使用 Camera2调用系统相机

使用Camera2 API调用系统相机需要以下步骤：

1.添加权限到 AndroidManifest.xml 文件中：

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

2.在布局文件中添加一个 SurfaceView 用于预览相机图像：

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

3.在 Activity 中获取 SurfaceView 对象并创建 CameraDevice：

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

private final CameraDevice.StateCallback stateCallback = new CameraDevice.StateCallback() {
    @Override
    public void onOpened(CameraDevice camera) {
        cameraDevice = camera;
        createCameraPreview();
    }

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

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

private void openCamera() {
    CameraManager manager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
    try {
        String cameraId = manager.getCameraIdList()[0];
        CameraCharacteristics characteristics = manager.getCameraCharacteristics(cameraId);
        Size[] outputSizes = characteristics.get(CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP)
                .getOutputSizes(SurfaceHolder.class);
        Size size = outputSizes[0];

        manager.openCamera(cameraId, stateCallback, null);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

4.为 CameraDevice 创建预览会话：

private void createCameraPreview() {
    try {
        SurfaceHolder holder = surfaceView.getHolder();
        holder.setFixedSize(size.getWidth(), size.getHeight());
        holder.setKeepScreenOn(true);

        final Surface surface = holder.getSurface();

        captureRequestBuilder = cameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        captureRequestBuilder.addTarget(surface);

        cameraDevice.createCaptureSession(Arrays.asList(surface), new CameraCaptureSession.StateCallback() {
            @Override
            public void onConfigured(CameraCaptureSession session) {
                try {
                    CaptureRequest captureRequest = captureRequestBuilder.build();
                    session.setRepeatingRequest(captureRequest, null, null);
                } catch (CameraAccessException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void onConfigureFailed(CameraCaptureSession session) {
                Toast.makeText(MainActivity.this, "Failed to configure camera", Toast.LENGTH_SHORT).show();
            }
        }, null);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

5.在 Activity 的 onResume() 方法中调用 openCamera() 方法：

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

6.在 Activity 的 onPause() 方法中释放相机资源：

@Override
protected void onPause() {
    super.onPause();
    if (cameraDevice != null) {
        cameraDevice.close();
        cameraDevice = null;
    }
}