合成gif时每帧之间间隔的属性。
kCGImagePropertyGIFDelayTime限定了最小的事件间隔范围，如果小于50ms，则是100ms。
kCGImagePropertyGIFUnclampedDelayTime没有这个限制，可以从0ms开始起。
关于获取每帧间隔的方法, stackflow上给出的解决办法
+ (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;
}
