ErrorHierarchy is a small framework designed to use the SwiftUI view hierarchy as a responder chain for error 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 Error 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 Error objects.

This is a sibling framework to EventHierarchy, but specialized in error handling.

Errors are triggered using the reportError closure added to EnvironmentValues.

Example:

struct MyError: Error {}

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

There are three kinds of operations that can be applied to an Error 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()
			.receiveError { .notHandled }
			.transformError { MyError() }
			.handleError {}
	}
}

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

struct ContentView: View {
	var body: some View {
		TriggerView()
			.handleError(MyError.self) {}
	}
}

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

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

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

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

AlertableError is a protocol that user-facing errors can conform to. The only non-optional requirement is a message String that will be displayed to the user.

By using the .handleAlertErrors() modifier, any alertable errors will be handled by displaying an alert to the user with a single dismiss button.

Note: If the error is triggered by a View contained in a sheet, this modifier needs to be called inside the body of the sheet, or it will fail to display the alert. This also applies to popovers and other similar modifiers.