[scrollable_positioned_list] Expose ScrollController to be observed by Scrollbar

[nabil6391]

Description

This PR exposes the primary listview's scrollController inside the ScrollablePositionedList from the ItemScrollController so that it can be observer by ScrollBars.

This also allows passing the scrollController to the ItemScrollController.

As a result we will finally be able to use like this

    Scrollbar(
        thickness: 8,
        radius: Radius.circular(4),
        controller: itemScrollController.scrollController,
        child: ScrollablePositionedList.builder(
              itemCount: totalItems,
              itemScrollController: itemScrollController,
              itemBuilder: (context, index) {
                return Container(
                    color: getItemColor(index, context),
                    child: buildDropdownList(context, index),
                );
              },
          ),
     )

Related Issues

#273
#303
#278
#235
#175

Checklist

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• I signed the [CLA].
• All tests from running flutter test pass.
• flutter analyze does not report any problems on my PR.
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[nabil6391]

@tarobins please have a look at this PR at your convenience

[tarobins]

Has this been well tested in the use case you are trying it with? Does the scrollbar behave as expected when you jump around etc?

[nabil6391]

I used the above code into your example main.dart and wrapped the ScrollablePositionedList with the Scrollbar.

Here are the results:

Horizontal around 30

Horizontal around 100

Vertical around 1000

Vertical around 5000

According to my manual tests, both jump and scroll to also works fine and the scroll bar automatically is updated.

[yokemuratakeshi]

I incorporated this change into my project and it's working pretty well.

[yokemuratakeshi]

It looked like working, but when performing a long distance scroll my listener seems to have lost track of the scroll.
Is it because the internal behavior that it uses two ListView's to make the long scroll smooth?

[lff5]

Hi. Any progress on this?

[nabil6391]

I am happily using it in my app after forking, depends on the authors to decide if they want to merge this or not.

[korvan]

There is a problem with exposed scrollController.
After list is scrolled to a long distance, primary and secondary _ListDisplayDetails are swapped. After such scroll, exposed ItemScrollController.scrollController is not attached to any ScrollPosition and can't be used any more.

[TheCarpetMerchant]

@korvan As long as that is properly documented in-code (so on ItemScrollController.scrollController) , I do not think this to be an issue as this is purely added functionality.
Also, I'm not sure why this PR exists, as scroll notifications are passed up the tree if not intercepted. My Scrollbar works perfectly fine.

But I would like to be able to scroll a precise number of pixels, and for that I need access to ScrollController. It could also be added as another API to ItemScrollController (ie jumpToPixel for example).
I'll see about making my own PR for that, but merging this one would also solve my problem.

[nabil6391]

In my case, I required exposing the scrollController to jump to a position and also use it with a custom ScrollBar such as DraggableScrollbar. Thats the reason for me to create this PR, in order to expose it for whatever requirements we need.

[TheCarpetMerchant]

@nabil6391 Ah of course, that makes sense then.
In the meantime I made #388 for my own needs. Oh well.

[saibotma]

Will this be merged eventually @tarobins?

[felipecastrosales]

👁️