• videoURL：視頻本地路徑
• gifOutputPath：生成的GIF存儲路徑

- (NSURL *)gifThumbnailURLFromVideo:(NSURL *)videoURL gifOutputPath:(NSString *)gifOutputPath
{
CGFloat increment = 0.1;
AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];

AVAssetImageGenerator *generator = [AVAssetImageGenerator assetImageGeneratorWithAsset:asset];

float movieLength = (float)asset.duration.value /asset.duration.timescale;
int frameCount = movieLength / increment;
float tolerance = 0.01f;

CFURLRef gifOutputURL = (__bridge CFURLRef) [NSURL fileURLWithPath:gifOutputPath];
CGImageDestinationRef destination = CGImageDestinationCreateWithURL(gifOutputURL, kUTTypeGIF, frameCount >= 24 ? 10 : frameCount,nil);

generator.requestedTimeToleranceBefore = CMTimeMakeWithSeconds(tolerance, 600) ;
generator.requestedTimeToleranceAfter = CMTimeMakeWithSeconds(tolerance, 600);
generator.appliesPreferredTrackTransform = YES;

NSDictionary *frameProperties = [NSDictionary dictionaryWithObject:[NSDictionary dictionaryWithObject:[NSNumber numberWithFloat:increment]
                                                                                               forKey:(NSString *)kCGImagePropertyGIFDelayTime]
                                                            forKey:(NSString *)kCGImagePropertyGIFDictionary];
NSDictionary *gifProperties = [NSDictionary dictionaryWithObject:[NSDictionary dictionaryWithObject:[NSNumber numberWithInteger:0]
                                                                                             forKey:(NSString *)kCGImagePropertyGIFLoopCount]
                                                          forKey:(NSString *)kCGImagePropertyGIFDictionary];

CGImageDestinationSetProperties(destination, (CFDictionaryRef)gifProperties);

CGSize videoSize = [[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] naturalSize];

float imageWidth = 200.0f;
generator.maximumSize = CGSizeMake( imageWidth , imageWidth/videoSize.width * videoSize.height);

NSError *error = nil;

for (int i = 0 ; i <= 20; ) {
    
    CMTime imageTime = CMTimeMakeWithSeconds(i*increment, 600);
    
    CGImageRef image = [generator copyCGImageAtTime:imageTime actualTime:nil error:&error];
    if (error) {
        return nil;
    }
    
    CGImageDestinationAddImage(destination, image,  (CFDictionaryRef)frameProperties);
    
    CFRelease(image);
    
    i += 4;
}

//Inverse
for (int i = 16 ; i >= 4; ) {
    
    CMTime imageTime = CMTimeMakeWithSeconds(i*increment, 600);
    
    CGImageRef image = [generator copyCGImageAtTime:imageTime actualTime:nil error:&error];
    if (error) {
        return nil;
    }
    
    CGImageDestinationAddImage(destination, image,  (CFDictionaryRef)frameProperties);
    
    CFRelease(image);
    
    i -= 4;
}

CGImageDestinationFinalize(destination);
CFRelease(destination);

return [NSURL fileURLWithPath:gifOutputPath];
}