以前编写【魔拍】应用的时候，发现一个问题，设置相机按每秒20帧的速度输出视频。开始时一切正常，但是切换摄像头后，帧率就变成动态的了且小于20帧/s。解决方式：

[_captureSession stopRunning];        [_captureSession beginConfiguration];                [_captureSession removeInput:videoInput];        if ([_captureSession canAddInput:newVideoInput]){            videoInput = newVideoInput;            [_captureSession addInput:videoInput];        }                [self setFrameRate];                [_captureSession commitConfiguration];        [_captureSession startRunning];

1 -(void) setFrameRate; 2 { 3      4     float rate = currentCameraPosition == 1 ? 20 : 30; 5      6     if ([_inputCamera respondsToSelector:@selector(activeVideoMinFrameDuration)]) { 7         [_inputCamera lockForConfiguration:nil]; 8         _inputCamera.activeVideoMinFrameDuration = CMTimeMake(1, rate); 9         _inputCamera.activeVideoMaxFrameDuration = CMTimeMake(1, rate);10         [_inputCamera unlockForConfiguration];11         12     }else{13         AVCaptureConnection *conn = [[_captureSession.outputs lastObject] connectionWithMediaType:AVMediaTypeVideo];14         if (conn.supportsVideoMinFrameDuration)15             conn.videoMinFrameDuration = CMTimeMake(1,rate);16         if (conn.supportsVideoMaxFrameDuration)17             conn.videoMaxFrameDuration = CMTimeMake(1,rate);18         19     }20 }

Tips: 在切换摄像头的时候Stop Seesion,然后重新设置FrameRate，最后重新Run Seesion。