CODEWITHSUNDEEP
dartflutter
daedalus
daedalus

Reputation: 10923

Crashing ListView.builder

I am trying to set up a search page in a Flutter app. This widget presents a search field, monitors what the user is typing, and updates a list of suggested hits with every change in the search field. The relevant snippet is here:

List<Text> suggestList = [new Text('')];

@override
Widget build(BuildContext context) {
  return Column(
    children: <Widget>[
      new Form(
        child: new TextFormField(
          // _controller listens for changes and updates suggestList
          controller: _controller, 
          autocorrect: false,
          decoration: InputDecoration(labelText: 'Search here'),
          onSaved: (str) => print(str),
        ),
      ),
      new ListView.builder(
        itemBuilder: (context, index) => suggestList[index],
        itemCount: suggestList.length,
      )
    ],
  );
}

This widget crashes with an error message that is pages long, but includes this text that suggests my error is a layout issue. I am new to Flutter and Dart and unable to really apply the advice here meaningfully to my problem.

══╡ EXCEPTION CAUGHT BY RENDERING LIBRARY ╞═════════════════════════════════════════════════════════
The following assertion was thrown during performResize():
Vertical viewport was given unbounded height.
Viewports expand in the scrolling direction to fill their container.
In this case, a vertical
viewport was given an unlimited amount of vertical space in which to expand. 
This situation
typically happens when a scrollable widget is nested inside another 
scrollable widget.
If this widget is always nested in a scrollable widget there is no 
need to use a viewport because
there will always be enough vertical space for the children. 
In this case, consider using a Column
instead. Otherwise, consider using the "shrinkWrap" property 
(or a ShrinkWrappingViewport) to size
the height of the viewport to the sum of the heights of its children.

In the snippet below, I try to simulate the effect I want by presenting a succession of Text widgets:

@override
Widget build(BuildContext context) {
  return Column(
    children: <Widget>[
      new Form(
        child: new TextFormField(
          controller: _controller,
          autocorrect: false,
          decoration: InputDecoration(labelText: 'Search here'),
          onSaved: (str) => print(str),
        ),
      ),
      // Present the two first items manually as Text widgets
      suggestList[0],
      suggestList.length > 1 ? suggestList[1] : new Text('...'),
    ],
  );
}

This works perfectly but looks ugly and is clearly a hack that I do not want in the final app. Also I have confirmed that my logic is working perfectly -- I have checked that suggestList is always populated with at least one Text widget suitable for display. Any ideas on how to repair the ListView approach?

Upvotes: 1

Views: 2123

Answers (1)

Richard Heap
Richard Heap

Reputation: 51682

One way is to just build the Column children like this

  @override
  Widget build(BuildContext context) {
    List<Widget> columnChildren = [
      new Form(
        child: new TextFormField(
          // _controller listens for changes and updates suggestList
          //controller: _controller,
          autocorrect: false,
          decoration: InputDecoration(labelText: 'Search here'),
          onSaved: (str) => print(str),
        ),
      )
    ];
    columnChildren.addAll(suggestList);

    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Column(
        children: columnChildren,
      ),
    );
  }

or better still just keep the suggestList as a List

  List<String> suggestList = [
    'hello',
    'world',
  ];

  @override
  Widget build(BuildContext context) {
    List<Widget> columnChildren = [
      new Form(
        child: new TextFormField(
          // _controller listens for changes and updates suggestList
          //controller: _controller,
          autocorrect: false,
          decoration: InputDecoration(labelText: 'Search here'),
          onSaved: (str) => print(str),
        ),
      )
    ];
    columnChildren.addAll(suggestList.map((s) => new Text(s)).toList());

Alternatively, wrap the ListView in an Expanded

  body: new Column(
    children: <Widget>[
      new Form(
        child: new TextFormField(
          // _controller listens for changes and updates suggestList
          //controller: _controller,
          autocorrect: false,
          decoration: InputDecoration(labelText: 'Search here'),
          onSaved: (str) => print(str),
        ),
      ),
      new Expanded(
        child: new ListView.builder(
          itemBuilder: (context, i) => new Text(wordList[i]),
          itemCount: wordList.length,
        ),
      ),
    ],
  ),

This allows an infinite scrolling list below a fixed Form.

Upvotes: 2

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