Presenting multiple children from a feature

[ysmaliak]

Description

There is a bug in the modal presentation behavior when replacing child views. Specifically:

1. When a modal is presented.
2. The modal displays a first child.
3. An attempt is made to replace the first child with a second child.

The replacement operation is not working as expected. The behavior is inconsistent and incorrect, resulting in improper display or failure to update the modal's content.

Note: this is a UIKit project.

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

The first child should be dismissed, and the second child should be presented.

Simulator.Screen.Recording.-.iPhone.15.Pro.-.2024-09-09.at.18.00.02.mp4

Actual behavior

Sometimes (randomly), the parent modal is dismissed when trying to present the second child.

Simulator.Screen.Recording.-.iPhone.15.Pro.-.2024-09-09.at.17.59.49.mp4

Steps to reproduce

1. Present Parent as a modal.
2. Present the First Child as a modal from the Parent.
3. Tap a button on the First Child; the Parent should be able to receive this action from the delegate and replace the destination with a Second Child.

Sample project: https://github.com/ysmaliak/tca-navigation-test

The Composable Architecture version information

1.14.0

Destination operating system

iOS 17.5

Xcode version information

15.4

Swift Compiler version information

swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4)
Target: arm64-apple-macosx14.0

[mbrandonw]

Hi @ysmaliak, this is not a bug with the library, but rather just how UIKit (and even SwiftUI) work. You cannot simultaneously dismiss and present controllers, and you can't even simultaneously present two sheets at once. There is even some logs in the console of your app letting you know this wrong:

Attempt to present <tca_navigation_test.SecondChildViewController: 0x13af15120> on <tca_navigation_test.ParentViewController: 0x13ad0a270> (from <tca_navigation_test.ParentViewController: 0x13ad0a270>) whose view is not in the window hierarchy.

The fix is to space out the destination mutations. You can first nil out the destination, wait a small amount of time, and then mutate the destination to the next screen.

However, I also tested this code in iOS 18.1 and things seem to work as intended. So perhaps UIKit has fixed some bugs. I'm not sure it will always work, but at least this simple use case seems to work as expected.

Since this isn't an issue with the library I am going to convert it to a discussion. Please feel free to continue the conversation over there!