如何在 Android 平台上编写一个实现了摄像头拍照和录像功能，并能切换前后摄像头的代码示例（Demo）？

在Android平台上编写一个简单的相机应用，可以使用`CameraX`库，它提供了一套现代、简单易用的API来进行设备访问。下面是一个简化的代码示例，展示了如何打开摄像头，切换前后摄像头以及拍照录像的基本流程：

import android.content.Context;
import android.graphics.ImageReader;
import android.hardware.camera2.CameraAccessException;
import android.hardware.camera2.CameraCaptureSession;
import android.hardware.camera2.CaptureRequest;
import android.hardware.camera2.TotalCaptureResult;
import android.media.MediaRecorder;
import android.os.Bundle;
import android.util.Size;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.lifecycle.LifecycleOwner;

import com.google.android.gms.vision.Frame;

public class CameraActivity extends AppCompatActivity implements ImageReader.OnImageAvailableListener {

    private static final int REQUEST_CAMERA_PERMISSION = 1;
    private CameraManager cameraManager;
    private CameraXPreview cameraxPreview;
    private MediaRecorder mediaRecorder;
    private String cameraId;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, REQUEST_CAMERA_PERMISSION);
        setupCamera();
    }

    private void setupCamera() {
        try {
            cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);
            cameraId = cameraManager.getCameraIdList()[0]; // 获取可用的第一个摄像头

            cameraManager.openCamera(cameraId, new CameraDevice.StateCallback() {
                @Override
                public void onOpened(@NonNull CameraDevice camera) {
                    createCameraXPreview(camera);
                }

                // 更多回调...

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

                // 更多回调...
            }, null);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

    private void createCameraXPreview(CameraDevice camera) {
        // 创建CameraX preview
        cameraxPreview = new CameraXPreview(this, camera, Size.of(640, 480)); // 使用默认大小，可以根据需要调整
        setContentView(cameraxPreview);

        // 初始化媒体录音
        initMediaRecorder();
    }

    private void initMediaRecorder() {
        mediaRecorder = new MediaRecorder();
        mediaRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);
        mediaRecorder.setOutputFormat(MediaRecorder.OutputFormat.MPEG_4);
        mediaRecorder.setVideoSource(MediaRecorder.VideoSource.CAMERA);
        mediaRecorder.setVideoSize(640, 480); // 视频尺寸与preview一致
        try {
            mediaRecorder.prepare(); // 设置好录制参数后可以立即开始录制
        } catch (IllegalStateException | IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onImageAvailable(ImageReader reader) {
        // 在这里处理接收到的帧，例如通过Vision API进行图像识别
        Frame frame = reader.acquireNextFrame();
        if (frame != null) {
            // 进行图像分析...
            frame.close();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case REQUEST_CAMERA_PERMISSION:
                if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    setupCamera();
                } else {
                    Toast.makeText(this, "权限请求未通过", Toast.LENGTH_SHORT).show();
                    finish();
                }
                break;
            default:
                super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }

    // 更多方法如切换前后摄像头、拍照、停止录像等需在此添加相应的逻辑...