http://joeyio.com/ios/2014/03/28/run-loop-vs-thread/
http://blog.ibireme.com/2015/05/18/runloop/#base
http://naotu.baidu.com/viewshare.html?shareId=axfrcjcxyqgc
runloop 的model
http://naotu.baidu.com/viewshare.html?shareId=axfrxkoiguwc
RunLoop 的内部逻辑
http://naotu.baidu.com/viewshare.html?shareId=axfse9uhlfs4
runloop 孙源视频总结