使用setActiveVideoMinFrameDuration设置帧率为60fps时应用Crash

用如下方式设置帧率为60fps时，app 会crash

// 会崩溃
AVCaptureDevice *captureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
[captureDevice lockForConfiguration:NULL];
[captureDevice setActiveVideoMinFrameDuration:CMTimeMake(1, frameRate)];
[captureDevice unlockForConfiguration];

故更改设置fps的写法

    AVCaptureDevice *videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];
    for(AVCaptureDeviceFormat *vFormat in [videoDevice formats] ) {
        CMFormatDescriptionRef description= vFormat.formatDescription;
        float maxRate = ((AVFrameRateRange*) [vFormat.videoSupportedFrameRateRanges objectAtIndex:0]).maxFrameRate;
        if (maxRate > frameRate - 1 &&
            CMFormatDescriptionGetMediaSubType(description)==kCVPixelFormatType_420YpCbCr8BiPlanarFullRange) {
            if ([videoDevice lockForConfiguration:nil]) {
                videoDevice.activeFormat = vFormat;
                [videoDevice setActiveVideoMinFrameDuration:CMTimeMake(10, frameRate * 10)];
                [videoDevice setActiveVideoMaxFrameDuration:CMTimeMake(10, frameRate * 10)];
                [videoDevice unlockForConfiguration];
                break;
            }
        }
    }