AVAssetImageGenerator 视频生成图片过程中有丢失

最近在搞视频相关的东东，遇到个很棘手的问题，我在调用 [generator generateCGImagesAsynchronouslyForTimes:times completionHandler:^(CMTime requestedTime, CGImageRef cgImage,
CMTime actualTime,
AVAssetImageGeneratorResult result,
NSError error) 这个方法的时候，生成的图片数量不够，NSString resultStr;
switch (result)
{
case AVAssetImageGeneratorSucceeded: resultStr = @"AVAssetImageGeneratorSucceeded"; break;
case AVAssetImageGeneratorFailed: resultStr = @"AVAssetImageGeneratorFailed"; break;
case AVAssetImageGeneratorCancelled: resultStr = @"AVAssetImageGeneratorCancelled"; break;
}
CFStringRef requestedTimeString = CMTimeCopyDescription(kCFAllocatorDefault, requestedTime);
CFStringRef actualTimeString = CMTimeCopyDescription(kCFAllocatorDefault, actualTime);
NSLog(@"%@ requestedTime:%@ actualTime:%@ error:%@", resultStr, (__bridge NSString*)requestedTimeString, (__bridge NSString*)actualTimeString, error);打印出的结构，后面的实际时间是无效的，肿么办

图像无法打开，看下是不是格式的问题或者解码器没有安装

安装解码器？能不能详细点