Swift语法3.03(类型Types)
https://www.jianshu.com/p/839f9bc4b9a3
https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Generics.html#//apple_ref/doc/uid/TP40014097-CH26-ID179
https://theswiftpost.co/generics-swift-4/
https://blog.csdn.net/crazyeveryday/article/details/46490149