IOS 多线程与异步的区别 转:http://wenku.baidu.com/link?url=qJDMdA-KjlkTlEXKGqH6BJ-SNQn7ix2szY31kWXV8dtN98kJMHt0OxuE4Wz5d46zcCOsVIuTIDmLEmnfKuw2iC9el9Z4kXFKEJIeCZAiyDS