使用camera2 实现相机预览

使用Camera2 Api实现相机预览需要以下步骤：

1. 实现CameraDevice.StateCallback接口，当相机设备状态发生变化时会回调该接口。
2. 获取相机管理器，并申请相机权限。
3. 打开相机，并设置预览Surface和相机回调。
4. 获取预览数据，进行相应的渲染和处理。

以下是示例代码：

public class CameraPreviewActivity extends AppCompatActivity implements
        SurfaceHolder.Callback, CameraDevice.StateCallback {

    private CameraManager mCameraManager;
    private String mCameraId;

    private CameraDevice mCameraDevice;
    private CameraCaptureSession mCaptureSession;
    private SurfaceHolder mSurfaceHolder;

    private Size mPreviewSize;

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

        SurfaceView surfaceView = findViewById(R.id.surface_view);
        mSurfaceHolder = surfaceView.getHolder();
        mSurfaceHolder.addCallback(this);

        mCameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);

        try {
            mCameraId = mCameraManager.getCameraIdList()[0];
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }

        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.CAMERA)
                != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, 1);
        } else {
            openCamera();
        }
    }

    private void openCamera() {
        try {
            mCameraManager.openCamera(mCameraId, this, null);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onOpened(CameraDevice camera) {
        mCameraDevice = camera;

        try {
            List<Surface> surfaces = new ArrayList<>();

            // 设置预览Surface
            Surface previewSurface = mSurfaceHolder.getSurface();
            surfaces.add(previewSurface);

            // 创建相机会话
            mCameraDevice.createCaptureSession(surfaces, new CameraCaptureSession.StateCallback() {
                @Override
                public void onConfigured(CameraCaptureSession session) {
                    mCaptureSession = session;
                    startPreview();
                }

                @Override
                public void onConfigureFailed(CameraCaptureSession session) {
                    Log.e("CameraPreview", "createCaptureSession failed");
                }
            }, null);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

    private void startPreview() {
        try {
            // 创建预览请求
            CaptureRequest.Builder previewBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
            previewBuilder.addTarget(mSurfaceHolder.getSurface());

            //设置自动对焦模式
            previewBuilder.set(CaptureRequest.CONTROL_AF_MODE,
                    CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);

            // 开始预览
            mCaptureSession.setRepeatingRequest(previewBuilder.build(), null, null);

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

    @Override
   public void onSurfaceCreated(SurfaceHolder holder) {
    }
    
    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
        // 获取预览大小
        try {
            CameraCharacteristics characteristics = mCameraManager.getCameraCharacteristics(mCameraId);
            StreamConfigurationMap map = characteristics.get(CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);
            mPreviewSize = map.getOutputSizes(SurfaceHolder.class)[0];

            // 更新Surface大小
            int rotatedPreviewWidth = width;
            int rotatedPreviewHeight = height;
            int maxWidth = mSurfaceHolder.getSurfaceFrame().width();
            int maxHeight = mSurfaceHolder.getSurfaceFrame().height();
            if (maxWidth > 0 && maxHeight > 0) {
                rotatedPreviewWidth = Math.min(maxWidth, maxHeight);
                rotatedPreviewHeight = Math.max(maxWidth, maxHeight);
            }
            mSurfaceHolder.setFixedSize(mPreviewSize.getWidth(), mPreviewSize.getHeight());
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        mCameraDevice.close();
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        if (requestCode == 1) {
            if (grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                openCamera();
            } else {
                Toast.makeText(this, "Camera permission not granted", Toast.LENGTH_SHORT).show();
            }
        }
    }

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

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

在AndroidManifest.xml中添加相机权限：

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

在布局文件中添加SurfaceView：

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