解释camera api2步骤与最简单的完整demo,包括xml

Camera API2是Android系统提供的相机框架，它允许应用程序完全控制相机设置和图像捕获过程。以下是使用Camera API2的基本步骤：

1. 获取相机管理器：使用getSystemService()方法获取相机管理器(CameraManager)实例。

2. 获取相机设备ID：通过CameraManager的getCameraIdList()方法获取相机设备ID，这个ID可以用于打开特定的相机设备。

3. 打开相机：使用CameraManager.openCamera()方法打开相机设备。在打开相机时需要传入一个CameraDevice.StateCallback回调，以便在相机打开成功或者失败时得到通知。

4. 创建预览会话：在相机打开成功后，创建一个预览会话(CameraCaptureSession)。预览会话可以用于展示相机捕获的实时预览图像，也可以用于拍照。

5. 配置预览：创建一个CaptureRequest.Builder实例，用于设置预览请求参数。其中，可以设置预览尺寸、预览帧率、自动对焦模式等等。

6. 开始预览：使用CaptureSession.setRepeatingRequest()方法开始预览。

下面是一个最简单的完整demo，包括XML：

activity_main.xml：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

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

</RelativeLayout>

MainActivity.java：

public class MainActivity extends AppCompatActivity {

    private TextureView mTextureView;
    private CameraDevice mCameraDevice;
    private CameraCaptureSession mCaptureSession;
    private CaptureRequest.Builder mPreviewRequestBuilder;

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

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

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

                @Override
                public void onSurfaceTextureUpdated(SurfaceTexture surfaceTexture) {
                }
            };

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

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

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

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

        mTextureView = findViewById(R.id.texture_view);
        mTextureView.setSurfaceTextureListener(mSurfaceTextureListener);
    }

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

        if (mTextureView.isAvailable()) {
            openCamera();
        } else {
            mTextureView.setSurfaceTextureListener(mSurfaceTextureListener);
        }
    }

    @Override
    protected void onPause() {
        closeCamera();

        super.onPause();
    }

    private void openCamera() {
        CameraManager cameraManager = (CameraManager) getSystemService(Context.CAMERA_SERVICE);

        try {
            String cameraId = cameraManager.getCameraIdList()[0];

            if (ActivityCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, 0);
                return;
            }

            cameraManager.openCamera(cameraId, mStateCallback, null);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }

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

        if (mCameraDevice != null) {
            mCameraDevice.close();
            mCameraDevice = null;
        }
    }

    private void createPreviewSession() {
        SurfaceTexture surfaceTexture = mTextureView.getSurfaceTexture();
        surfaceTexture.setDefaultBufferSize(mTextureView.getWidth(), mTextureView.getHeight());
        Surface previewSurface = new Surface(surfaceTexture);

        try {
            mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
            mPreviewRequestBuilder.addTarget(previewSurface);

            mCameraDevice.createCaptureSession(Collections.singletonList(previewSurface),
                    new CameraCaptureSession.StateCallback() {
                        @Override
                        public void onConfigured(CameraCaptureSession cameraCaptureSession) {
                            mCaptureSession = cameraCaptureSession;

                            try {
                                mCaptureSession.setRepeatingRequest(mPreviewRequestBuilder.build(), null, null);
                            } catch (CameraAccessException e) {
                                e.printStackTrace();
                            }
                        }

                        @Override
                        public void onConfigureFailed(CameraCaptureSession cameraCaptureSession) {
                        }
                    }, null);
        } catch (CameraAccessException e) {
            e.printStackTrace();
        }
    }
}

这个demo展示了如何在TextureView上预览相机捕获的实时图像。在打开相机时，我们通过getCameraIdList()方法获取可用的相机设备ID列表，然后使用openCamera()方法打开相机设备。在相机打开成功后，我们创建了一个预览会话，并且设置了CaptureRequest.Builder的预览请求参数。最后，通过setRepeatingRequest()方法开始预览。