Example of a simple Swift alternative to KVO. Inspired by reactive cocoa's MutableProperty. Weakly-referenced subscribers and all.

Property.swift

import Foundation

// Simple observable property
// Careful: Not thread safe! You should use this only from the main thread, or modify to
// have locks.
class Property<T> {
    private var _value: T
    var value: T {
        get { return _value }
        set {
            _value = newValue
            tellSubscribers()
        }
    }
    
    init(_ value: T) {
        _value = value
    }
    
    var subscriptions = [Subscription<T>]()
    func subscribe(subscriber: AnyObject, next: T -> Void) {
        subscriptions.append(Subscription(subscriber: subscriber, next: next))
        next
    }
    
    private func tellSubscribers() {
        subscriptions = subscriptions.flatMap(tellAndFilterSubscription)
    }
    
    private func tellAndFilterSubscription(subscription: Subscription<T>) -> Subscription<T>? {
        if subscription.subscriber != nil { // Subscriber is still around.
            subscription.next(_value)
            return subscription
        } else { // Subscriber has gone, so cull this subscription.
            return nil
        }
    }
}

struct Subscription<T> {
    weak var subscriber: AnyObject?
    let next: T -> Void
}

// Tests / examples of how to use it.

class Foo { // The one with the property.
    let property = Property("Blah") // Here's how you declare a property.
}
class Bar { // The observer.
    var string: String?
}
class Test {
    var string: String?
    func test() {
        // Setup bar subscribed to foo's property.
        let foo = Foo()
        var bar: Bar? = Bar()
        foo.property.subscribe(bar!) {
            [weak bar] newValue in
            bar?.string = newValue
        }
        
        // Example of how to subscribe.
        foo.property.subscribe(self) {
            [weak self] newValue in
            self?.string = newValue
        }
        
        // Test it calls the subscriber.
        foo.property.value = "Test"
        print(bar!.string) // Should be "Test"
        print(string) // Should also be "Test"
        
        // Test it culls nilled subscribers.
        print(foo.property.subscriptions.count) // Should be 1.
        bar = nil
        foo.property.value = "GiveItAChanceToCull"
        print(foo.property.subscriptions.count) // Should be 0.
    }
}
Test().test()