Question 235506 in Flutter Question posted in
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Flutter – ListView with a single expanded tile at a time

I am trying to create a ListView widget that contains ExpansionTiles and I want to have only a single tile expanded at a time. I am using quite a bit of data, so I am using ListView.builder I am getting an error:

Error: Assertion failed:
_state != null

Documentation also specifies that you cannot call controller methods inside a build method.

My code looks like this:

return ListView.builder(
          itemCount: data.length,
          itemBuilder: (context, index) {
            return ExpansionTile(
              controller: expansionControllers[index],
              leading: Text(data[index].code),
              title: Text(data[index].name),
              onExpansionChanged: (value) async {
                if (value) {
                  setState(() {
                    prevIndexExpanded = index;
                  });

                  //Do stuff
                }
              },
              children: [...]
            );
          },
        );

where I have a prevIndexExpanded which tracks the currently expanded tile and I have generated a list of ExpansionTileController() for each tile.

final expansionControllers = List.generate(
          data.length,
          (index) => ExpansionTileController(),
        );

So to be clear: when selecting a tile it should expand and if there is another already expanded tile, it should collapse, so there is only one expanded at a time.

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    As it turns out, I was making the mistake of generating the list inside of the build method of the Widget, therefore it was generating a new list on every rebuild and the controllers wouldn't match. After taking it outside, everything is working as expected.

    Quick note: If you are using a ListView.builder with a lot of ExpansionTiles, you should specify the initiallyExpanded property, since while scrolling, the ListView rebuilds and it will collapse expanded tiles otherwise.


  2. 0

    I don’t think you want a ListView but instead, I think you might want to use a Silver List
    SliverList & SliverGrid

    If you still want to follow the ListView approach, you can look up this link on how to Create lists with different types of items

    You must define your own build method, in order to define the structure of your items.

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