https://www.jianshu.com/p/116b1354b058
https://www.jianshu.com/p/c7d525dca398
https://blog.csdn.net/Cehae/article/details/78112712?utm_source=blogxgwz5
http://www.cocoachina.com/ios/20180308/22509.html
原文：https://www.cnblogs.com/dengchaojie/p/10077377.html