A replacement for NSNotificationCenter#addObserver and NSObject#addObserver that is type-safe and not verbose.

import EmitterKit

// A generic event emitter (but type-safe)!
var event = Event<T>()

// Any emitted data must be the correct type.
event.emit(data)

// This listener will only be called once.
// You are *not* required to retain it.
event.once { data: T in
  print(data)
}

// This listener won't stop listening;
// unless you stop it manually,
// or its Event<T> is deallocated.
// You *are* required to retain it.
var listener = event.on { data: T in
  print(data)
}

// Stop the listener manually.
listener.isListening = false

// Restart the listener (if it was stopped).
listener.isListening = true

A target allows you to associate a specific AnyObject with an emit call. This is useful when emitting events associated with classes you can't add properties to (like UIView).

When calling emit with a target, you must also call on or once with the same target in order to receive the emitted event.

let myView = UIView()
let didTouch = Event<UITouch>()

didTouch.once(myView) { touch in
  print(touch)
}

didTouch.emit(myView, touch)

The Notifier class helps when you are forced to use NSNotificationCenter (for example, if you want to know when the keyboard has appeared).

// You are **not** required to retain this after creating your listener.
var event = Notifier(UIKeyboardWillShowNotification)

// Handle NSNotifications with style!
listener = event.on { (userInfo: NSDictionary) in
  print(userInfo)
}

// Any NSObject descendant will work.
var view = UIView()

// "Make KVO great again!" - Donald Trump
listener = view.on("bounds") { (change: Change<CGRect>) in
  print(change)
}

• Swift 3.0 + Xcode 8.0 beta 6 support

• The Signal class was removed. (use Event<Void> instead)

• The Emitter abstract class was removed.

• The EmitterListener class was renamed EventListener<T>.

• The Event<T> class no longer has a superclass.

• The Notification class was renamed Notifier (to prevent collision with Foundation.Notification).

• The on and once methods of Event<T> now return an EventListener<T> (instead of just a Listener)

• The on and once methods of Notifier now return an NotificationListener (instead of just a Listener)

• The on and once methods of NSObject now return an ChangeListener<T> (instead of just a Listener)

• The keyPath, options, and object properties of ChangeListener<T> are now public.

• A listenerCount: Int computed property was added to the Event<T> class.

• An event: Event<T> property was added to the EventListener<T> class.

The changelog for older versions can be found here.

• The ChangeListener class may throw an exception (looking for help):

An -observeValueForKeyPath:ofObject:change:context: message was received but not handled.```