CustomScrollView scroll behavior changes when scrollController is passed

Question

I'm trying to auto scroll to the end of a SliverList that is inside a CustomScrollView. SliverList itself doesn't have a controller property so I have to pass a ScrollController to the CustomScrollView.

The problem is when I pass a controller to the CustomScrollView its behavior changes and it's no longer scrolls the outer list and would not cause a collapsed SliverAppBar widget. How can I auto scroll SliverList and also keep the behavior of CustomScrollView as before?

Here is my code:

class _MyHomePageState extends State<MyHomePage> {
  ScrollController _scrollController = ScrollController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(widget.title),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            _scrollController.animateTo(
              _scrollController.position.maxScrollExtent,
              curve: Curves.easeOut,
              duration: const Duration(seconds: 1),
            );
          },
        ),
        body: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return <Widget>[
              SliverAppBar(
                expandedHeight: 230.0,
                pinned: true,
                flexibleSpace: FlexibleSpaceBar(
                  title: Text('SliverAppBar Expand'),
                ),
              )
            ];
          },
          body: CustomScrollView(
                //When controller is passed to CustomScrollView, its behavior changes
                // controller: _scrollController,  
                slivers: [
                  //Some widgets are here
                  SliverList(
                    delegate: SliverChildBuilderDelegate(
                      (context, index) {
                        return Container(
                          height: 80,
                          color: Colors.primaries[index % Colors.primaries.length],
                          alignment: Alignment.center,
                          child: Text(
                            'Item : $index',
                          ),
                        );
                      },
                      childCount: 20,
                    ),
                  ),
                ],
              ),
            ) ,
         );
      }
    }

yellowgray · Accepted Answer · 2020-12-22 09:05:59Z

5

I think you may notice the explanation of the example here: NestedScrollView class

// The "controller" and "primary" members should be left
// unset, so that the NestedScrollView can control this
// inner scroll view.
// If the "controller" property is set, then this scroll
// view will not be associated with the NestedScrollView.
// The PageStorageKey should be unique to this ScrollView;
// it allows the list to remember its scroll position when
// the tab view is not on the screen.

and

// This Builder is needed to provide a BuildContext that is
// "inside" the NestedScrollView, so that
// sliverOverlapAbsorberHandleFor() can find the
// NestedScrollView.

The correct way is to get the controller used by CustomScrollView, not to add a new one to it.

It can be done by adding a Builder above the CustomScrollView) and assign the controller to _scrollController.

...
  body: NestedScrollView(
    ...
    body: Builder(
      builder: (context){
        _scrollController = PrimaryScrollController.of(context);
        return CustomScrollView(
          ...
      },
    ),
  ),
...

answered Dec 22, 2020 at 9:05

yellowgray

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1 Comment

RaSha Over a year ago

Thank you for such a detailed answer!

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