dispatch_sync：As an optimization, this function invokes the block on the current thread when possible
为什么使用dispatch_sync
如何安全使用dispatch_sync
dispatch_sync
Dispatch Queues and Thread Safety
高阶函数-参数与返回值
In Swift, typedef is called typealias:
偏函数应用(Partial Application）和函数柯里化(Currying)
Centos下添加用户到用户组