Assertion failure in RawScrollbarState when dynamically assigning a controller and setting thumbVisibility: true #172614
Description
Steps to reproduce
- A stateful widget builds a
Scrollbar, wrapping its child. This child is expected to contain aSingleChildScrollView. Initially, the widget has no scroll controller, so itsScrollbaris built withcontroller: null,thumbVisibility: falseandinteractive: false. - In a next frame, a scroll controller becomes available, so
Scrollbaris rebuilt with a non-nullcontroller,thumbVisibility: trueandinteractive: true. The controller is not yet attached at this point, as the descendantSingleChildScrollViewstill has to be rebuilt with the new controller, too.
Expected results
The scrollbar is now working interactively (i.e. can be used to change the current scroll offset through the provided controller).
Actual results
The scrollbar is made interactive, as expected, but first an assertion failure is triggered in the RawScrollbarState underlying the Scrollbar widget:
══╡ EXCEPTION CAUGHT BY ANIMATION LIBRARY ╞══════════════════════
The following assertion was thrown while notifying status
listeners for AnimationController:
The Scrollbar's ScrollController has no ScrollPosition attached.
A Scrollbar cannot be painted without a ScrollPosition.
The Scrollbar attempted to use the provided ScrollController.
This ScrollController should be associated with the ScrollView
that the Scrollbar is being applied to.
When providing your own ScrollController, ensure both the
Scrollbar and the Scrollable widget use the same one.
When the exception was thrown, this was the stack
The AnimationController notifying status listeners was:
AnimationController#ab7ae(▶ 0.000)
═════════════════════════════════════════════════════════════════
This assertion is checked synchronously at build time, in RawScrollbarState._debugCheckHasValidScrollPosition, called from RawScrollbarState._validateInteractions, which is a status listener of RawScrollbarState._fadeOutAnimationController. This animation controller is started at build time, in RawScrollbarState.didUpdateWidget, causing the assertion check to be performed at build time as well.
There is another assertion to verify the same thing, but that is scheduled as a post-frame-callback (in RawScrollbarState._debugScheduleCheckHasValidScrollPosition), when you can more reasonably assume that a controller will have been attached.
On a side note, I'm not sure whether this claim that "A Scrollbar cannot be painted without a ScrollPosition." is true. Painting the scrollbar seems to be based on information from Scroll(Metrics)Notification. Maybe complaining about an unattached ScrollController could be delayed up to the point where one is really needed: actual scrollbar interaction?
Code sample
See https://dartpad.dev/d564e246c3fccfe638a7b9b5e0ec9ced. Perform step 2 (the assignment of a scroll controller, thumbVisibility: true and interactive: true) by flipping the switch at the top.
Screenshots or Video
No response
Logs
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