什么是Camera2

Google在2014年发布了Android5.0 ， 在Android5.0的API中我们可以发现，以往的Camera类已经被遗弃，不再建议使用，如果继续在5.0以后的系统版本中使用Camera类开发相机，会出现各种各样的兼容问题，那么我们该怎么办呢？
不必担心，Google在新的API中为我们提供了新的相机类，Camera2。该类能够提供更丰富的功能以及更高的性能。相对的，因为全程异步的关系，它的使用难度也有所提高，并且网络上相关的中文教程并不多。下面的示例代码为使用Camera2实现预览与闪关灯的功能。

@TargetApi(Build.VERSION_CODES.LOLLIPOP)
public class FlashActivity extends Activity implements TextureView.SurfaceTextureListener,Runnable {
private ImageButton mSwitchBtn;
private boolean isOpen;
private CameraManager mCameraManager;
private TextureView mPreviewView;
private Handler mHandler;
private CameraCaptureSession mSession;
private HandlerThread mHandlerThread;
private Size mPreviewSize;
private CaptureRequest.Builder mPreviewBuilder;
private boolean sos;
private TextView mSosBtn;
private TextView mScreenBtn;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_flash);

    init();
    initLooper();
    bindEvents();
}

/**
 * 初始化控件
 */
private void init() {
    mSwitchBtn = (ImageButton) findViewById(R.id.switchBtn);
    mPreviewView = (TextureView) findViewById(R.id.texture);
    mSosBtn = (TextView) findViewById(R.id.sosBtn);
    mScreenBtn = (TextView) findViewById(R.id.screenBtn);
    mPreviewView.setSurfaceTextureListener(this);
}

private void bindEvents() {
    mSwitchBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (!isOpen) {
                openFlash();
                isOpen = true;
                mSwitchBtn.setBackgroundResource(R.mipmap.open);
            } else {
                closeFlash();
                isOpen = false;
                mSwitchBtn.setBackgroundResource(R.mipmap.close);
            }
        }
    });

    mSosBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if (!sos) {
                sos = true;
                mSosBtn.setText("SOS ON");
            } else {
                sos = false;
                mSosBtn.setText("SOS OFF");
            }

            new Thread(FlashActivity.this).start();
        }
    });


    mScreenBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            startActivity(new Intent(FlashActivity.this, ColorActivity.class));
        }
    });
}

private void closeFlash() {
    mPreviewBuilder.set(CaptureRequest.FLASH_MODE,
            CaptureRequest.FLASH_MODE_OFF);
    updatePreview(mSession);
}

private void openFlash() {
    mPreviewBuilder.set(CaptureRequest.FLASH_MODE,
            CaptureRequest.FLASH_MODE_TORCH);
    updatePreview(mSession);
}

//需要一个子线程的looper，因为camera2是全程异步的
public void initLooper() {
    mHandlerThread = new HandlerThread("Camera2");
    mHandlerThread.start(); //开启handle线程
    mHandler = new Handler(mHandlerThread.getLooper());//用handler线程中获取的looper，实例化一个handler
}

@TargetApi(Build.VERSION_CODES.LOLLIPOP)
@Override
public void onSurfaceTextureAvailable(SurfaceTexture surface, int width, int height) {
    //获得CameraManager
    mCameraManager = (CameraManager) this.getSystemService(CAMERA_SERVICE);
    //获得属性
    CameraCharacteristics characteristics = null;
    try {
        characteristics = mCameraManager.getCameraCharacteristics(0 + "");
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
    //支持的STREAM CONFIGURATION
    StreamConfigurationMap map = characteristics.get(CameraCharacteristics.SCALER_STREAM_CONFIGURATION_MAP);
    //显示的size
    mPreviewSize = map.getOutputSizes(SurfaceTexture.class)[0];
    //打开相机
    try {
        mCameraManager.openCamera(0 + "", mCameraDeviceStateCallback, mHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

@Override
public void onSurfaceTextureSizeChanged(SurfaceTexture surface, int width, int height) {

}

@Override
public boolean onSurfaceTextureDestroyed(SurfaceTexture surface) {
    return false;
}

@Override
public void onSurfaceTextureUpdated(SurfaceTexture surface) {

}

private CameraDevice.StateCallback mCameraDeviceStateCallback = new CameraDevice.StateCallback() {
    @Override
    public void onOpened(CameraDevice camera) {
        startPreview(camera); //在相机被打开的回调里 调用startPreview传进相机对象 开始预览
    }

    @Override
    public void onDisconnected(CameraDevice camera) {

    }

    @Override
    public void onError(CameraDevice camera, int error) {

    }
};

private void startPreview(CameraDevice camera) {
    //获取SurfaceTexture对象用于预览相机图像
    SurfaceTexture surfaceTexture = mPreviewView.getSurfaceTexture();
    //设置预览的尺寸
    surfaceTexture.setDefaultBufferSize(mPreviewView.getWidth(), mPreviewView.getHeight());
    Surface surface = new Surface(surfaceTexture);

    try {
        //获得图像信息，参数代表是预览图像
        mPreviewBuilder = camera.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
        mPreviewBuilder.addTarget(surface); //设置页面预览的位置
        //创建会话
        camera.createCaptureSession(Arrays.asList(surface), mSessionStateCallback, mHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

private CameraCaptureSession.StateCallback mSessionStateCallback = new CameraCaptureSession.StateCallback() {
    @Override
    public void onConfigured(CameraCaptureSession session) {
        mSession = session; //将session对象，作用域扩大至全局，以便开关闪光灯使用
        updatePreview(session);//更新信息
    }

    @Override
    public void onConfigureFailed(CameraCaptureSession session) {

    }
};

private void updatePreview(CameraCaptureSession session) {
    try {
        session.setRepeatingRequest(mPreviewBuilder.build(), null, mHandler);
    } catch (CameraAccessException e) {
        e.printStackTrace();
    }
}

@Override
public void run() {
    while (sos) {
        try {
            Thread.sleep(200); //线程休眠
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        if (!isOpen) {
            openFlash();
            isOpen = true;
        } else {
            closeFlash();
            isOpen = false;
        }
    }
}  }