Classes to ease observer pattern implementation in Swift inspired by C# events


License: MIT

Classes to ease observer pattern implementation in Swift inspired by C# events

This package is built arround the following principes :

  1. Genericity : every classes supports generic

  2. Simplicity : inspired by C# event swift-event supports subscribe (via += and subscribe()), unsubcribe (via -= and unsubscribe()) and notify via invoke.

  3. Encapsulation : invoke is not accessible outside of declaration scope, preventing other classes from breaking internal logic.

Installation - Getting started - Samples - Docs - Q&A - Changelog


Swift Package Manager

You can use The Swift Package Manager to install swift-event by adding the proper description

To your Package.swift file:

dependencies: [
        .package(url: "https://github.com/MarcAlx/swift-event.git", from: "1.0.1"),

Or via Xcode

From your .xcodeproj file select your project then go to Swift Packages tab then add (via +) : https://github.com/MarcAlx/swift-event.git

Copy source code

You can find source code here : https://github.com/MarcAlx/swift-event/blob/master/Sources/swift-event/swift_event.swift

All commented it's <100 sloc just copy and paste it into your code.

Getting started

0. Import package

import swift_event

1. Create an event

let tmp = Event<String>.create()
//tmp holds the event in 'event' and a pointer to 'invoke' method

2. Create an handler and subscribe

var handler =  EventHandler<String>(handle: { sender, args in
tmp.event += handler

3. Invoke the event

tmp.invoke(self,"Hello world !")
//handler should print "Hello world !"

4. (optional) Unsubscribe

tmp.event -= handler



let tmp = Event<String>.create()
tmp.event += EventHandler<String>(handle: { sender, args in
tmp.invoke(self,"Hello world !")
//handler should print "Hello world !"

Via functions

let tmp = Event<String>.create()
let dispose = tmp.event.subscribe(EventHandler<String>(handle: { sender, args in
tmp.invoke(self,"Hello world !")
//handler should print "Hello world !"

Inside a class


import swift_event

class Test {
    private var _somethingHappened:Event<String>
    public var somethingHappened: Event<String> {
        return self._somethingHappened
    private var _somethingHappenedInvoke:Delegate<String>
    public init() {
        let tmp = Event<String>.create()
        self._somethingHappened = tmp.event
        self._somethingHappenedInvoke = tmp.invoke
    public func doSomething() {
        self._somethingHappenedInvoke(self, "Hello !")


var test = Test()

var handler = EventHandler<String>(handle: {sender, args in print(args)})

test.somethingHappened += handler

test.doSomething() //should raise event, thus leading in an handler call

test.somethingHappened -= handler

test.doSomething() //should print nothing, as handler has been unsubscribe


Documentation is also provided as a .doccarchive that includes some interractive tutorials, here : ./Doc/swift-event.doccarchive

class Event<T>

static func create() -> (invoke:Delegate<T>,event:Event<T>)

Create an Event and return it along with its invoke method

note: this is the only way to instantiate an Event, this way only the class that call this method has access to invoke

returns: A tuple containing the created Event along with a pointer to its private invoke

operator overload +=

Subscribe to an event by adding an handler

operator overload -=

Unsubsribe from an event by removing handler

func subscribe (handler: EventHandler<T>)-> () -> Void

Subscribe to an event by adding an handler

note: shorthand to +=

parameter handler: the EventHandler to add

returns: a function that is a shorthand to -=, to ease unsubscribe

func unsubscribe (handler: EventHandler<T>) -> Void

Unsubsribe from an event by removing handler

note: shorthand to -=

class EventHandler<T>

Typed EventHandler for typed Event

note: this class is needed as Swift doesn't allow func equity via ===

var handle:Delegate<T>

The Delegate that will handle the Event

init(handle:@escaping Delegate<T>)

Instanciate a new typed EventHandler

parameter handle: the associated Delegate to handle the Event

typealias Delegate<T> = (_ sender:AnyObject?,_ args:T) -> Void

Shorthand for Event delegation

parameter sender: Sender of the event

parameter args: Event args


Q. Why EventHandler is a class instead of typealias over func

A. Because in swift func are not equatable and doesn't supports === thus leaving unsubscribe and -= operator unimplemented.

Q. Why can't Event.init() is private ?

A. To avoid call to invoke() outside of creation scope. (like invoke() in c#)


You can contribute to this repo via pull requests, be sure to follow the philosophy of this repo and to update documentation.



First version of the package.


  • Swift Tools 5.5.0
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Last updated: Thu Nov 23 2023 01:59:45 GMT-1000 (Hawaii-Aleutian Standard Time)