1. 概述

最近在做一些關(guān)于人臉識別的項目，需要用到 Android 相機的預(yù)覽功能。網(wǎng)上查閱相關(guān)資料后，發(fā)現(xiàn) Android 5.0 及以后的版本中，原有的 Camera API 已經(jīng)被 Camera2 API 所取代。

全新的 Camera2 在 Camera 的基礎(chǔ)上進(jìn)行了改造，大幅提升了 Android 系統(tǒng)的拍照功能。它通過以下幾個類與方法來實現(xiàn)相機預(yù)覽時的工作過程：

?CameraManager ：攝像頭管理器，主要用于檢測系統(tǒng)攝像頭、打開系統(tǒng)攝像頭等；
?CameraDevice : 用于描述系統(tǒng)攝像頭，可用于關(guān)閉相機、創(chuàng)建相機會話、發(fā)送拍照請求等；
?CameraCharacteristics ：用于描述攝像頭所支持的各種特性；
?CameraCaptureSession ：當(dāng)程序需要預(yù)覽、拍照時，都需要先通過 CameraCaptureSession 來實現(xiàn)。該會話通過調(diào)用方法 setRepeatingRequest() 實現(xiàn)預(yù)覽；
?CameraRequest ：代表一次捕獲請求，用于描述捕獲圖片的各種參數(shù)設(shè)置；
?CameraRequest.Builder ：負(fù)責(zé)生成 CameraRequest 對象。

2. 相機預(yù)覽

下面通過源碼來講解如何使用 Camera2 來實現(xiàn)相機的預(yù)覽功能。

2.1 相機權(quán)限設(shè)置

2.2 App 布局

?activity_main.xml

?fragment_camera.xml
?

2.3 相機自定義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?
2.4 動態(tài)申請相機權(quán)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);
?}
?}?//?權(quán)限判斷，當(dāng)系統(tǒng)版本大于23時，才有必要判斷是否獲取權(quán)限
?private?boolean?hasPermission()?{?if?(Build.VERSION.SDK_INT?>=?Build.VERSION_CODES.M)?{?return?checkSelfPermission(Manifest.permission.CAMERA)?==?PackageManager.PERMISSION_GRANTED;
?}?else?{?return?true;
?}
?}?//?請求相機權(quán)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);
?}
?}?//?啟動相機Fragment
?private?void?setFragment()?{
?getSupportFragmentManager()
?.beginTransaction()
?.replace(R.id.container,?CameraFragment.newInstance())
?.commitNowAllowingStateLoss();
?}
}
2.5 開啟相機預(yù)覽
首先，在onResume()中，我們需要開啟一個 HandlerThread，然后利用該線程的 Looper 對象構(gòu)建一個 Handler 用于相機回調(diào)。
@Overridepublic?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());
}
同時，在?onPause()?中有對應(yīng)的?HandlerThread?關(guān)閉方法。

當(dāng)屏幕關(guān)閉后重新開啟，SurfaceTexture?已經(jīng)就緒，此時不會觸發(fā)?onSurfaceTextureAvailable?回調(diào)。因此，我們判斷?mTextureView?如果可用，則直接打開相機，否則等待?SurfaceTexture?回調(diào)就緒后再開啟相機。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);
?}
}
開啟相機時，我們首先判斷是否具備相機權(quán)限，然后調(diào)用?setUpCameraOutputs?函數(shù)對相機參數(shù)進(jìn)行設(shè)置（包括指定攝像頭、相機預(yù)覽方向以及預(yù)覽尺寸的設(shè)定等），接下來調(diào)用?configureTransform?函數(shù)對預(yù)覽圖片的大小和方向進(jìn)行調(diào)整，最后獲取?CameraManager?對象開啟相機。因為相機有可能會被其他進(jìn)程同時訪問，所以在開啟相機時需要加鎖。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();
?}
?}
};
相機開啟時還會指定相機的狀態(tài)變化回調(diào)函數(shù) mStateCallback，如果相機成功開啟，則開始創(chuàng)建相機預(yù)覽會話。
private?void?createCameraPreviewSession()?{?try?{?//?獲取?texture?實例
?SurfaceTexture?texture?=?mTextureView.getSurfaceTexture();?assert?texture?!=?null;?//?設(shè)置?TextureView?緩沖區(qū)大小
?texture.setDefaultBufferSize(mPreviewSize.getWidth(),?mPreviewSize.getHeight());?//?獲取?Surface?顯示預(yù)覽數(shù)據(jù)
?Surface?surface?=?new?Surface(texture);?//?構(gòu)建適合相機預(yù)覽的請求
?mPreviewRequestBuilder?=?mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);?//?設(shè)置?surface?作為預(yù)覽數(shù)據(jù)的顯示界面
?mPreviewRequestBuilder.addTarget(surface);?//?創(chuàng)建相機捕獲會話用于預(yù)覽
?mCameraDevice.createCaptureSession(Arrays.asList(surface),?new?CameraCaptureSession.StateCallback()?{??@Override
??public?void?onConfigured(@NonNull?CameraCaptureSession?cameraCaptureSession)?{??//?如果相機關(guān)閉則返回
??if?(null?==?mCameraDevice)?{??return;
??}??//?如果會話準(zhǔn)備好則開啟預(yù)覽
??mCaptureSession?=?cameraCaptureSession;??try?{??//?自動對焦
??mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE,
???CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE);
??mPreviewRequest?=?mPreviewRequestBuilder.build();??//?設(shè)置反復(fù)捕獲數(shù)據(jù)的請求，預(yù)覽界面一直顯示畫面
??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 實現(xiàn)相機預(yù)覽的主要過程。有什么問題歡迎一起交流討論
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