AVCaptureDevice中通過調用VideoZoomFactor方法調整焦距實現拉近拉遠鏡頭進行拍照錄制視頻（動畫縮放畫面，不閃屏）

做人臉識別的項目中用到，順便做了下筆記

//上鎖

-(void)changeDevicePropertySafety:(void (^)(AVCaptureDevice *captureDevice))propertyChange{

    AVCaptureDevice *captureDevice= [_videoInput device];
 
    if ([captureDevice lockForConfiguration:nil]) {
        propertyChange(captureDevice);
        [captureDevice unlockForConfiguration];
    }

}

//縮放的實現點擊

-(void)FocalLength{
    NSLog(@"調整焦距");
    [self changeDevicePropertySafety:^(AVCaptureDevice *captureDevice) {
      if (captureDevice.videoZoomFactor == 1.0) {

            //縮放倍數

            CGFloat current = 1.5;
            if (current < captureDevice.activeFormat.videoMaxZoomFactor) {
                [captureDevice rampToVideoZoomFactor:current withRate:10];
            }
        }else{

            //1.0 表示縮放回到原來的大小（復原）

            [captureDevice rampToVideoZoomFactor:1.0 withRate:10];
        }
    }];
    
}