Fix memory leaks in Redwood UIView components #2899

jhollida24 · 2026-01-08T20:01:20Z

This PR fixes multiple memory leaks in Redwood's iOS/UIView implementation that prevented UI components from being deallocated after they were removed from the view hierarchy (e.g., after logout).

I am not a Kotlin, GC, or Treehouse expert. This is the result of a local redwood -> cash-ios build, a test case of opening several receipts, and a conversation with an LLM that can see both sets of source about the references to leaked objects I can see using the iOS memory graph.

This does get rid of a large number of leaked objects on the iOS side.

Root Cause
Kotlin/Native uses reference counting similar to ARC, but lambdas and anonymous objects create strong references by default. Several components were creating retain cycles that prevented proper cleanup.

Fixes

UIViewBox and UIViewFlexContainer - Used WeakReference to break retain cycles in SizeListener callbacks that were capturing parent containers strongly.
RedwoodUIView - Used WeakReference in the sizeListener to prevent capturing valueRootView strongly.
HostProtocolAdapter - The widgetSystem was being retained even after close() was called, creating a Kotlin→Swift reference chain that kept Swift objects alive. Made widgetSystem nullable and cleared it in close() to break this chain.

Impact
These leaks affected all Treehouse-based screens in Cash iOS. After logout, UI components and their associated graphics memory (VM: Memory Tag objects) remained in memory indefinitely. With these fixes, all components are now properly deallocated when screens are dismissed.

This was tested with Receipts on the activity tab.

jhollida24 · 2026-01-08T20:01:51Z

redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewBox.kt

    var sizeListener: ResizableWidget.SizeListener? = null
    private val measurer = Measurer()

+    @OptIn(kotlin.experimental.ExperimentalNativeApi::class)


I am not clear if this is an okay thing to do, and I'm hoping that someone who knows this area better than I can weigh in.

This seems reasonable to me--WeakReference was added as an internal ExperimentalNativeApi in #2191 cc @JakeWharton

Yep it should be fine to use ExperimentalNativeApis as an implementation detail (like here).

Awesome. Thanks for verifying!

jhollida24 · 2026-01-08T20:13:16Z

Build methodology, if you wanted to try this out locally:

1. Configure cash-treehouse to use local Redwood

Create or edit ~/Development/cash-treehouse/local.properties:

# Enable Kotlin/Native to build iOS framework
treehouse.kotlin.native.enabled=true

# Point to your local Redwood repository
redwoodPath=/Users/jholliday/Development/redwood

2. Build cash-treehouse with local Redwood changes

cd ~/Development/cash-treehouse
./gradlew debugSpm --no-configuration-cache

This will:

Build the CashKt XCFramework with your local Redwood changes
Automatically update cash-ios/ThirdParty/SPM/Package.swift to point to the local build
Output the framework to platform/host-ios/build/XCFrameworks/debug/CashKt.xcframework

3. Resolve SPM dependencies in cash-ios

cd ~/Development/cash-ios
cash spm resolve

4. Build and run Cash iOS

Build and run from Xcode

dnagler · 2026-01-08T20:19:11Z

...od-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewFlexContainer.kt

-  private val enclosing: UIViewFlexContainer,
+  enclosing: UIViewFlexContainer,
 ) : SizeListener {
+  private val weakEnclosing = WeakReference(enclosing)


Ooh, great catch here, this would be a strict retain cycle otherwise

dnagler

These changes make sense to me! I think it's worth getting at least one more set of eyes on them just to be safe though.

Before merging, it'd be good to add a note in CHANGELOG.md. I think @colinrtwhite should be able to create a 0.19.1 release?

jhollida24 · 2026-01-08T21:13:05Z

These changes make sense to me! I think it's worth getting at least one more set of eyes on them just to be safe though.

Before merging, it'd be good to add a note in CHANGELOG.md. I think @colinrtwhite should be able to create a 0.19.1 release?

Added a 0.19.1 section in the Changelog

dellisd

This looks good to me

redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewBox.kt

colinrtwhite

Looks good to me, though I'm curious why the size listeners aren't being freed when the associated UI component is removed from the view hierarchy - maybe some ARC intricacies I don't understand.

HostProtocolAdapter - The widgetSystem was being retained even after close() was called, creating a Kotlin→Swift reference chain that kept Swift objects alive

Is it possible that this change is enough to fix the issue on its own?

colinrtwhite · 2026-01-08T20:53:57Z

redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewBox.kt

    var sizeListener: ResizableWidget.SizeListener? = null
    private val measurer = Measurer()

+    @OptIn(kotlin.experimental.ExperimentalNativeApi::class)


Yep it should be fine to use ExperimentalNativeApis as an implementation detail (like here).

jhollida24 · 2026-01-08T22:30:19Z

Looks good to me, though I'm curious why the size listeners aren't being freed when the associated UI component is removed from the view hierarchy - maybe some ARC intricacies I don't understand.

HostProtocolAdapter - The widgetSystem was being retained even after close() was called, creating a Kotlin→Swift reference chain that kept Swift objects alive

Is it possible that this change is enough to fix the issue on its own?

I tried testing with just that commit, rebasing the others away. The leaks of the UIViewBox (and a whole bunch of other objects) reappear when I do this.

(The Backtrace here is when this object was allocated)

Use WeakReference to break retain cycles in UIViewBox and UIViewFlexContainer. In Kotlin/Native, lambdas and anonymous objects create strong references by default. The SizeListener callbacks were capturing their parent containers strongly, creating retain cycles that prevented deallocation: - UIViewBox.View → children → insert lambda → SizeListener → View - UIViewFlexContainer → NodeSizeListener → enclosing container This caused UI components to remain in memory after they should have been deallocated (e.g., after logout), along with their associated graphics buffers (VM: Memory Tag objects). The fix uses WeakReference for the captured parent references, allowing proper cleanup when the parent containers are no longer needed.

Use WeakReference to break retain cycle in RedwoodUIView's sizeListener. The sizeListener was capturing valueRootView strongly, creating a retain cycle: RedwoodUIView → valueRootView → sizeListener → valueRootView This prevented RedwoodUIView instances from being deallocated after they were removed from the view hierarchy. The fix uses WeakReference for the captured valueRootView reference, allowing proper cleanup when the view is no longer needed.

The HostProtocolAdapter was retaining a reference to the WidgetSystem even after close() was called. This created a leak chain: HostProtocolAdapter (Kotlin) → WidgetSystem (Kotlin) → RealTreehouseWidgetFactory (Swift) → ContentListener (Swift) This prevented Swift objects from being deallocated even though they had no strong references visible in the memory graph, because they were being retained by Kotlin/Native's reference counting. The fix makes widgetSystem nullable and clears it in close(), breaking the Kotlin→Swift reference chain and allowing proper cleanup.

JakeWharton

Curious as to why the built-in leak detection didn't catch these cases. Normally I'd want that blind spot fixed to prevent regression, but it's probably not worth the effort at this point.

JakeWharton · 2026-01-08T23:12:29Z

CHANGELOG.md

+## [0.19.1] - 2026-01-08
+[0.19.1]: https://github.com/cashapp/redwood/releases/tag/0.19.1


Suggested change

## [0.19.1] - 2026-01-08

[0.19.1]: https://github.com/cashapp/redwood/releases/tag/0.19.1

## Unreleased

Normally there's already an unreleased section here, but I didn't add it when releasing 0.19.0 because I don't expect there to ever be another public release. The version, date, and link are inserted as part of the release process.

You're probably better off just doing an internal release to use this and any future changes. I would not bother doing public releases. But I'm still supportive of documenting the changes using the normal process.

JakeWharton · 2026-01-08T23:15:04Z

CHANGELOG.md

+- Fix memory leaks in UIView layout components (`UIViewBox`, `UIViewFlexContainer`, `RedwoodUIView`) where `SizeListener` callbacks were creating retain cycles by capturing parent containers strongly. Now use `WeakReference` to break these cycles.
+- Fix memory leak in `HostProtocolAdapter` where the `widgetSystem` was retained after `close()` was called, creating a Kotlin→Swift reference chain that prevented Swift objects from being deallocated. The `widgetSystem` is now cleared in `close()` to break this chain.
+
+These leaks affected some Treehouse-based screens and prevented UI components and their associated memory from being released after dismissal.


Suggested change

- Fix memory leaks in UIView layout components (`UIViewBox`, `UIViewFlexContainer`, `RedwoodUIView`) where `SizeListener` callbacks were creating retain cycles by capturing parent containers strongly. Now use `WeakReference` to break these cycles.

- Fix memory leak in `HostProtocolAdapter` where the `widgetSystem` was retained after `close()` was called, creating a Kotlin→Swift reference chain that prevented Swift objects from being deallocated. The `widgetSystem` is now cleared in `close()` to break this chain.

These leaks affected some Treehouse-based screens and prevented UI components and their associated memory from being released after dismissal.

- Fix memory leaks in UIView layout components (`UIViewBox`, `UIViewFlexContainer`, `RedwoodUIView`) where `SizeListener` callbacks were creating retain cycles by capturing parent containers strongly.

- Fix memory leak in `HostProtocolAdapter` where the `widgetSystem` was retained after `close()` was called, creating a Kotlin→Swift reference chain that prevented Swift objects from being deallocated.

This log is for users who aren't expected to know implementation details. Those are better served in the commit message for us developers.

Condensed. The same details are indeed in the individual commits.

jhollida24 · 2026-01-08T23:40:47Z

Curious as to why the built-in leak detection didn't catch these cases. Normally I'd want that blind spot fixed to prevent regression, but it's probably not worth the effort at this point.

Yeah, probably not.

Thank you for taking a look at this. It's very much appreciated 🙏 .

jhollida24 requested review from JakeWharton, dnagler, swankjesse and underscoretang January 8, 2026 20:01

jhollida24 commented Jan 8, 2026

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jhollida24 requested review from colinrtwhite and jingibus January 8, 2026 20:06

dnagler requested a review from dellisd January 8, 2026 20:08

dnagler reviewed Jan 8, 2026

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dnagler approved these changes Jan 8, 2026

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dellisd approved these changes Jan 8, 2026

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redwood-layout-uiview/src/commonMain/kotlin/app/cash/redwood/layout/uiview/UIViewBox.kt Outdated Show resolved Hide resolved

colinrtwhite approved these changes Jan 8, 2026

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jhollida24 added 3 commits January 8, 2026 14:36

jhollida24 force-pushed the jholliday/ios-leaks branch from bb8d4c1 to 4ce075f Compare January 8, 2026 22:38

JakeWharton approved these changes Jan 8, 2026

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Add CHANGELOG items for the leaks

e8c07bd

jhollida24 force-pushed the jholliday/ios-leaks branch from 4ce075f to e8c07bd Compare January 8, 2026 23:39

jhollida24 enabled auto-merge (squash) January 8, 2026 23:41

jhollida24 disabled auto-merge January 8, 2026 23:41

jhollida24 enabled auto-merge (squash) January 8, 2026 23:41

jhollida24 merged commit 5c49a0b into trunk Jan 9, 2026
14 checks passed

jhollida24 deleted the jholliday/ios-leaks branch January 9, 2026 00:19

		## [0.19.1] - 2026-01-08
		[0.19.1]: https://github.com/cashapp/redwood/releases/tag/0.19.1

	## [0.19.1] - 2026-01-08
	[0.19.1]: https://github.com/cashapp/redwood/releases/tag/0.19.1
	## Unreleased

Fix memory leaks in Redwood UIView components #2899

Fix memory leaks in Redwood UIView components #2899

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jhollida24 commented Jan 8, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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jhollida24 commented Jan 8, 2026

1. Configure cash-treehouse to use local Redwood

2. Build cash-treehouse with local Redwood changes

3. Resolve SPM dependencies in cash-ios

4. Build and run Cash iOS

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