removeDuplicates in WithViewStore not working?

[miguelkennedy]

Hey guys...

Maybe someone can shed some light into an issue I'm running using SwiftUI and TCA.

I'm implementing removeDuplicates in a WithViewStore initializer in order to avoid re-rendering my view when state doesn't change (the view relies heavily on custom paths and shapes) but I can't seem to make it work. The view still re-renders even though state hasn't change. This ends up affecting the overall performance of my app.

Here's my WithViewStore initializer:

WithViewStore(
	self.store.scope(
		state: { $0.chartYAxis },
		action: MainChartFeatureAction.chartYAxis
	),
	removeDuplicates: isDuplicate(lhs:rhs:)
) { viewStore in

REST OF THE VIEW'S CODE GOES HERE

And here's how I implemented the isDuplicate function:

func isDuplicate(
	lhs: ChartYAxis.ViewState,
	rhs: ChartYAxis.ViewState
) -> Bool {
	return lhs.dummyMode == rhs.dummyMode &&
               lhs.yAxisLineWidth == rhs.yAxisLineWidth &&
	       lhs.scaleYFactor == rhs.scaleYFactor &&
	       lhs.minAxisValue == rhs.minAxisValue &&
	       lhs.maxAxisValue == rhs.maxAxisValue
}

By using print() in my code I can see that the isDuplicate function is being called correctly before each render and even though it returns true (state hasn't changed) all views wrapped inside WithViewStore still re-render.

If I redo the entire view using standard SwiftUI code without TCA and using local state and conform the view to Equatable I can avoid re-renders all together as intended.

Can anyone help? What am I doing wrong here? :)

Thanks in advance!

[SilverTab]

Was just about to post this exact question but found this one instead, so here's a shameless bump! Even just returning true in removeDuplicates doesn't seem to have any effect on how often the views inside the WithViewStore gets re-rendered, am I perhaps misunderstanding the point of removeDuplicates?

Any help would be appreciated!

[lightandshadow68]

One way to tell what's causing re-rendering is to use...

var body: some View {
    Self._printChanges()
    return VStack {
        // .. other changes
    }
}

Also, you can use ._printChanges() at the end of a store to print differences as they occur. it's really useful for debugging.

SignInView(store: Store(initialState: SignInFeature.State(), reducer: SignInFeature()._printChanges()))

See this article for additional details and strategies: https://www.avanderlee.com/swiftui/debugging-swiftui-views/

[SilverTab]

Hmm so, after trying that, it looks like no matter what I return in removeDuplicates: (wether it's always true, or always false, or some other logic), the output is exactly the same, and Self._printChanges() prints the following, the exact same amount of times in each case: MyView: @self changed. 🤔

I was using _printChanges() on my store already, so I'm already aware of state changes are occurring, what I don't understand is that I'm purposefully ignoring these state changes in removeDuplicate to avoid a re-render, yet it is still happening. Hence why I'm asking if removeDuplicate is really what I think it is (i.e. a way to avoid a re-render of the view by returning true).

[stephencelis]

@SilverTab Without a project to look at it'll be very difficult to debug the issue for you, but if the view is always rendering, then it may be the result of a parent view always updating.

[SilverTab]

@stephencelis Totally understandable! And your comment about the parent view always updating is making me think 🤔
Will keep that in mind as I take another look at this code, and try to extract a minimal working example to share if possible!

That being said; I'd love to confirm that my understanding of removeDuplicate's purposes is indeed correct! In theory, should returning true here prevent the view from getting redrawn?

[fabstu]

I found my render instrumentation does a pretty good job of showing why a view re-renders due to TCA. Just applying

.tcaSettings(
  .withRerenderLogging(.statistics)
  .withInitialStateHideDiffs(true)
)

to the top view shows why ALL the TCA views below re-render.

#1695

@stephencelis Are you guys planning to implement something similarly recursive? Currently occasionally rebased to 0.52.11 for my own use, but would love to have something similar inside TCA's debugging-tooling.

[SilverTab]

Ok I figured it out! 😅

Turns out @stephencelis was right; the problem was my very root view's ViewStore observing { $0 } which was causing a lot of extra re-renders, so pretty much all the child views where I was trying to narrow the ViewState and the removeDuplicates was getting ignored/bypassed. By reconsidering exactly what the root view needed to observe so as to avoid any unnecessary re-renders, the child views started behaving as I expected!