swift实现dispatch_once

一、封装一个once函数

public extension DispatchQueue {

    private static var _onceTracker = [String]()

    /**
     Executes a block of code, associated with a unique token, only once.  The code is thread safe and will
     only execute the code once even in the presence of multithreaded calls.

     - parameter token: A unique reverse DNS style name such as com.vectorform.<name> or a GUID
     - parameter block: Block to execute once
     */
    public class func once(token: String, block: (()->Void)) {
        objc_sync_enter(self); defer { objc_sync_exit(self) }

        if _onceTracker.contains(token) {
            return
        }

        _onceTracker.append(token)
        block()
    }
}

二. 带立即执行闭包初始化器的全局变量

let dispatchOnce: () = {
    print("dispatchOnce-------")
}()

/// 使用Aspects做交换: 用一个全局的常量来实现的dispatchOnce效果
let doSwizzles: () = {
    print("doSwizzles----")
    let oriSel1 = #selector(UIViewController.viewWillAppear(_:))
    let wrappedBlock: @convention(block) (AspectInfo, Bool) -> Void = { aspectInfo, _ in
        print("wrappedBlock---")
    }
    _ = try? UIViewController.aspect_hook(oriSel1, with: AspectOptions.positionBefore, usingBlock: wrappedBlock)

}()

// 之后在合适的时机使用变量，比如：
_ = dispatchOnce