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：Camera2Preview
4. 参考
• https://github.com/googlesamples/android-Camera2Basic
总结
以上所述是小编给大家介绍的Android Camera2 实现预览功能，希望对大家有所帮助！
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