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I want to create an infinite scrollable list like it is possible to do with ListView.builder :

ListView.builder(
  itemBuilder: (context, index) => Text(index.toString()),
);

But doing so only creates an infinite list in "one direction" (down): index can only be 0 <= index.

I would like to also be able to scroll up which would be equivalent to having an index being possibly negative.

Is there any built-in widget for that? I couldn't find anything, even among the slivers:

Valentin Vignal
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2 Answers2

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I don't know if that's the sleekest solution, but you could add a listener to your ScrollController, check if _controller.position.pixels <= _controller.position.minScrollExtent, then add the data you want to have 'above' the current index to the list of the ListView.builder and then use the ScrollController to jump down to where you were before you inserted the data.

To make it easier to scroll back to the right item you could use the scrollable_positioned_list package like here.

I didn't test if that results in visible jitter or not, but maybe worth a try.

Max de Boer
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No programming language allows us to use a negative index value. But they have different functionalities in different languages (in Python you are refering with -1 to the last object and -2 to the second last). But Dart doesnt support negative indexes, so the lists do not do it either.

I think what you wan to do is to combine two "infinite" Lists/arrays. I would advise you to use two lists. Use shrinkWrap: true and physics: const NeverScrollableScrollPhysics() in both lists and Wrap them with SingleChildScrollView.

To give you a better help, I would need more Informations. But this should do it for the start.

MrOrhan
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  • I don't think using `shrinkWrap: true` and `physics: const NeverScrollableScrollPhysics()` in both lists and Wrap them with SingleChildScrollView. is a viable solution, especially with an infinite list. `shrinkWrap` is very expensive and `SingleChildScrollView` would force to build all the widgets. If the list is infinite, it has to be lazy. The `index < 0` part was just to illustrate what I'm trying to do. The issue here is not whether or not dart supports negative index, but rather whether or not it is possible ot have an infinite lazy scrollable list in both directions of an axis. – Valentin Vignal Jun 05 '23 at 07:19
  • Yeah It is not easy to achieve. This is the only possible way I can imagine! Otherways, you have to combine the lists in one and start at the middle. I have done it once with Pageviewbuilder, there you can set the controller where it should start. But also there is this, where they have two Lists in one: https://stackoverflow.com/questions/57886393/how-to-make-listview-builder-builder-that-starts-from-the-center. If you want an direct answer to your question 'Is there any built-in widget for that?': NO, there is not. – MrOrhan Jun 05 '23 at 10:05