我们可以这样理解，一个大任务或者多个大任务，利用单一的线程去下载的话，肯定会很慢，效率也不高。如果我们把一个大任务去拆分一个一个的小任务， 利用多个线程去下载一个一个的小任务的话，每个小任务的进度叠加起来，这样做且不下载的速度很快了，效率也提升了。在下载的过程中， 我们又需要考虑特殊的情况，用户可能需要中止下载，这时我们需要保存下下载的进度，下次进来时，读取上次的下载进度，告诉服务器，接着下载。
Copyright 2018 StevenYan88
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