异步文档：
https://developer.apple.com/library/content/documentation/General/Conceptual/ConcurrencyProgrammingGuide/Introduction/Introduction.html#//apple_ref/doc/uid/TP40008091
thread文档：
https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/Introduction/Introduction.html#//apple_ref/doc/uid/10000057i-CH1-SW1
Threads are one of several technologies that make it possible to execute multiple code paths concurrently inside a single application. Although newer technologies such as operation objects and Grand Central Dispatch (GCD) provide a more modern and efficient infrastructure for implementing concurrency, OS X and iOS also provide interfaces for creating and managing threads.
线程是几种技术之一，它使得在单应用程序中同时执行多段代码称为可能。尽管新技术像NSOperation和GCD提供了一种更加现代并且有效的办法来实现并发，但是OS X 和 iOS还是提供了接口来创建和管理线程。
This document provides an introduction to the thread packages available in OS X and shows you how to use them. This document also describes the relevant technologies provided to support threading and the synchronization of multithreaded code inside your application.
这篇文档告诉你thread可以应用于OS X以及告诉你怎样使用它们。这篇文档也描绘了相关的技术来支持threading和同步的多线程代码。
