ClickToGIF:NSOperation子类，该子类在动画GIF的第一帧之外构建UIImage

下载动画gif的第一帧，而不是完整的东西。 用法示例： func example () { let url = NSBundle. mainBundle (). URLForResource ( " puppy " , withExtension : " gif " ) ! let operation = CQIntroductoryGIFFrameOperation ( URL : url) // note: `completionBlock` is also available from NSOperation, although it // works based off of KVO states, and will take an extra runloop or two to fire. operation. target = self operat