Cache. Cache ——Level 3 cache(memory, sdcard, network), abstract asynchronous time-consuming operation for any object, main thread callback, chained call. Get the first frame of the video, audio duration, imageloader, image compress; 3级缓存（内存, 本地, 网络）, 针对任意对象，抽象异步耗时操作，主线程回调，链式调用. 获取视频第一帧/缩略图，音频时长, 图片加载，图片压缩等