Android Camera2 预览功能实现

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。

3. Demo 源码

Github：https://github.com/lightweh/Camera2Preview

4. 参考

• https://github.com/googlesamples/android-Camera2Basic

本文链接：https://www.cnblogs.com/lightweh/p/9994927.html，转载请注明出处。