截取单帧

AVAsset *myAsset = <#An asset#>];
AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc] initWithAsset:myAsset];
     
Float64 durationSeconds = CMTimeGetSeconds([myAsset duration]);
CMTime midpoint = CMTimeMakeWithSeconds(durationSeconds/2.0, 600);
NSError *error;
CMTime actualTime;
     
CGImageRef halfWayImage = [imageGenerator copyCGImageAtTime:midpoint actualTime:&actualTime error:&error];
     
if (halfWayImage != NULL) {
  NSString *actualTimeString = (NSString *)CMTimeCopyDescription(NULL, actualTime);
  NSString *requestedTimeString = (NSString *)CMTimeCopyDescription(NULL, midpoint);
  NSLog(@"Got halfWayImage: Asked for %@, got %@", requestedTimeString, actualTimeString);
     
  // Do something interesting with the image.
  CGImageRelease(halfWayImage);
}

截取多帧

    AVAsset *myAsset = <#An asset#>];
    // Assume: @property (strong) AVAssetImageGenerator *imageGenerator;
    self.imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:myAsset];
    
    Float64 durationSeconds = CMTimeGetSeconds([myAsset duration]);
    CMTime firstThird = CMTimeMakeWithSeconds(durationSeconds/3.0, 600);
    CMTime secondThird = CMTimeMakeWithSeconds(durationSeconds*2.0/3.0, 600);
    CMTime end = CMTimeMakeWithSeconds(durationSeconds, 600);
    NSArray *times = @[NSValue valueWithCMTime:kCMTimeZero],
    [NSValue valueWithCMTime:firstThird], [NSValue valueWithCMTime:secondThird],
    [NSValue valueWithCMTime:end]];
    
    [imageGenerator generateCGImagesAsynchronouslyForTimes:times
                                         completionHandler:^(CMTime requestedTime, CGImageRef image, CMTime actualTime,
                                                             AVAssetImageGeneratorResult result, NSError *error) {
        
        NSString *requestedTimeString = (NSString *)
        CFBridgingRelease(CMTimeCopyDescription(NULL, requestedTime));
        NSString *actualTimeString = (NSString *)
        CFBridgingRelease(CMTimeCopyDescription(NULL, actualTime));
        NSLog(@"Requested: %@; actual %@", requestedTimeString, actualTimeString);
        
        if (result == AVAssetImageGeneratorSucceeded) {
            // Do something interesting with the image.
        }
        
        if (result == AVAssetImageGeneratorFailed) {
            NSLog(@"Failed with error: %@", [error localizedDescription]);
        }
        if (result == AVAssetImageGeneratorCancelled) {
            NSLog(@"Canceled");
        }
    }];

引用参考：

Apple文档