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：
/*
run like:
swift once.swift
swift once.swift run
to see both cases
*/
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.swift
Note how it's run lazily, so you won't see the "Behold" text Now.
~/W/WhenDoesStaticDefaultRun> swift once.swift run
Behold! Once runs!
Called twice, but only printed "Behold" once, as desired.
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