https://onevcat.com/2013/08/ios7-background-multitask/
http://zhidao.baidu.com/link?url=NUOMrLGB6OdrEGtdxwHTCsGJ24f9FmW5x4bGh28r03U3I_vgESk2dfmi018FHa2gwTUDXOvvllpSA0OCfH5cbsk1FOmLWUzJRqHT5lMynky
http://www.kuqin.com/shuoit/20150512/346010.html
http://www.cnblogs.com/Neo-joke/p/4019677.html
http://blog.sina.com.cn/s/blog_71715bf80101d4z0.html