Add rx.event arg to cancel previous background tasks

[tim-haselhoff]

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a. Give a descriptive title to your PR.
Add rx.event arg to cancel previous background tasks

b. Describe your changes.
Add rx.event arg to cancels runs of previous background tasks when a new one is scheduled.

c. Put `closes #XXXX` in your comment to auto-close the issue that your PR fixes (if such).

[benedikt-bartscher]

Looks good to me, thanks Tim!

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

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_{Comparing tim-haselhoff:cancel-previous-background-tasks (95fd0ce) with main (f79d011)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[greptile-apps]

Greptile Summary

This PR adds a cancel_previous_task=True option to rx.event that automatically cancels a still-running background task for the same client whenever a new dispatch of the same handler arrives. The implementation is clean and correct.

• O(1) lookup via _cancel_keys: A secondary dict[(token, event_name) → txid] is introduced, populated in _create_event_task and cleaned up in _finish_task (with a guard that avoids deleting a newer task's entry) and in stop().
• Validation at decoration time: cancel_previous_task=True is rejected with a clear ValueError unless background=True is also set; the is_cancel_previous_task property on EventHandler propagates the marker to the processor.
• Tests: A new async integration test exercises the cancellation path end-to-end and inspects _cancel_keys directly, verifying both start, cancel, and completion ordering.

Confidence Score: 5/5

Safe to merge; the cancellation bookkeeping is correct, cleanup is properly guarded, and the new behavior is well-tested.

The cancellation bookkeeping handles all edge cases correctly — task already done when superseded, task replaced before its _finish_task fires, and shutdown ordering. No runtime defects were found in the changed code.

No files require special attention.

Important Files Changed

Filename	Overview
packages/reflex-base/src/reflex_base/event/processor/event_processor.py	Adds _cancel_keys dict for O(1) cancel-key lookup; cancellation logic in _create_event_task is correct; _finish_task cleanup correctly guards against stomping a newer task's entry; shutdown clears the dict after tasks are stopped.
packages/reflex-base/src/reflex_base/event/init.py	Adds cancel_previous_task parameter and CANCEL_PREVIOUS_TASK_MARKER constant; validation correctly requires background=True to be co-set; is_cancel_previous_task property is a clean addition to EventHandler.
tests/units/test_state.py	Adds CancelPreviousTaskState and test_background_task_cancel_previous; module-level mutable lists accessed inside the background handler are safe for sequential runs but would conflict under parallel test execution.
packages/reflex-base/news/6793.feature.md	Changelog entry with minor wording inaccuracy ("scheduled events" vs "running background tasks").

_{Reviews (6): Last reviewed commit: "Merge branch 'main' into cancel-previous..." | Re-trigger Greptile}

[benedikt-bartscher]

this sadly probably won't work as expected with redis :/