Index scrolling problem with PageView.Builder()

Question

i´m trying to make a horizontal scrollable widget. it must be infinit (or kind of), so I´ve done this.

Widget build(BuildContext context) {
    final int initialPage = 1000;

    return Container(
      margin: const EdgeInsets.all(10.0),
      height: 400,
      child: PageView.builder(
        controller: PageController(
          keepPage: true,
          initialPage: initialPage,
        ),

        itemBuilder: (context, position) {

          print(position);

          return Container(child:Text(position),
          );
        },
        itemCount: null,
      ),
    );
  }

Then i scroll right 3 times and the console shows this:

I/flutter (12616): 1000
I/flutter (12616): 1001
I/flutter (12616): 1002
I/flutter (12616): 1003

but then when i go backwards, i have to scroll 4 times to see some results, but in the middle it doesn´t show anything.

I/flutter (12616): 1000
I/flutter (12616): 1001
I/flutter (12616): 1002
I/flutter (12616): 1003
I/flutter (12616): 999
I/flutter (12616): 1004
I/flutter (12616): 1005

That was 3 times right, 4 times left and 6 to the right again. Why itemBuilder function doesn´t run sometimes?

Answer 1

Well, I think that the reason is because the print() runs when the PageView builder has to build a new page.

When you go back (or forward), and you go to a page that was already built, the builder don't run again. (Nothing to build)

The pages 1003 1002 1001 1000 were already created when going back.

---

Please read this, from the documentation of PageView.builder constructor

Creates a scrollable list that works page by page using widgets that are created on demand.

This constructor is appropriate for page views with a large (or infinite) number of children because the builder is called only for those children that are actually visible.