Demo project. Reproduces the issue with IfLetStore and effect cancellation when using The Composable Architecture.
- Clone the repository
- Open
Demo.xcodeprojin Xcode 11.5 - Run the app in iPhone Simulator using
Demobuild scheme - Tap "Present Detail"
- Observe timer on the screen
- Tap "Dismiss Detail"
- Detail view disappears
- Timer effect is canceled
- App crashes due to error:
Fatal error: "DetailAction.stopTimer" was received by an optional reducer when its state was "nil". This can happen for a few reasons:
* The optional reducer was combined with or run from another reducer that set "DetailState" to "nil" before the optional reducer ran. Combine or run optional reducers before reducers that can set their state to "nil". This ensures that optional reducers can handle their actions while their state is still non-"nil".
* An active effect emitted this action while state was "nil". Make sure that effects for this optional reducer are canceled when optional state is set to "nil".
* This action was sent to the store while state was "nil". Make sure that actions for this reducer can only be sent to a view store when state is non-"nil". In SwiftUI applications, use "IfLetStore".
I assume the issue is caused by the fact, that DetailAction.stopTimer action is sent to a store using .onDisappear modifier on a SwiftUI view that is already removed from the hierarchy by the IfLetStore view.
I don't think it's a bug in The Composable Architecture, I think it's rather misuse of it. I am not sure if expected behavior can be achieved in the current implementation of the library.
The problem seems to be common, as it will be present whenever we send actions using .onDisappear view modifier, when the view is embedded in IfLetStore view. This concrete use case is probably a common one when implementing long-time-running effects that needs to be eventually canceled (when view disappears).
Lifecycle Reducer - Solution proposed by Brandon Williams on Swift Forums is available on solution-lifecycle branch. There is also an open pull request.
Copyright © 2020 Dariusz Rybicki Darrarski
License: GNU GPLv3