LazyBox.swift

import Foundation


// Based on http://oleb.net/blog/2015/12/lazy-properties-in-structs-swift/
final class LazyBox<T> {

    init(computation: () -> T) {
        self.lazyValue = .NotYetComputed(computation)
    }

    private var lazyValue: LazyValue<T>
    private let queue = dispatch_queue_create("LazyBox.Queue.Serial", DISPATCH_QUEUE_SERIAL)

    var value: T {
        var value: T!
        switch self.lazyValue {
        case .NotYetComputed(let computation):
            dispatch_sync(self.queue) {
                // Check if value has been computed already in case of multi-thread access
                if case .Computed(let result) = self.lazyValue {
                    value = result
                } else {
                    let result = computation()
                    self.lazyValue = .Computed(result)
                    value = result
                }
            }
        case .Computed(let result):
            value = result
        }
        return value
    }
}


private enum LazyValue<T> {
    case NotYetComputed(() -> T)
    case Computed(T)
}