iOS 中正确切换摄像头&正确实现设置帧率的方式

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];
        

-(void) setFrameRate;
{
    
    float rate = currentCameraPosition == 1 ? 20 : 30;
    
    if ([_inputCamera respondsToSelector:@selector(activeVideoMinFrameDuration)]) {
        [_inputCamera lockForConfiguration:nil];
        _inputCamera.activeVideoMinFrameDuration = CMTimeMake(1, rate);
        _inputCamera.activeVideoMaxFrameDuration = CMTimeMake(1, rate);
        [_inputCamera unlockForConfiguration];
        
    }else{
        AVCaptureConnection *conn = [[_captureSession.outputs lastObject] connectionWithMediaType:AVMediaTypeVideo];
        if (conn.supportsVideoMinFrameDuration)
            conn.videoMinFrameDuration = CMTimeMake(1,rate);
        if (conn.supportsVideoMaxFrameDuration)
            conn.videoMaxFrameDuration = CMTimeMake(1,rate);
        
    }
}

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。