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//前后摄像头的切换
- (void)changeDeviceAction
{
    [self.session beginConfiguration];
    //改变会话的配置前一定要先开启配置，配置完成后提交配置改变
    AVCaptureDevice *currentDevice = [self.imageInput device];
    AVCaptureDevice *toChangeDevice;
    if([currentDevice position] == AVCaptureDevicePositionBack){
        toChangeDevice = [self cameraWithPosition:AVCaptureDevicePositionFront];
    }else{
        toChangeDevice = [self cameraWithPosition:AVCaptureDevicePositionBack];
    }
    //获得要调整的设备输入对象

    AVCaptureDeviceInput *toChangeDeviceInput = [[AVCaptureDeviceInput alloc]initWithDevice:toChangeDevice error:nil];
    //移除原有输入对象
    [self.session removeInput:self.imageInput];
    //添加新的输入对象
    if ([self.session canAddInput:toChangeDeviceInput]) {
        [self.session addInput:toChangeDeviceInput];
        self.imageInput = toChangeDeviceInput;
    }
    //提交会话配置
    [self.session commitConfiguration];
}
//定向获取摄像头设备
- (AVCaptureDevice *)cameraWithPosition:(AVCaptureDevicePosition)position
{
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];

    for (AVCaptureDevice *device in devices)
    {
        if ([device position] == position)
            return device;
    }
    return nil;
}