Extension(Swift)中的延迟加载属性

如果您不需要参考 self 您可以使用 static var :

extension String {
    static var count = 0
    static var laughingOutLoud : String = {
        count++

        return "LOL: \(count)"
    }()
}

String.laughingOutLoud // outputs "LOL: 1"
String.laughingOutLoud // outputs "LOL: 1"
String.laughingOutLoud // outputs "LOL: 1"

(你不需要 count 在您的实施中;这只是为了显示它只执行一次。)

可以使用computed属性和associatedObject。通过这种方式,您可以模拟存储的属性。因此延迟var模拟将是:

// global var's address used as key
var #PropertyKey# : UInt8 = 0
var #varName# : #type# {
    get {
        if let v = objc_getAssociatedObject(self, & #PropertyKey#) as #type# {
            return v
        }else {
            // the val not exist, init it and set it. then return it

            // this way doesn't support nil option value. 
            // if you need nil option value, you need another associatedObject to indicate this situation.
        }
    }
    set {
        objc_setAssociatedObject(self, & #PropertyKey#, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
    }
}