swift单例

In Swift, you can simply use a static type property, which is guaranteed to be lazily initialized only once, even when accessed across multiple threads simultaneously:

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class Singleton {
    static let sharedInstance = Singleton()
}

If you need to perform additional setup beyond initialization, you can assign the result of the invocation of a closure to the global constant:

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class Singleton {
    static let sharedInstance: Singleton = {
        let instance = Singleton()
        // setup code
        return instance
    }()
}