如何使用Android的camera2 API 实现一个实时二维码扫码功能？

要实现实时二维码扫描功能，可以使用 Android 的 camera2 API 和 Google 的 ZXing 库。

首先，在 AndroidManifest.xml 文件中添加相机和网络权限：

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

然后，在 build.gradle 文件中添加 ZXing 库的依赖项：

implementation 'com.google.zxing:core:3.4.0'
implementation 'com.journeyapps:zxing-android-embedded:3.6.0'

接下来，在布局文件中添加一个 TextureView 组件用于预览相机图像，并在代码中实例化 CameraManager 和 TextureView：

<TextureView
    android:id="@+id/textureView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

private CameraManager cameraManager;
private TextureView textureView;

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

    cameraManager = (CameraManager) getSystemService(CAMERA_SERVICE);
    textureView = findViewById(R.id.textureView);
}

然后，实现 TextureView.SurfaceTextureListener 接口，以在 TextureView 就绪时打开相机，开始预览图像：

private String cameraId;
private CameraDevice cameraDevice;
private Size previewSize;
private CaptureRequest.Builder captureRequestBuilder;
private CameraCaptureSession cameraCaptureSession;
private ImageReader imageReader;

private final TextureView.SurfaceTextureListener surfaceTextureListener = new TextureView.SurfaceTextureListener() {
    @Override
    public void onSurfaceTextureAvailable(SurfaceTexture surfaceTexture, int width, int height) {
        openCamera();
    }

    @Override
    public void onSurfaceTextureSizeChanged(SurfaceTexture surfaceTexture, int width, int height) {
        configureTransform(width, height);
    }

    @Override
    public boolean onSurfaceTextureDestroyed(SurfaceTexture surfaceTexture) {
        return false;
    }

    @Override
    public void onSurfaceTextureUpdated(SurfaceTexture surfaceTexture) {

    }
};

在 openCamera() 方法中，获取可用的相机列表并选择一个相机，设置预览尺寸和输出格式，然后打开相机：

private void openCamera() {
    try {
        CameraCharacteristics characteristics = cameraManager.getCameraCharacteristics(cameraId);
        StreamConfigurationMap map = characteristics.get(CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);
        assert map != null;
        previewSize = getPreferredPreviewSize(map.getOutputSizes(SurfaceTexture.class), textureView.getWidth(), textureView.getHeight());
        imageReader = ImageReader.newInstance(previewSize.getWidth(), previewSize.getHeight(), ImageFormat.YUV_420_888, 2);
        imageReader.setOnImageAvailableListener(imageReaderListener, backgroundHandler);

        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
            return;
        }
        cameraManager.openCamera(cameraId, stateCallback, backgroundHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

在 stateCallback 中处理相机设备的状态，如果打开成功，创建一个 CaptureRequest.Builder 对象，用于构建捕获请求：

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

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

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

private void createPreviewSession() {
    try {
        SurfaceTexture surfaceTexture = textureView.getSurfaceTexture();
        assert surfaceTexture != null;
        surfaceTexture.setDefaultBufferSize(previewSize.getWidth(), previewSize.getHeight());
        Surface previewSurface = new Surface(surfaceTexture);
        Surface imageReaderSurface = imageReader.getSurface();
        captureRequestBuilder = cameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        captureRequestBuilder.addTarget(previewSurface);
        captureRequestBuilder.addTarget(imageReaderSurface);
        cameraDevice.createCaptureSession(Arrays.asList(previewSurface, imageReaderSurface),
                new CameraCaptureSession.StateCallback() {
                    @Override
                    public void onConfigured(CameraCaptureSession session) {
                        cameraCaptureSession = session;
                        updatePreview();
                    }

                    @Override
                    public void onConfigureFailed(CameraCaptureSession session) {

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

在 updatePreview() 方法中，设置捕获请求的参数并开始预览：

private void updatePreview() {
    try {
        captureRequestBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
        cameraCaptureSession.setRepeatingRequest(captureRequestBuilder.build(), null, backgroundHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

最后，在 imageReaderListener 中处理相机输出的图像数据，解码二维码并将结果显示在界面上：

private final ImageReader.OnImageAvailableListener imageReaderListener = new ImageReader.OnImageAvailableListener() {
    @Override
    public void onImageAvailable(ImageReader reader) {
        Image image = reader.acquireLatestImage();
        ByteBuffer buffer = image.getPlanes()[0].getBuffer();
        byte[] data = new byte[buffer.remaining()];
        buffer.get(data);
        image.close();

        try {
            PlanarYUVLuminanceSource source = new PlanarYUVLuminanceSource(data, previewSize.getWidth(), previewSize.getHeight(), 0, 0, previewSize.getWidth(), previewSize.getHeight(), false);
            BinaryBitmap bitmap = new BinaryBitmap(new HybridBinarizer(source));
            Result result = new MultiFormatReader().decode(bitmap);
            runOnUiThread(() -> textView.setText(result.getText()));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
};

完整的代码示例可以参考 https://github.com/edwardw1987/Camera2QRCodeScanner 。注意，在实际开发中，可能需要进行更多的错误处理和性能优化。