在Swift 3中调度一次

swift 3中的dispatch_once的新语法是什么？旧版本如下。

var token: dispatch_once_t = 0
func test() {
    dispatch_once(&token) {
    }
}

这里是changes到libdispatch的Swift 3。

从 doc：

dispatch The free function dispatch_once is no longer available in
Swift. In Swift,you can use lazily initialized globals or static
properties and get the same thread-safety and called-once guarantees
as dispatch_once provided. Example:

let myGlobal = { … global contains initialization in a call to a closure … }() _ = myGlobal // using myGlobal will invoke the initialization code only the first time it is used.