1. 概述
最近在做一些關于人臉識別的項目，需要用到 Android 相機的預覽功能。網上查閱相關資料后，發現 Android 5.0 及以后的版本中，原有的 Camera API 已經被 Camera2 API 所取代。
全新的 Camera2 在 Camera 的基礎上進行了改造，大幅提升了 Android 系統的拍照功能。它通過以下幾個類與方法來實現相機預覽時的工作過程：
•CameraManager ：攝像頭管理器，主要用于檢測系統攝像頭、打開系統攝像頭等；
•CameraDevice : 用于描述系統攝像頭，可用于關閉相機、創建相機會話、發送拍照請求等；
•CameraCharacteristics ：用于描述攝像頭所支持的各種特性；
•CameraCaptureSession ：當程序需要預覽、拍照時，都需要先通過 CameraCaptureSession 來實現。該會話通過調用方法 setRepeatingRequest() 實現預覽；
•CameraRequest ：代表一次捕獲請求，用于描述捕獲圖片的各種參數設置；
•CameraRequest.Builder ：負責生成 CameraRequest 對象。
2. 相機預覽
下面通過源碼來講解如何使用 Camera2 來實現相機的預覽功能。
2.1 相機權限設置
<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 < 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 動態申請相機權限
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); } } // 權限判斷，當系統版本大于23時，才有必要判斷是否獲取權限 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 對象構建一個 Handler 用于相機回調。
@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() 中有對應的 HandlerThread 關閉方法。
當屏幕關閉后重新開啟，SurfaceTexture 已經就緒，此時不會觸發 onSurfaceTextureAvailable 回調。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限，然后調用 setUpCameraOutputs 函數對相機參數進行設置（包括指定攝像頭、相機預覽方向以及預覽尺寸的設定等），接下來調用 configureTransform 函數對預覽圖片的大小和方向進行調整，最后獲取 CameraManager 對象開啟相機。因為相機有可能會被其他進程同時訪問，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變化回調函數 mStateCallback，如果相機成功開啟，則開始創建相機預覽會話。
private void createCameraPreviewSession() { try { // 獲取 texture 實例 SurfaceTexture texture = mTextureView.getSurfaceTexture(); assert texture != null; // 設置 TextureView 緩沖區大小 texture.setDefaultBufferSize(mPreviewSize.getWidth(), mPreviewSize.getHeight()); // 獲取 Surface 顯示預覽數據 Surface surface = new Surface(texture); // 構建適合相機預覽的請求 mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW); // 設置 surface 作為預覽數據的顯示界面 mPreviewRequestBuilder.addTarget(surface); // 創建相機捕獲會話用于預覽 mCameraDevice.createCaptureSession(Arrays.asList(surface), new CameraCaptureSession.StateCallback() { @Override public void onConfigured(@NonNull CameraCaptureSession cameraCaptureSession) { // 如果相機關閉則返回 if (null == mCameraDevice) { return; } // 如果會話準備好則開啟預覽 mCaptureSession = cameraCaptureSession; try { // 自動對焦 mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_CONTINUOUS_PICTURE); mPreviewRequest = mPreviewRequestBuilder.build(); // 設置反復捕獲數據的請求，預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 實現相機預覽的主要過程。
3. Demo 源碼
Github：Camera2Preview
4. 參考
• https://github.com/googlesamples/android-Camera2Basic
總結
以上所述是小編給大家介紹的Android Camera2 實現預覽功能，希望對大家有所幫助，如果大家有任何疑問請給我留言，小編會及時回復大家的。在此也非常感謝大家對VEVB武林網網站的支持！
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