EventHierarchy is a small framework designed to use the SwiftUI view hierarchy as a responder chain for event handling.

A more detailed explanation can be found in this article.

TL;DR: Using a closure contained in EnvironmentValues, View objects that are lower in the hierarchy send Event objects up the view hierarchy, while views that are higher in the hierarchy use one of the modifiers to register themselves as a responder to receive, transform, or handle the Event objects.

This is a sibling framework to ErrorHierarchy, but specialized in event handling.

Event is requirement-less protocol that identifies a type as an event that can be sent up the SwiftUI view hierarchy.

It can be of any type and contain any kind of additional information. It exists to avoid annotating the types used by methods in this framework as Any.

Events are triggered using the triggerEvent closure added to EnvironmentValues.

Example:

struct MyEvent: Event {}

struct TriggerView: View {
	@Environment(\.triggerEvent) var triggerEvent
	
	var body: some View {
		Button("Trigger") {
			triggerEvent(MyEvent())
		}
	}
}

There are three kinds of operations that can be applied to an Event that has been triggered. All of these are executed by registering a closure the same way you would apply a view modifier to a View.

struct ContentView: View {
	var body: some View {
		TriggerView()
			.receiveEvent { .notHandled }
			.transformEvent { MyEvent() }
			.handleEvent {}
	}
}

All of these functions have a generic version that receives the type of an Event as the first parameter, only events matching the provided type will be acted on, any other event will be propagated up the view hierarchy.

struct ContentView: View {
	var body: some View {
		TriggerView()
			.handleEvent(MyEvent.self) {}
	}
}

When receiving an Event, it's up to the registered closure to determine whether the Event has been fully handled or not.

If the registered closure returns .handled, the Event will no longer be propagated up the view hierarchy. If it returns .unhandled instead, the Event will continue to be propagated.

Any event that is handled will no longer be propagated up the view hierarchy. This is equivalent to using a receiveEvent closure that always returns .handled.

Transforming functions can be used to replace the received Event. It could be an Event of a different type, or an Event of the same type but with different values.