1. 概述

最近在做一些关于人脸识别的项目，需要用到 Android 相机的预览功能。网上查阅相关资料后，发现 Android 5.0 及以后的版本中，原有的 Camera API 已经被 Camera2 API 所取代。

全新的 Camera2 在 Camera 的基础上进行了改造，大幅提升了 Android 系统的拍照功能。它通过以下几个类与方法来实现相机预览时的工作过程：

•CameraManager ：摄像头管理器，主要用于检测系统摄像头、打开系统摄像头等；
•CameraDevice : 用于描述系统摄像头，可用于关闭相机、创建相机会话、发送拍照请求等；
•CameraCharacteristics ：用于描述摄像头所支持的各种特性；
•CameraCaptureSession ：当程序需要预览、拍照时，都需要先通过 CameraCaptureSession 来实现。该会话通过调用方法 setRepeatingRequest() 实现预览；
•CameraRequest ：代表一次捕获请求，用于描述捕获图片的各种参数设置；
•CameraRequest.Builder ：负责生成 CameraRequest 对象。

2. 相机预览

下面通过源码来讲解如何使用 Camera2 来实现相机的预览功能。

2.1 相机权限设置

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

2.2 App 布局

•activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:id="@+id/container"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:background="#000"
 tools:context=".MainActivity">
</FrameLayout>
•fragment_camera.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 tools:context=".CameraFragment">
 <com.lightweh.camera2preview.AutoFitTextureView
 android:id="@+id/textureView"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_centerVertical="true"
 android:layout_centerHorizontal="true" />
</RelativeLayout>

2.3 相机自定义View

public class AutoFitTextureView extends TextureView {
 private int mRatioWidth = 0;
 private int mRatioHeight = 0;
 public AutoFitTextureView(Context context) {
 this(context, null);
 }
 public AutoFitTextureView(Context context, AttributeSet attrs) {
 this(context, attrs, 0);
 }
 public AutoFitTextureView(Context context, AttributeSet attrs, int defStyle) {
 super(context, attrs, defStyle);
 }
 public void setAspectRatio(int width, int height) {
 if (width < 0 || height < 0) {
 throw new IllegalArgumentException("Size cannot be negative.");
 }
 mRatioWidth = width;
 mRatioHeight = height;
 requestLayout();
 }
 @Override
 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
 super.onMeasure(widthMeasureSpec, heightMeasureSpec);
 int width = MeasureSpec.getSize(widthMeasureSpec);
 int height = MeasureSpec.getSize(heightMeasureSpec);
 if (0 == mRatioWidth || 0 == mRatioHeight) {
 setMeasuredDimension(width, height);
 } else {
 if (width < height * mRatioWidth / mRatioHeight) {
 setMeasuredDimension(width, width * mRatioHeight / mRatioWidth);
 } else {
 setMeasuredDimension(height * mRatioWidth / mRatioHeight, height);
 }
 }
 }
}

2.4 动态申请相机权限

public class MainActivity extends AppCompatActivity {
 private static final int REQUEST_PERMISSION = 1;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 if (hasPermission()) {
 if (null == savedInstanceState) {
 setFragment();
 }
 } else {
 requestPermission();
 }
 }
 @Override
 public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
 if (requestCode == REQUEST_PERMISSION) {
 if (grantResults.length == 1 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
 setFragment();
 } else {
 requestPermission();
 }
 } else {
 super.onRequestPermissionsResult(requestCode, permissions, grantResults);
 }
 }
 // 权限判断，当系统版本大于23时，才有必要判断是否获取权限
 private boolean hasPermission() {
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
 return checkSelfPermission(Manifest.permission.CAMERA) == PackageManager.PERMISSION_GRANTED;
 } else {
 return true;
 }
 }
 // 请求相机权限
 private void requestPermission() {
 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
 if (shouldShowRequestPermissionRationale(Manifest.permission.CAMERA)) {
 Toast.makeText(MainActivity.this, "Camera permission are required for this demo", Toast.LENGTH_LONG).show();
 }
 requestPermissions(new String[]{Manifest.permission.CAMERA}, REQUEST_PERMISSION);
 }
 }
 // 启动相机Fragment
 private void setFragment() {
 getSupportFragmentManager()
 .beginTransaction()
 .replace(R.id.container, CameraFragment.newInstance())
 .commitNowAllowingStateLoss();
 }
}

2.5 开启相机预览

首先，在onResume()中，我们需要开启一个 HandlerThread，然后利用该线程的 Looper 对象构建一个 Handler 用于相机回调。

@Override
public void onResume() {
 super.onResume();
 startBackgroundThread();

 // When the screen is turned off and turned back on, the SurfaceTexture is 
 // already available, and "onSurfaceTextureAvailable" will not be called. In 
 // that case, we can open a camera and start preview from here (otherwise, we 
 // wait until the surface is ready in the SurfaceTextureListener).
 if (mTextureView.isAvailable()) {
 openCamera(mTextureView.getWidth(), mTextureView.getHeight());
 } else {
 mTextureView.setSurfaceTextureListener(mSurfaceTextureListener);
 }
}
private void startBackgroundThread() {
 mBackgroundThread = new HandlerThread("CameraBackground");
 mBackgroundThread.start();
 mBackgroundHandler = new Handler(mBackgroundThread.getLooper());
}
同时，在 onPause() 中有对应的 HandlerThread 关闭方法。

当屏幕关闭后重新开启，SurfaceTexture 已经就绪，此时不会触发 onSurfaceTextureAvailable 回调。因此，我们判断 mTextureView 如果可用，则直接打开相机，否则等待 SurfaceTexture 回调就绪后再开启相机。

private void openCamera(int width, int height) {
 if (ContextCompat.checkSelfPermission(getActivity(), Manifest.permission.CAMERA)
 != PackageManager.PERMISSION_GRANTED) {
 return;
 }
 setUpCameraOutputs(width, height);
 configureTransform(width, height);
 Activity activity = getActivity();
 CameraManager manager = (CameraManager) activity.getSystemService(Context.CAMERA_SERVICE);
 try {
 if (!mCameraOpenCloseLock.tryAcquire(2500, TimeUnit.MILLISECONDS)) {
 throw new RuntimeException("Time out waiting to lock camera opening.");
 }
 manager.openCamera(mCameraId, mStateCallback, mBackgroundHandler);
 } catch (CameraAccessException e) {
 e.printStackTrace();
 } catch (InterruptedException e) {
 throw new RuntimeException("Interrupted while trying to lock camera opening.", e);
 }
}
开启相机时，我们首先判断是否具备相机权限，然后调用 setUpCameraOutputs 函数对相机参数进行设置（包括指定摄像头、相机预览方向以及预览尺寸的设定等），接下来调用 configureTransform 函数对预览图片的大小和方向进行调整，最后获取 CameraManager 对象开启相机。因为相机有可能会被其他进程同时访问，所以在开启相机时需要加锁。

private final CameraDevice.StateCallback mStateCallback = new CameraDevice.StateCallback() {
 @Override
 public void onOpened(@NonNull CameraDevice cameraDevice) {
 mCameraOpenCloseLock.release();
 mCameraDevice = cameraDevice;
 createCameraPreviewSession();
 }
 @Override
 public void onDisconnected(@NonNull CameraDevice cameraDevice) {
 mCameraOpenCloseLock.release();
 cameraDevice.close();
 mCameraDevice = null;
 }
 @Override
 public void onError(@NonNull CameraDevice cameraDevice, int error) {
 mCameraOpenCloseLock.release();
 cameraDevice.close();
 mCameraDevice = null;
 Activity activity = getActivity();
 if (null != activity) {
 activity.finish();
 }
 }
};

相机开启时还会指定相机的状态变化回调函数 mStateCallback，如果相机成功开启，则开始创建相机预览会话。

private void createCameraPreviewSession() {
 try {
 // 获取 texture 实例
 SurfaceTexture texture = mTextureView.getSurfaceTexture();
 assert texture != null;
 // 设置 TextureView 缓冲区大小
 texture.setDefaultBufferSize(mPreviewSize.getWidth(), mPreviewSize.getHeight());
 // 获取 Surface 显示预览数据
 Surface surface = new Surface(texture);
 // 构建适合相机预览的请求
 mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
 // 设置 surface 作为预览数据的显示界面
 mPreviewRequestBuilder.addTarget(surface);
 // 创建相机捕获会话用于预览
 mCameraDevice.createCaptureSession(Arrays.asList(surface),
 new CameraCaptureSession.StateCallback() {
  @Override
  public void onConfigured(@NonNull CameraCaptureSession cameraCaptureSession) {
  // 如果相机关闭则返回
  if (null == mCameraDevice) {
  return;
  }
  // 如果会话准备好则开启预览
  mCaptureSession = cameraCaptureSession;
  try {
  // 自动对焦
  mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE,
   CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
  mPreviewRequest = mPreviewRequestBuilder.build();
  // 设置反复捕获数据的请求，预览界面一直显示画面
  mCaptureSession.setRepeatingRequest(mPreviewRequest,
   null, mBackgroundHandler);
  } catch (CameraAccessException e) {
  e.printStackTrace();
  }
  }
  @Override
  public void onConfigureFailed(
  @NonNull CameraCaptureSession cameraCaptureSession) {
  showToast("Failed");
  }
 }, null
 );
 } catch (CameraAccessException e) {
 e.printStackTrace();
 }
}

以上便是 Camera2 API 实现相机预览的主要过程。

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