1、最常用的在一个异步队列中获取数据，然后返回主线程更新UI
DispatchQueue.global().async {
DispatchQueue.main.async {
self.label?.text ="finished"
}
}
2、优先级的改进
DISPATCH_QUEUE_PRIORITY_HIGH: .userInitiated
DISPATCH_QUEUE_PRIORITY_DEFAULT: .default
DISPATCH_QUEUE_PRIORITY_LOW: .utility
DISPATCH_QUEUE_PRIORITY_BACKGROUND: .background
eg：
DispatchQueue.global(qos: .userInitiated).async {
}
3、DispatchWorkItem 的使用
et queue = DispatchQueue(label:"swift.queue")
let workItem = DispatchWorkItem(qos: .userInitiated, flags: .assignCurrentContext) {
}
queue.async(execute: workItem)
相当于：使用Global Queue
DispatchQueue.global(qos:.userInitiated).async {
}
4、对dispatch_time_t的改进
let delay = DispatchTime.now() + .seconds(60)
DispatchQueue.main.after(when: delay) {
// Do something
}
