feat: Resolve CSS transition timelines for Android and bridge them over JNI

[MatiPl01]

Owns the per-property state CSS reversing needs: it detects a reversal against the in-flight transition's start value, reverse-shortens the duration through the easing curve, and hands the resolved timeline to the Kotlin animator from #10090.

Elapsed time crosses the boundary instead of an absolute start. Reverse-shortening backdates the start and transition-delay pushes it forward, and computing it here keeps Kotlin off a second clock that would drift from the loop's timestamp.

Easings cross as the same normalized point arrays the loop uses, so nothing reimplements the easing spec on the other side of the boundary.

Nothing routes to this yet; it is inert until the next PR flips the gate.

Stacked on #10090.

[coderabbitai]

Note

Reviews paused

It looks like this branch is under active development. To avoid overwhelming you with review comments due to an influx of new commits, CodeRabbit has automatically paused this review. You can configure this behavior by changing the reviews.auto_review.auto_pause_after_reviewed_commits setting.

Use the following commands to manage reviews:

• @coderabbitai resume to resume automatic reviews.
• @coderabbitai review to trigger a single review.

Use the checkboxes below for quick actions:

• ▶️ Resume reviews
• 🔍 Trigger review

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: f3a3adb7-a633-4453-ac89-aaf64ec47f21

📥 Commits

Reviewing files that changed from the base of the PR and between 0ec3bc1 and d87aa6d.

📒 Files selected for processing (1)

• packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt

---

📝 Walkthrough

Summary by CodeRabbit

• New Features

  • Added Android support for CSS transitions on scalar properties.
  • Supports cubic-bezier, steps, and linear-stop easing configurations.
  • Transitions can preserve their final animated values after completion.
  • Added smooth handling for transition reversals and value reconciliation.

• Bug Fixes

  • Improved transition lifecycle management, including reliable cleanup when transitions are removed or replaced.

Walkthrough

Adds Android C++ CSS transition management with easing conversion, scalar transition application, reversal handling, and removal. Wires transition callbacks through NativeProxy JNI to the Kotlin manager, which preserves completed values for persistent transitions.

Changes

Android CSS transitions

Layer / File(s)	Summary
Transition contract and easing representation packages/react-native-reanimated/android/src/main/cpp/reanimated/android/CSS/CSSPlatformTransitions.h, packages/react-native-reanimated/android/src/main/cpp/reanimated/android/CSS/CSSPlatformTransitions.cpp	Defines PlatformEasing, transition callbacks, active-transition storage, and CSS easing conversion.
Transition application and removal packages/react-native-reanimated/android/src/main/cpp/reanimated/android/CSS/CSSPlatformTransitions.cpp	Applies scalar transitions, handles reversals, stores active state, and removes transitions by view and property.
NativeProxy and Java manager wiring packages/react-native-reanimated/android/src/main/cpp/reanimated/android/NativeProxy.h, packages/react-native-reanimated/android/src/main/cpp/reanimated/android/NativeProxy.cpp, packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/NativeProxy.kt, packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSEasing.kt	Forwards transition values, easing points, persistence, and removal operations between C++ and Kotlin.
Persistent transition lifecycle packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/css/CSSPlatformTransitionsManager.kt	Retains completed values for persistent transitions and removes animator state for non-persistent transitions.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant CSSPlatformTransitions
  participant NativeProxy
  participant CSSPlatformTransitionsManager
  participant AndroidAnimator
  CSSPlatformTransitions->>NativeProxy: cssAnimateTransition with values and easing
  NativeProxy->>CSSPlatformTransitionsManager: animateTransition
  CSSPlatformTransitionsManager->>AndroidAnimator: start transition
  AndroidAnimator-->>CSSPlatformTransitionsManager: completion callback
  CSSPlatformTransitionsManager->>CSSPlatformTransitionsManager: retain or remove completed state
  CSSPlatformTransitions->>NativeProxy: cssRemoveTransition
  NativeProxy->>CSSPlatformTransitionsManager: removeTransition

Loading

Possibly related PRs

• software-mansion/react-native-reanimated#10057: Introduces the unified platform transition hook used by this Android backend.
• software-mansion/react-native-reanimated#10090: Extends the shared Android transition manager with persistent-transition behavior.
• software-mansion/react-native-reanimated#10169: Refactors the same CSS transition and JNI APIs.

Suggested reviewers: pawicao

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the Android CSS transition timeline resolution and JNI bridging implemented by the pull request.
Description check	✅ Passed	The description accurately explains per-property state, reversal handling, timeline resolution, easing transfer, and the planned follow-up activation.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch @matipl01/android-css-platform-transition-backend

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In
`@packages/react-native-reanimated/android/src/main/cpp/reanimated/android/CSS/CSSPlatformTransitions.cpp`:
- Around line 94-103: In the transition loop around animate_, remove or cancel
the existing platform transition for the current key before returning false when
animate_ fails. Ensure both the prior animator and its active_ entry are cleaned
up so loop fallback cannot conflict with the old Android animator or stale
reversal state.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: a6563d7c-8083-404d-a0ba-7b6bf6789f16

📥 Commits

Reviewing files that changed from the base of the PR and between 102d063 and 224ada3.

📒 Files selected for processing (5)

• packages/react-native-reanimated/android/src/main/cpp/reanimated/android/CSS/CSSPlatformTransitions.cpp
• packages/react-native-reanimated/android/src/main/cpp/reanimated/android/CSS/CSSPlatformTransitions.h
• packages/react-native-reanimated/android/src/main/cpp/reanimated/android/NativeProxy.cpp
• packages/react-native-reanimated/android/src/main/cpp/reanimated/android/NativeProxy.h
• packages/react-native-reanimated/android/src/main/java/com/swmansion/reanimated/NativeProxy.kt