.Callback() {
    @Override
    public void surfaceCreated(SurfaceHolder surfaceHolder) {
        openCamera();
    }

    @Override
    public void surfaceChanged(SurfaceHolder surfaceHolder, int i, int i1, int i2) {

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder surfaceHolder) {
        closeCamera();
    }
};

private void openCamera() {
    CameraManager cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
    try {
        String cameraId = cameraManager.getCameraIdList()[0];
        cameraManager.openCamera(cameraId, cameraDeviceStateCallback, null);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

private void closeCamera() {
    if (cameraDevice != null) {
        cameraDevice.close();
        cameraDevice = null;
    }
}

private void createCameraPreviewSession() {
    Surface surface = surfaceView.getHolder().getSurface();
    try {
        captureRequestBuilder = cameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        captureRequestBuilder.addTarget(surface);

        cameraDevice.createCaptureSession(Arrays.asList(surface), new CameraCaptureSession.StateCallback() {
            @Override
            public void onConfigured(@NonNull CameraCaptureSession session) {
                if (cameraDevice == null) {
                    return;
                }

                cameraCaptureSession = session;

                try {
                    captureRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
                    cameraCaptureSession.setRepeatingRequest(captureRequestBuilder.build(), null, null);
                } catch (CameraAccessException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onConfigureFailed(@NonNull CameraCaptureSession session) {

            }
        }, null);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    surfaceView = findViewById(R.id.surfaceView);
    surfaceView.getHolder().addCallback(surfaceHolderCallback);
}

4. 在 Activity 的 onResume() 和 onPause() 方法中打开和关闭相机：

@Override
protected void onResume() {
    super.onResume();
    openCamera();
}

@Override
protected void onPause() {
    closeCamera();
    super.onPause();
}

这样，你就可以在 Android 中打开相机预览了。