fix(app): support register_lifespan_task as a decorator

[BABTUNA]

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Type of change

• Bug fix (non-breaking change which fixes an issue)

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Description

This fixes App.register_lifespan_task so it can be used as a decorator.

Before this change:

• @app.register_lifespan_task bound the function name to None because
  the method did not return the task.
• @app.register_lifespan_task(timeout=10) raised because the method
  required task as a positional arg.

What changed:

• register_lifespan_task now accepts task: Callable | asyncio.Task | None = None.
• If called with no task and kwargs only, it returns a decorator via
  functools.partial(...).
• When registration succeeds, it now returns the original task callable.
• Internal registration behavior is unchanged: kwargs still produce a
  wrapped functools.partial that is stored in _lifespan_tasks.

Tests

Added:

• tests/units/app_mixins/test_lifespan.py
  • test_register_lifespan_task_can_be_used_as_decorator
  • test_register_lifespan_task_with_kwargs_can_be_used_as_decorator

Validation run:

• uv run pytest tests/units/app_mixins/test_lifespan.py -q
• uv run ruff check reflex/app_mixins/lifespan.py tests/units/app_mixins/ test_lifespan.py

Closes #6330

[greptile-apps]

Greptile Summary

This PR fixes register_lifespan_task so it can be used as a plain decorator (@app.register_lifespan_task) or as a decorator factory (@app.register_lifespan_task(timeout=10)). The fix makes task optional, returns a functools.partial when called with kwargs only, and returns the original callable on registration so the decorated name is correctly preserved.

Confidence Score: 5/5

Safe to merge — the logic is correct, tests cover both decorator patterns, and internal registration behavior is unchanged.

Both decorator patterns work correctly: the plain case returns the original callable and stores it directly, and the kwargs case returns a functools.partial decorator that then triggers registration with proper guard checks. The _lifespan_tasks_started guard and generator-function validation are preserved on the registration path. No P0/P1 issues found.

No files require special attention.

Important Files Changed

Filename	Overview
reflex/app_mixins/lifespan.py	Added decorator support to register_lifespan_task: accepts task=None, returns a functools.partial decorator when called with kwargs only, and returns the original callable on direct registration.
tests/units/app_mixins/test_lifespan.py	New test file with two tests covering the plain-decorator and decorator-with-kwargs patterns; both are well-structured and verify registration behavior and callable identity correctly.

Sequence Diagram

sequenceDiagram
    participant U as User Code
    participant R as register_lifespan_task
    participant T as _lifespan_tasks dict

    Note over U,T: Plain decorator usage
    U->>R: @app.register_lifespan_task (task=fn)
    R->>R: task is not None → proceed
    R->>R: check _lifespan_tasks_started
    R->>R: check isgeneratorfunction
    R->>T: _lifespan_tasks[fn] = None
    R-->>U: returns fn (original callable)

    Note over U,T: Decorator-with-kwargs usage
    U->>R: @app.register_lifespan_task(timeout=10)
    R->>R: task is None → return partial(register_lifespan_task, timeout=10)
    R-->>U: returns partial (decorator)
    U->>R: partial(fn) → register_lifespan_task(fn, timeout=10)
    R->>R: check _lifespan_tasks_started
    R->>R: check isgeneratorfunction
    R->>R: registered_task = partial(fn, timeout=10)
    R->>T: _lifespan_tasks[registered_task] = None
    R-->>U: returns fn (original callable)

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_{Reviews (1): Last reviewed commit: "Support register_lifespan_task decorator..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

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_{Comparing BABTUNA:fix-6330-register-lifespan-decorator (8b97bdd) with main (cc7436c)}