class WriterProxy<T> {
    var write:T {
        didSet {
            updateClosure(write)
        }
    }
    let updateClosure:(T)->()
    fileprivate init(value:T, updateClosure:@escaping (T)->()) {
        self.write = value
        self.updateClosure = updateClosure
    }
}

class SingleWriter<T> {
    private var value:T
    private weak var proxy:WriterProxy<T>?
    var read:T { return value }
    var writer:WriterProxy<T>? {
        get {
            if self.proxy != nil { return nil }
            let proxy = WriterProxy<T>(value:value) {
                self.value = $0
            }
            self.proxy = proxy
            return proxy
        }
    }
    init(value:T) {
        self.value = value
    }
}