perf: avoid per-read emptiness scan getCollectionData

[TMisiukiewicz]

Details

Problem

OnyxCache.getCollectionData() calls isEmptyObject(snapshot) on every read to decide whether to return the collection or the empty-collection fallback. Profiling the Search page (4× CPU throttle) showed this check is a significant cost:

• In a DevTools trace, isEmptyObject accounted for ~919 ms of self-time, called almost entirely from getCollectionData.
• Instrumenting getCollectionData per collection key confirmed the emptiness check dominates read time and that its cost scales with collection size:

collection	reads	total time (ms)	emptiness-check time (ms)	ms per read (emptiness check)
report_	180	201.3	192.8	~1.07
transactions_	114	131.5	130.2	~1.14
reportActions_	84	98.9	93.9	~1.12
transactionViolations_ (small)	699	35.6	34.1	~0.05

Rebuilds were negligible (≤4 across the run), so the cost is the per-read check itself, not snapshot rebuilding.

Solution

Store the shared frozen-empty object (FROZEN_EMPTY_COLLECTION) as the snapshot for empty collections instead of a fresh {}. The read path then:

• returns undefined for a collection that was never stored,
• detects "empty" with a single snapshot === FROZEN_EMPTY_COLLECTION reference comparison (no key scan),
• and for non-empty collections returns the snapshot directly, so isEmptyObject no longer runs on populated data at all.

Emptiness is now determined once, at snapshot-build time (rebuildCollectionSnapshot, which is dirty-gated and rare), rather than on every read.

Performance improvement:
On large account and staging, I was able to record 500-1000ms spent on isEmptyObject in getCollectionData. On dev, it was 400-700ms before and 11ms after.

Related Issues

$ Expensify/App#96855

Linked E/App PR

Expensify/App#96853

Automated Tests

Unit tests already cover it, added only one missing

Manual Tests

1. Open the app
2. Open multiple cold/warm reports
3. Verify every report opens correctly and shows all the data properly

Author Checklist

• I linked the correct issue in the ### Related Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android / native
  • Android / Chrome
  • iOS / native
  • iOS / Safari
  • MacOS / Chrome / Safari
  • MacOS / Desktop
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
  • I verified that any callback methods that were added or modified are named for what the method does and never what callback they handle (i.e. toggleReport and not onIconClick)
  • I verified that the left part of a conditional rendering a React component is a boolean and NOT a string, e.g. myBool && .
  • I verified that comments were added to code that is not self explanatory
  • I verified that any new or modified comments were clear, correct English, and explained "why" the code was doing something instead of only explaining "what" the code was doing.
  • I verified proper file naming conventions were followed for any new files or renamed files. All non-platform specific files are named after what they export and are not named "index.js". All platform-specific files are named for the platform the code supports as outlined in the README.
  • I verified the JSDocs style guidelines (in STYLE.md) were followed
• If a new code pattern is added I verified it was agreed to be used by multiple Expensify engineers
• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
• I verified all code is DRY (the PR doesn't include any logic written more than once, with the exception of tests)
• I verified any variables that can be defined as constants (ie. in CONST.js or at the top of the file that uses the constant) are defined as such
• I verified that if a function's arguments changed that all usages have also been updated correctly
• If a new component is created I verified that:
  • A similar component doesn't exist in the codebase
  • All props are defined accurately and each prop has a /** comment above it */
  • The file is named correctly
  • The component has a clear name that is non-ambiguous and the purpose of the component can be inferred from the name alone
  • The only data being stored in the state is data necessary for rendering and nothing else
  • If we are not using the full Onyx data that we loaded, I've added the proper selector in order to ensure the component only re-renders when the data it is using changes
  • For Class Components, any internal methods passed to components event handlers are bound to this properly so there are no scoping issues (i.e. for onClick={this.submit} the method this.submit should be bound to this in the constructor)
  • Any internal methods bound to this are necessary to be bound (i.e. avoid this.submit = this.submit.bind(this); if this.submit is never passed to a component event handler like onClick)
  • All JSX used for rendering exists in the render method
  • The component has the minimum amount of code necessary for its purpose, and it is broken down into smaller components in order to separate concerns and functions
• If any new file was added I verified that:
  • The file has a description of what it does and/or why is needed at the top of the file if the code is not self explanatory
• If the PR modifies a generic component, I tested and verified that those changes do not break usages of that component in the rest of the App (i.e. if a shared library or component like Avatar is modified, I verified that Avatar is working as expected in all cases)
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.
• I have checked off every checkbox in the PR author checklist, including those that don't apply to this PR.

Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[abzokhattab]

I will review this by the end of the day