说明：camera子系统系列文章针对Android10.0系统，主要针对 camera API2 + HAL3 框架进行解读。

1 预览流程简要解读

@1 由 CameraManager.openCamera 方法，打开摄像头。该方法传入三个参数:

String CameraId:传入要打开的摄像头的 Id
CameraDevice.stateCallback:即 CameraDevice.stateCallback 的实例
Handler:代表执行callback的handler，如果程序希望直接在当前线程中执行callback，则可以将handler参数设为null

关键代码如下所示：

//camera process，step1 打开camera
mCameraManager.openCamera(mCameraId, mStateCallback, mCameraHandler);

@2 实例化CameraDevice.statCallback，并重写 onOpened 方法，该方法在摄像头完成打开动作后调用，以下动作均在 onOpened 方法中完成，onOpened方法，会返回当前使用的CameraDevice。关键代码如下所示：

//camera process，step2 mStateCallback 实例化private CameraDevice.StateCallback mStateCallback = new CameraDevice.StateCallback() {@Overridepublic void onOpened(CameraDevice camera) {//重写onOpened方法，最为关键mCameraOpenCloseLock.release();mCameraDevice = camera;startCaptureSession();}@Overridepublic void onDisconnected(CameraDevice camera) {//...}@Overridepublic void onError(CameraDevice camera, int error) {//...}};

@3 调用 CameraDevice.CreateCapture(int templateType)，创建一个 CaptureRequest.Builder，templateType来区分是拍照还是预览，当预览时，我们传入CameraDevice.TEMPLATE_PREVIEW参数，该方法回返回一个对于返回的对象，我们声明一个 CaptureRequest.Builder类型的mCaptureRequestBuilder变量来接收。关键代码如下所示：

//camera process，step3 创建一个 CaptureRequest.Builder，templateType来区分是拍照还是预览
mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);

@4 调用mCaptureRequestBuilder.addTarget()方法，将显示预览用的surface的实例传入，即将显示预览用的 surface 的实例，作为一个显示层添加到该请求的目标列表中。关键代码如下所示：

//camera process，step4 将显示预览用的surface的实例传入，即将显示预览用的 surface 的实例，作为一个显示层添加到该 请求的目标列表中
mPreviewRequestBuilder.addTarget(mImageReader.getSurface());

@5 调用 CameraDevice.CreateCaptureSession()方法，该方法传入三个参数:

List<Surface>:可用于捕获目标图像数据的 Surface 集合，此处传入用于显示预览图像的 Surface 即可，即 Arrays.asList(surface)
CameraCaptureSession.stateCallback:该回调的实例
Handler:一个句柄，代表执行 callback 的 handler，如果程序希望在当前线程执行callback，可以将 Handler 参数设为 null。

//camera process，step5 将显示预览用的surface的实例传入，即将显示预览用的surface的实例，作为一个显示层添加到该请求的目标列表中
surfaceList = Arrays.asList(surface, mImageReader.getSurface());

@6 调用 CameraDevice.CreateCaptureSession()方法。该方法传入三个参数:

List<Surface>:可用于捕获目标图像数据的Surface集合，此处传入用于显示预览图像的 Surface 即可，即 Arrays.asList(surface) CameraCaptureSession.
stateCallback:该回调的实例
Handler:代表执行callback的handler，如果程序希望在当前线程执行callback，可以将 Handler 参数设为 null

关键代码整合在8中，因为 6 7 8 代码紧密相关。

@7 实例化 CameraCaptureSession.stateCallback，并重写 onConfigured 方法，该方法在摄像头设备完成自身配置的时候调用，该方法会从底层传回一个 CameraCaptureSession,该 session 可以开始处理捕获请求。

关键代码整合在8中，因为 6 7 8 代码紧密相关。

@8 调用 CameraCaptureSession.setRepeatingRequest()方法创建预览，此方法接收三个参数:

CaptureReqeust:无限重复的请求，此处传入上一步中中创建的captureRequest对象，即mCaptureRequest
CameraCaptureSession.CaptureCallback:callback的回调，不过预览中该回调中不用进行任何处理
Handler:代表执行callback的handler，如果程序希望直接在当前线程中执行callback，则可以将handler参数设为 null

关键代码如下所示：

//camera process，step6 & 7
// 6 执行createCaptureSession方法
// 7 参数中实例化 CameraCaptureSession.stateCallback，并重写 onConfigured 方法
mCameraDevice.createCaptureSession(surfaceList, new CameraCaptureSession.StateCallback() {@Overridepublic void onConfigured(CameraCaptureSession session) {mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_OFF);mCaptureSession = session;//camera process，step8 用 CameraCaptureSession.setRepeatingRequest()方法创建预览mCaptureSession.setRepeatingRequest(mPreviewRequestBuilder.build(), null, null);}@Overridepublic void onConfigureFailed(CameraCaptureSession session) {//...}@Overridepublic void onClosed(CameraCaptureSession session) {//...}
}, mCameraHandler);

2 camera预览流程代码完整解读

2.1 java源码部分（草稿）

Camera流程相关代码如下所示：

class CameraCoreManager {private static final String TAG = "CameraDemo";private Context mContext;private CameraManager mCameraManager;private String mCameraId;private HandlerThread mCameraThread;private Handler mCameraHandler;private Handler mImageHandler;private ImageReader mImageReader;private CameraDevice mCameraDevice;private CameraCharacteristics mCameraCharacteristics;//Max preview width&height that is guaranteed by Camera2 APIprivate static final int MAX_PREVIEW_WIDTH = 1920;private static final int MAX_PREVIEW_HEIGHT = 1080;//A Semaphore to prevent the app from exiting before closing the camera.private Semaphore mCameraOpenCloseLock = new Semaphore(1);private Size mPreviewSize = new Size(1920, 1080);private CaptureRequest.Builder mPreviewRequestBuilder;private CameraCaptureSession mCaptureSession;private int mFacing = CameraCharacteristics.LENS_FACING_BACK;private Choreographer.FrameCallback mFrameCallback;private SurfaceTexture mSurfaceTexture;private ImageReader.OnImageAvailableListener mImageAvailableListener = new ImageReader.OnImageAvailableListener() {@Overridepublic void onImageAvailable(ImageReader reader) {Image image = reader.acquireNextImage();Log.d(TAG, "##### onFrame: " + image.getPlanes());image.close();}};//camera process，step2 mStateCallback 实例化private CameraDevice.StateCallback mStateCallback = new CameraDevice.StateCallback() {@Overridepublic void onOpened(CameraDevice camera) {//重写onOpened方法，最为关键mCameraOpenCloseLock.release();mCameraDevice = camera;startCaptureSession();}@Overridepublic void onDisconnected(CameraDevice camera) {mCameraOpenCloseLock.release();camera.close();mCameraDevice = null;}@Overridepublic void onError(CameraDevice camera, int error) {Log.e("DEBUG", "onError: " + error);mCameraOpenCloseLock.release();camera.close();mCameraDevice = null;Log.e("DEBUG", "onError:  restart camera");stopPreview();startPreview();}};public CameraCoreManager(Context context) {mContext = context;mCameraManager = (CameraManager) context.getSystemService(Context.CAMERA_SERVICE);}public void startPreview() {if (!chooseCameraIdByFacing()) {Log.e(TAG, "Choose camera failed.");return;}mCameraThread = new HandlerThread("CameraThread");mCameraThread.start();mCameraHandler = new Handler(mCameraThread.getLooper());prepareImageReader();openCamera();}public void stopPreview() {closeCamera();if (mCameraThread != null) {mCameraThread.quitSafely();mCameraThread = null;}mCameraHandler = null;}private void prepareImageReader() {if (mImageReader != null) {mImageReader.close();}mImageReader = ImageReader.newInstance(mPreviewSize.getWidth(), mPreviewSize.getHeight(), ImageFormat.YUV_420_888, 1);mImageReader.setOnImageAvailableListener(mImageAvailableListener, mCameraHandler);}private boolean chooseCameraIdByFacing() {try {String ids[] = mCameraManager.getCameraIdList();if (ids.length == 0) {Log.e(TAG, "No available camera.");return false;}for (String cameraId : mCameraManager.getCameraIdList()) {CameraCharacteristics characteristics = mCameraManager.getCameraCharacteristics(cameraId);StreamConfigurationMap map = characteristics.get(CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);if (map == null) {continue;}Integer level = characteristics.get(CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL);if (level == null || level == CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_LEGACY) {continue;}Integer internal = characteristics.get(CameraCharacteristics.LENS_FACING);if (internal == null) {continue;}if (internal == mFacing) {mCameraId = cameraId;mCameraCharacteristics = characteristics;return true;}}mCameraId = ids[1];mCameraCharacteristics = mCameraManager.getCameraCharacteristics(mCameraId);Integer level = mCameraCharacteristics.get(CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL);if (level == null || level == CameraCharacteristics.INFO_SUPPORTED_HARDWARE_LEVEL_LEGACY) {return false;}Integer internal = mCameraCharacteristics.get(CameraCharacteristics.LENS_FACING);if (internal == null) {return false;}mFacing = CameraCharacteristics.LENS_FACING_BACK;} catch (CameraAccessException e) {e.printStackTrace();}return true;}@SuppressLint("MissingPermission")public void openCamera() {if (TextUtils.isEmpty(mCameraId)) {Log.e(TAG, "Open camera failed. No camera available");return;}try {if (!mCameraOpenCloseLock.tryAcquire(2500, TimeUnit.MILLISECONDS)) {throw new RuntimeException("Time out waiting to lock camera opening.");}//camera process，step1 打开cameramCameraManager.openCamera(mCameraId, mStateCallback, mCameraHandler);} catch (InterruptedException | CameraAccessException e) {Log.e(TAG, e.getMessage());}}private void closeCamera() {try {mCameraOpenCloseLock.acquire();if (mCaptureSession != null) {mCaptureSession.close();mCaptureSession = null;}if (mCameraDevice != null) {mCameraDevice.close();mCameraDevice = null;}if (mImageReader != null) {mImageReader.close();mImageReader = null;}} catch (InterruptedException e) {throw new RuntimeException("Interrupted while trying to lock camera closing.", e);} finally {mCameraOpenCloseLock.release();}}private void startCaptureSession() {if (mCameraDevice == null) {return;}if ((mImageReader != null || mSurfaceTexture != null)) {try {//camera process，step3 创建一个 CaptureRequest.Builder，templateType来区分是拍照还是预览mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);//camera process，step4 将显示预览用的surface的实例传入，即将显示预览用的 surface 的实例，作为一个显示层添加到该 请求的目标列表中mPreviewRequestBuilder.addTarget(mImageReader.getSurface());List<Surface> surfaceList = Arrays.asList(mImageReader.getSurface());if (mSurfaceTexture != null) {mSurfaceTexture.setDefaultBufferSize(mPreviewSize.getWidth(), mPreviewSize.getHeight());Surface surface = new Surface(mSurfaceTexture);mPreviewRequestBuilder.addTarget(mImageReader.getSurface());mPreviewRequestBuilder.addTarget(surface);//camera process，step5 将显示预览用的surface的实例传入，即将显示预览用的surface的实例，作为一个显示层添加到该请求的目标列表中surfaceList = Arrays.asList(surface, mImageReader.getSurface());}Range<Integer>[] fpsRanges = mCameraCharacteristics.get(CameraCharacteristics.CONTROL_AE_AVAILABLE_TARGET_FPS_RANGES);Log.d("DEBUG", "##### fpsRange: " + Arrays.toString(fpsRanges));//camera process，step6 & 7// 6 执行createCaptureSession方法// 7 参数中实例化 CameraCaptureSession.stateCallback，并重写 onConfigured 方法mCameraDevice.createCaptureSession(surfaceList, new CameraCaptureSession.StateCallback() {@Overridepublic void onConfigured(CameraCaptureSession session) {if (mCameraDevice == null)return;mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_OFF);mCaptureSession = session;try {if (mCaptureSession != null)//camera process，step8 用 CameraCaptureSession.setRepeatingRequest()方法创建预览mCaptureSession.setRepeatingRequest(mPreviewRequestBuilder.build(), null, null);} catch (CameraAccessException | IllegalArgumentException | IllegalStateException | NullPointerException e) {e.printStackTrace();}}@Overridepublic void onConfigureFailed(CameraCaptureSession session) {Log.e(TAG, "Failed to configure capture session");}@Overridepublic void onClosed(CameraCaptureSession session) {if (mCaptureSession != null && mCaptureSession.equals(session)) {mCaptureSession = null;}}}, mCameraHandler);} catch (CameraAccessException e) {e.printStackTrace();Log.e(TAG, e.getMessage());} catch (IllegalStateException e) {stopPreview();startPreview();} catch (UnsupportedOperationException e) {e.printStackTrace();Log.e(TAG, e.getMessage());}}}public interface FrameCallback {void onFrame(Image data);}public Size getPreviewSize() {return mPreviewSize;}public void setPreviewSize(Size previewSize) {mPreviewSize = previewSize;}public FrameCallback getFrameCallback() {return (FrameCallback) mFrameCallback;}public void setFrameCallback(FrameCallback frameCallback) {mFrameCallback = (Choreographer.FrameCallback) frameCallback;}public void setSurfaceTexture(SurfaceTexture surfaceTexture) {mSurfaceTexture = surfaceTexture;}
}

Activity UI相关代码如下所示：

public class MainActivity extends AppCompatActivity implements View.OnClickListener{private ImageButton mTakePictureBtn;private CameraCoreManager manager;private TextureView mTextureView;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_camera);manager = new CameraCoreManager(this);mTakePictureBtn = findViewById(R.id.camera_take_picture);mTakePictureBtn.setOnClickListener(this);mTextureView = findViewById(R.id.texture_view);}@Overridepublic void onResume() {super.onResume();mTextureView.setSurfaceTextureListener(new TextureView.SurfaceTextureListener() {@Overridepublic void onSurfaceTextureAvailable(SurfaceTexture surface, int width, int height) {manager.setSurfaceTexture(surface);manager.startPreview();}@Overridepublic void onSurfaceTextureSizeChanged(SurfaceTexture surface, int width, int height) {}@Overridepublic boolean onSurfaceTextureDestroyed(SurfaceTexture surface) {return false;}@Overridepublic void onSurfaceTextureUpdated(SurfaceTexture surface) {}});}@Overridepublic void onPause() {super.onPause();}@Overrideprotected void onDestroy() {super.onDestroy();manager.stopPreview();}@Overridepublic void onClick(View v) {switch (v.getId()) {case R.id.camera_take_picture://take picturebreak;}}
}

2.2 layout文件

这里涉及布局文件主要为activity_camera.xml，xml文件内容如下：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"android:layout_width="match_parent"
android:layout_height="match_parent"><LinearLayoutandroid:layout_width="match_parent"android:layout_height="wrap_content"android:background="@color/gray"android:gravity="center"android:orientation="horizontal"android:padding="20dp"app:layout_constraintBottom_toBottomOf="parent"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintRight_toRightOf="parent"><ImageButtonandroid:id="@+id/camera_take_picture"android:layout_width="60dp"android:layout_height="60dp"android:background="@drawable/ic_camera" />
</LinearLayout><TextureViewandroid:id="@+id/texture_view"android:layout_width="320dp"android:layout_height="180dp"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintTop_toTopOf="parent" /></androidx.constraintlayout.widget.ConstraintLayout>

camera图标的矢量xml文件，这里特殊记录下，文件内容为：

<?xml version="1.0" encoding="utf-8"?>
<vector xmlns:android="http://schemas.android.com/apk/res/android"
android:width="24dp"
android:height="24dp"
android:viewportHeight="24.0"
android:viewportWidth="24.0">
<pathandroid:fillColor="#FFFFFFFF"android:pathData="M12,12m-3.2,0a3.2,3.2 0,1 1,6.4 0a3.2,3.2 0,1 1,-6.4 0"/>
<pathandroid:fillColor="#FFFFFFFF"android:pathData="M9,2L7.17,4L4,4c-1.1,0 -2,0.9 -2,2v12c0,1.1 0.9,2 2,2h16c1.1,0 2,-0.9 2,-2L22,6c0,-1.1 -0.9,-2 -2,-2h-3.17L15,2L9,2zM12,17c-2.76,0 -5,-2.24 -5,-5s2.24,-5 5,-5 5,2.24 5,5 -2.24,5 -5,5z"/>
</vector>

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Android APP Camera2应用（02）预览流程

1 预览流程简要解读

2 camera预览流程代码完整解读

2.1 java源码部分（草稿）

2.2 layout文件

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