ios - GIFS的DelayTime或UnclampedDelayTime

将Gif转换为动画UIImage时，从属性kCGImagePropertyGIFDelayTime或kCGImagePropertyGIFUnclampedDelayTime中提取每帧的延迟

像这样:

NSString *frameKeyPath = [NSString stringWithFormat:@"%@.%@",(NSString*)kCGImagePropertyGIFDictionary,kCGImagePropertyGIFUnclampedDelayTime];
CFDictionaryRef cfFrameProperties = CGImageSourceCopyPropertiesAtIndex(source,i,nil);
NSDictionary *frameProperties = (__bridge NSDictionary*)cfFrameProperties;
NSNumber *delayTimeProp = [frameProperties valueForKeyPath:frameKeyPath];

在以下两种情况下，Chrome都会选择正确的选项:
kCGImagePropertyGIFDelayTime = 0.1:http://i.imgur.com/tX9cjUO.gif
kCGImagePropertyGIFUnclampedDelayTime = 0.01:http://i.minus.com/iIOyK7SKp8TYc.gif

为每种情况选择一个会导致其中一个示例图像的动画太慢或太快

有没有一种方法可以确定要使用哪个属性？

谢谢

最佳答案

最终使用webkit方法选择，不固定，固定或默认0.1

+ (float)frameDurationAtIndex:(NSUInteger)index source:(CGImageSourceRef)source
{
    float frameDuration = 0.1f;
    CFDictionaryRef cfFrameProperties = CGImageSourceCopyPropertiesAtIndex(source,index,nil);
    NSDictionary *frameProperties = (__bridge NSDictionary*)cfFrameProperties;
    NSDictionary *gifProperties = frameProperties[(NSString*)kCGImagePropertyGIFDictionary];

    NSNumber *delayTimeUnclampedProp = gifProperties[(NSString*)kCGImagePropertyGIFUnclampedDelayTime];
    if(delayTimeUnclampedProp) {
        frameDuration = [delayTimeUnclampedProp floatValue];
    } else {

        NSNumber *delayTimeProp = gifProperties[(NSString*)kCGImagePropertyGIFDelayTime];
        if(delayTimeProp) {
            frameDuration = [delayTimeProp floatValue];
        }
    }

    // Many annoying ads specify a 0 duration to make an image flash as quickly as possible.
    // We follow Firefox's behavior and use a duration of 100 ms for any frames that specify
    // a duration of <= 10 ms. See <rdar://problem/7689300> and <http://webkit.org/b/36082>
    // for more information.

    if (frameDuration < 0.011f)
        frameDuration = 0.100f;

    CFRelease(cfFrameProperties);
    return frameDuration;
}