如何在Swift中执行一次代码？

由闭包初始化的静态属性是延迟运行的，最多只能运行一次，因此尽管被调用了两次，但它仅打印一次：

class once {    static let run: Void = {        print("Behold! (__FUNCTION__) runs!")        return ()    }()}if Process.arguments.indexOf("run") != nil {    let _ = Once.run    let _ = Once.run    print("Called twice, but only printed "Behold" once, as desired.")} else {    print("Note how it's run lazily, so you won't see the "Behold" text now.")}

示例运行：

~/W/WhenDoesStaticDefaultRun> swift once.swiftNote how it's run lazily, so you won't see the "Behold" text now.~/W/WhenDoesStaticDefaultRun> swift once.swift runBehold! once runs!Called twice, but only printed "Behold" once, as desired.