AVFoundation-高帧率捕捉

以高帧率（FPS）捕捉视频内容带来很多好处

AVCaptureDeviceFormat实例具有 videoSupportedFrameRateRanges 属性，它包含一个AVFrameRateRange对象数组，其中带有格式所支持的最小帧率、最大帧率和时长信息。

确定是否支持高帧率

@implementation AVCaptureDevice (THAdditions)

//是否支持高帧率
- (BOOL)supportsHighFrameRateCapture {
    if (![self hasMediaType:AVMediaTypeVideo]) {                            // 1
        return NO;
    }
    return [self findHighestQualityOfService].isHighFrameRate;              // 2
}

//查找设备所支持的帧率等信息
- (THQualityOfService *)findHighestQualityOfService {

    AVCaptureDeviceFormat *maxFormat = nil;
    AVFrameRateRange *maxFrameRateRange = nil;

    for (AVCaptureDeviceFormat *format in self.formats) {

        FourCharCode codecType =                                            // 3
            CMVideoFormatDescriptionGetCodecType(format.formatDescription);

        if (codecType == kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange) { // 4

            NSArray *frameRateRanges = format.videoSupportedFrameRateRanges;

            for (AVFrameRateRange *range in frameRateRanges) {              // 5
                if (range.maxFrameRate > maxFrameRateRange.maxFrameRate) {
                    maxFormat = format;
                    maxFrameRateRange = range;
                }
            }
        }
    }

    return [THQualityOfService qosWithFormat:maxFormat                      // 6
                              frameRateRange:maxFrameRateRange];

}
//开启高帧率
- (BOOL)enableMaxFrameRateCapture:(NSError **)error {

    THQualityOfService *qos = [self findHighestQualityOfService];

    if (!qos.isHighFrameRate) {                                             // 1
        if (error) {
            NSString *message = @"Device does not support high FPS capture";
            NSDictionary *userInfo = @{NSLocalizedDescriptionKey : message};

            NSUInteger code = THCameraErrorHighFrameRateCaptureNotSupported;

            *error = [NSError errorWithDomain:THCameraErrorDomain
                                         code:code
                                     userInfo:userInfo];
        }
        return NO;
    }


    if ([self lockForConfiguration:error]) {                                // 2

        CMTime minFrameDuration = qos.frameRateRange.minFrameDuration;

        self.activeFormat = qos.format;                                     // 3
        self.activeVideoMinFrameDuration = minFrameDuration;                // 4
        self.activeVideoMaxFrameDuration = minFrameDuration;

        [self unlockForConfiguration];
        return YES;
    }
    return NO;
}


@end

辅助类

@implementation THQualityOfService

+ (instancetype)qosWithFormat:(AVCaptureDeviceFormat *)format
               frameRateRange:(AVFrameRateRange *)frameRateRange {

    return [[self alloc] initWithFormat:format frameRateRange:frameRateRange];
}

- (instancetype)initWithFormat:(AVCaptureDeviceFormat *)format
                frameRateRange:(AVFrameRateRange *)frameRateRange {
    self = [super init];
    if (self) {
        _format = format;
        _frameRateRange = frameRateRange;
    }
    return self;
}
//大于30就看成高帧率
- (BOOL)isHighFrameRate {
    return self.frameRateRange.maxFrameRate > 30.0f;
}

@end