Dependency not overridden when using subscript

[abel3cl]

Description

First, apologies if this isn't the better place, perhaps this is more related to swift-dependencies, but I have used SyncUps sample code to reproduce it.
I am not sure if this is a Swift bug or a Dependencies bug.

Basically, an expected a date dependency to be overridden in a particular case and it's not.

You can find the sample project code changes here: abel3cl@5771735

I added 2 properties to SyncUpForm.State date and style (some custom enum with 3 cases)
I also added a button and 3 toggles to SyncUpFormView

The first, sends an action whereas the 3 toggles take a Binding derived from some Bindable store to that style. To derived the binding I am using the subscript technique described in the documentation.

Toggling one of the toggles, changes the style and sets a date range. For example all time or last week.
Tapping the button simulates setting the style to .lastWeek. However this is with a "traditional" domain action.

In addition, I added a new line to App.swift when AppFeature is created, in the withDependencies block I am overriding $0.date.now to some canned Date.

Lastly, I added an environment variable to the scheme to get the same behaviour as UI testing.

When I tap the button "Send with action":
Date is overridden correctly to 14/12/2021 9:12AM UK format

When I toggle last week:
Date is the current now, at 11/03/2025 11:03AM UK format

You can also run the UI tests to verify the behaviour

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

Date.now dependency is overridden and it returns "my" expected value

Actual behavior

Date.now dependency is NOT overridden and it returns the default value. In this case, the "real" now, i.e. present time.

Reproducing project

Find the changes in here: abel3cl@5771735

The Composable Architecture version information

main

Destination operating system

iOS 18.1

Xcode version information

Version 16.1 (16B40)

Swift Compiler version information

swift-driver version: 1.115 Apple Swift version 6.0.2 (swiftlang-6.0.2.1.2 clang-1600.0.26.4)
Target: arm64-apple-macosx15.0

[mbrandonw]

Hi @abel3cl, this is not considered a bug in the library, though we may be able to provide a better story to this in 2.0 of the library.

The problem is that dependencies are only set in the body of the reducer. But you have accessed a dependency outside the reducer:

mutating func handleToggleStyle(_ style: Style) {
  @Dependency(\.date.now) var now
  …
}

…which is used in subscript(style:), which in turn is used from the view:

Toggle(
  style.description,
  isOn: $store.state[style: style]
)

So the \.date dependency goes back to its default since it is not accessed from within the body of the reducer.

There are some improvements in this area we will be able to make in the future, but it will be a slight breaking change and so will have to wait for 2.0. But, for the purpose of UI tests there is an easier way. You can use the prepareDependencies tool at the entry point of the app:

https://github.com/pointfreeco/syncups/blob/a909279860b25cd59aa0ba5fef922b52a63c7830/SyncUps/SyncUpsApp.swift#L22-L29

That should achieve what you want.

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!

[abel3cl]

Thank you very much for your quick reply @mbrandonw and for all your efforts with TCA and all other libraries.
I was partly convinced it wouldn't be a bug but a misuse of some of the tools.

Amazing! I didn't know about prepareDependencies. Again, thank you