fix(assets): memoize selectAllTokensFlat for referential stability

[cursor]

Description

Memoizes selectAllTokensFlat so consumers do not re-render when the flattened token list content is unchanged.

selectAllTokensFlat previously used createSelector and returned a fresh [] for the empty case. Because the result function always builds a new array via reduce/concat, any recomputation produced a new reference even when token contents were identical.

This change:

1. Switches selectAllTokensFlat to createDeepEqualSelector (already used by sibling token selectors)
2. Returns a module-level frozen empty array (EMPTY_TOKENS) for the empty case
3. Adds referential-stability unit tests (same state + equal-content states)

Caller note (out of scope): the only production consumer found is app/util/sentry/tags/index.ts, which reads .length and does not immediately .map/.filter the result.

Changelog

CHANGELOG entry: null

Related issues

Fixes: #31345
Fixes: ASSETS-3721

Manual testing steps

Feature: token list selector stability

  Scenario: wallet token list does not churn on unrelated store updates
    Given a wallet with tokens across one or more networks

    When an unrelated Redux update occurs that does not change token contents
    Then token list components subscribed via selectAllTokensFlat should keep a stable selector result reference

Unit coverage: yarn jest app/selectors/tokensController (23 passed), including referential-stability assertions.

Screenshots/Recordings

N/A — selector-only performance fix; no UI changes.

Before

N/A

After

N/A

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Mobile Coding Standards.
• I've completed the PR template to the best of my ability
• I've included tests if applicable
• I've documented my code using JSDoc format if applicable
• I've applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Performance checks (if applicable)

• I've tested on Android
  • Ideally on a mid-range device; emulator is acceptable
• I've tested with a power user scenario
  • Use these power-user SRPs to import wallets with many accounts and tokens
• I've instrumented key operations with Sentry traces for production performance metrics
  • See trace() for usage and addToken for an example

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

  

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Note

Low Risk
Selector-only memoization with no auth, data, or API behavior changes; production use is limited to Sentry tag .length.

Overview
selectAllTokensFlat no longer returns a new array reference on every recompute when the flattened token list is unchanged, which cuts unnecessary re-renders for Redux subscribers.

The selector is switched from createSelector to createDeepEqualSelector (aligned with other token selectors), and the empty case returns a shared frozen EMPTY_TOKENS instead of a fresh []. Unit tests assert stable references for repeated calls, equal-content state, and empty token maps.

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[github-actions]

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[github-actions]

🔍 Smart E2E Test Selection

• Selected E2E tags: None (no tests recommended)
• Selected Performance tags: None (no tests recommended)
• Risk Level: low
• AI Confidence: 92%

click to see 🤖 AI reasoning details

E2E Test Selection:
The PR makes two changes:

1. tokensController.ts: Upgrades selectAllTokensFlat from createSelector to createDeepEqualSelector and introduces a frozen EMPTY_TOKENS constant for stable empty array references. This is a pure memoization/performance optimization — the functional output (token data) is identical, just with better reference stability to prevent unnecessary re-renders.

2. tokensController.test.ts: Adds unit tests to verify the stable reference behavior of the updated selector.

Impact analysis:

• selectAllTokensFlat is only consumed by app/util/sentry/tags/index.ts for tracking wallet.token_count in Sentry error tags — no React UI components use it directly (confirmed by grep showing zero .tsx importers).
• The change is a memoization improvement, not a behavioral change. The selector still returns the same token data.
• No user-facing flows are affected.
• No E2E smoke tests cover Sentry tag reporting.

Conclusion: No E2E smoke tags are needed. The change is a low-risk internal optimization with unit test coverage. No performance test tags are warranted either, as this is a selector-level memoization fix that doesn't affect any measured performance scenarios (onboarding, login, launch, swaps, asset loading, etc.).

Performance Test Selection:
The change optimizes memoization of the selectAllTokensFlat selector, which is only used for Sentry tag reporting (wallet.token_count). While better memoization could theoretically reduce unnecessary re-renders, this selector is not used in any UI component and doesn't affect any of the measured performance scenarios (launch, login, onboarding, swaps, asset loading, account list, perps, predictions). No performance test tags are warranted.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud