GCD

http://www.cocoachina.com/ios/20161031/17887.html

http://ios.jobbole.com/82622/

http://www.jianshu.com/p/bc45569adee2

http://www.cnblogs.com/ioshe/p/5937933.html

http://www.jianshu.com/p/ae786a4cf3b1

http://blog.csdn.net/demondev/article/details/52965236

http://www.cocoachina.com/ios/20160804/17291.html