A tiny event system for Swift (1 source file, 113 lines of code)

What's New


  • Fixed TinyEvent and TinyEventWithData not having public initializers which prevented them from being used in other modules


A tiny event system for Swift (1 source file, 113 lines of code)


import TinyEvents

Create an event and add a closure to call when the event is fired:

event = TinyEvent()
observer = event.add { print("Event fired") }

And then later fire it:


To remove an observer, just destroy it. Either explicitly:

observer = nil

or if the observer is stored as a property on some object of yours, it will be removed automatically when your object is destroyed:

class SomeViewController: UIViewController {
    // […]
    override func viewDidLoad() {
        // […]
        observer = someSystem.importantEvent.add { [unowned self] in
            self.importantEventLabel.isHidden = false
    var observer: TinyEventObserver? // I'll be destroyed automatically with the view controller.

You can pass data to the observers by using TinyEventWithData<T> instead of TinyEvent:

enum Option { case good, better, best }
userSelectedOption = TinyEventWithData<Option>()
// The observers will be of type `TinyEventWithDataObserver<Option>`
optionObserver = userSelectedOption.add { selectedOption in

Pass multiple pieces of data using tuples (or structs or any other kind of structure):

titleRatedByUser = TinyEventWithData<(String, Int)>()
observer = titleRatedByUser.add { title, rating in
    print("\(title) rated: \(rating)")
titleRatedByUser.fire(("Foo", 10))

Avoiding Strong Reference Cycles (Important)

In short, whenever you reference self in your closure and store the observer on that same object you should use [unowned self] in the closure's capture list to avoid creating a reference cycle (see the SomeViewController listing above for an example of this).

This is important, because if you didn't add the [unowned self] then you'd have created a reference cycle — in this example view controller -> observer -> closure -> view controller — and then even if the view controller would normally have been destroyed and deallocated, it would be kept alive by the reference in your closure and you'd leak memory.

Please see the Automatic Reference Counting chapter of The Swift Programming Language if you'd like to know more about this subject.


You can add TinyEvents to your project using the Swift Package Manager (or Xcode's integration with the Swift Package Manager) by adding this repository to your dependencies, or you can just drop the TinyEvents.swift file into your project.

Stability and Support

I'm using TinyEvents in production in my app Day Planner and I intend to keep it updated for any breaking changes in new Swift versions, but I don't have any plans for adding new features. If you'd like a slightly heavier event system with more GitHub stars I'd suggest having a look at emitter-kit or Observable-Swift.

Please use GitHub issues for questions, bug reports, suggestions, and patches.

Frequently Asked Questions

Why am I getting warnings about Immutable value 'someObserver' was never used; consider replacing with '_' or removing it or Variable 'someObserver' was written to, but never read?

Are you storing an observer in a local variable or constant instead of in a property? That's fine, but those warnings are hinting at something important, which is that the automatic reference counting system will consider unused local objects as being unnecessary and can destroy them immediately, and your closure won't get called if the observer is destroyed. The solution to this is to use the Swift standard library function withExtendedLifetime(_:_:) to tell the system explicitly that you want the observer to stay alive:

let event = TinyEvent()
let observer = event.add {
    // […]
withExtendedLifetime(observer) {

In my experience this doesn't come up frequently (if at all) in normal usage because it's unusual to have an event observer that is as short-lived as a function invocation, but this idiom is used extensively in the TinyEvent tests.


  • Swift Tools 5.1.0
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Last updated: Tue Mar 14 2023 14:54:15 GMT-0500 (GMT-05:00)