Swift 2.2

Docs (you may already be here)

HIPEventedProperty is a simple library of observables, intended to be the minimum viable support for a Model-View-ViewModel (MVVM) architecture. At Hipmunk, we’ve used it for exactly that, and have hardly changed anything over a year of app development.

Think of it as ReactiveCocoa stripped way, way down. It’s less than 150 lines long.

Design goals

  • Small stack traces
  • Easily understood memory model
  • Simple, clear code with no complex type system tricks
  • Obvious API

ReactiveCocoa is a lot more powerful, and even more succinct in most cases, but HIPEventedProperty asks very little of your brain.



The most basic class in the module is HIPEventSource. A HIPEventSource collects callbacks that are tied to the memory lifecycle of objects, and allows them to be fired iff their associated objects are still alive.

let p = HIPEventedProperty()
var x: NSObject? = NSObject()

p.subscribe(withObject: x) {
    print("Event fired")

p.fireEvent()  // prints "Event fired"
x = nil
p.fireEvent()  // nothing happens

99.9% of the time, you’ll be instantiating a HIPEventSource inside a class, and you’ll want its callback to be cleaned up whenever self is deallocated.

class MyViewController: UIViewController {
    var taps: HIPEventSource!
    @IBOutlet var tapLabel: UILabel!

    override func viewDidLoad() {
        taps = HIPEventSource()
        taps.subscribe(withObject: self) { [weak self] in
            self?.tapLabel.text += (self?.tapLabel.text ?? "") + "tap "


To store a value inside your event source, you’ll need to use one of the variants of the HIPEventedProperty class:

  • HIPEventedProperty: Equatable, non-optional
  • HIPEventedPropertyOptional: Equatable, optional
  • HIPEventedPropertyBasic: No restrictions, no skipping of duplicate values
// numTaps.value has type `Int`
let numTaps = HIPEventedProperty<Int>(0)

// rating.value has type `Int?`
let rating = HIPEventedPropertyOptional<Int>(nil)

// asyncImage.value has type `UIImage?`
let asyncImage = HIPEventedPropertyBasic<UIImage?>(nil)

// Properties have more methods. Here's one:

let x = NSObject()  // this is just an example, never deallocate this
rating.subscribeToValue(withObject: x) {
  if let r = $0 {
    print("Rating: \(r)")
  } else {
    print("Rating: no rating")