GCD 队列及并发

let mainQueue = DispatchQueue.main
let globalQueue = DispatchQueue.global()
let globalQueueWithQos = DispatchQueue.global(qos: .userInitiated)

http://www.jianshu.com/p/f042432e2d7d
http://blog.csdn.net/Hello_Hwc/article/details/54293280