OkDownload

In fact OkDownload is FileDownloader2, which extends all benefits from FileDownloader and beyond. FileDownloader framework is not easy to write unit-test, it is not a testable framework, so it is not stable enough. The core library of FileDownloader is too complex and not pure enough, so 6K+ star 1K+ fork with around 10 PR. Unit test coverage is very high which means reliable. Simpler interface. Task priority support. Uri file to store output-stream. The core library is Pure and light. More flexible callback mechanism and listener. More flexible to expand each part of OkDownload. Fewer threads to do the same thing without drop performance. File-IO thread pool is independent of Network-IO thread pool. Make sense auto filename from URL if can't find from response header. Cancel and Start is very effective especially for a bunch of tasks with tons of optimize.