lazy

lazy.swift

public class Lazy<T> {
    private let initializer: () -> T
    
    private var _value: T!
    public var value: T {
        get {
            // TODO: mutex
            switch _value {
            case .none:
                _value = initializer()
                return _value
            case .some(let v):
                return v
            }
        }
        set {
            _value = newValue
        }
    }
    
    public init(initializer: @escaping () -> T) {
        self.initializer = initializer
    }
}

public func lazy<T>(initializer: @escaping () -> T) -> Lazy<T> {
    return Lazy(initializer: initializer)
}

sample.swift

class B {
    let v1 = lazy { return nil as Optional<A> } // self making
    let v3 = lazy { () -> Int! in return 1 } // self making
    lazy var v2 = { return A() }() // swift's `lazy`
}

let b = B()
b.v3.value = 2
b.v3.value  // 2
b.v3.value = nil
b.v3.value  // nil