Flutter - 带 Appbar 和列表视图的脚手架 - 与 Expanded 决裂

Question

I have an issue with Scaffold. I have an AppBar with text and a button. The body hosts a ListView.builder. Inside the ListView is a container and a number of TextButton Widgets.

The Text part of the TextButton can be wide, and causes the warning bar to appear. So, I wrapped in an Expanded widget to wrap it.

This results in an error...

The following assertion was thrown during performLayout(): RenderFlex children have non-zero flex but incoming width constraints are unbounded.

When a row is in a parent that does not provide a finite width constraint, for example if it is in a horizontal scrollable, it will try to shrink-wrap its children along the horizontal axis. Setting a flex on a child (eg using Expanded) indicates that the child is to expand to fill the remaining space in the horizontal direction. These two directives are mutually exclusive. If a parent is to shrink-wrap its child, the child cannot simultaneously expand to fit its parent.

Consider setting mainAxisSize to MainAxisSize.min and using FlexFit.loose fits for the flexible children (using Flexible rather than Expanded). This will allow the flexible children to size themselves to less than the infinite remaining space they would otherwise be forced to take, and then will cause the RenderFlex to shrink-wrap the children rather than expanding to fit the maximum constraints provided by the parent.

and also the contents of my AppBar disappear, and I can no longer click the back arrow.

Here is the scaffold to build the page...

late StateSetter _setState;

@override
Widget build(BuildContext context) {

  return buildSubFormPage();

}


Widget buildSubPage() {

  return StatefulBuilder(
      builder: (bContext, StateSetter setBState) {
        _setState = setBState;
        return Scaffold(
            appBar: new AppBar(
              title: new Text("My list of items"),
              actions: [
                Padding(
                  padding: EdgeInsets.only(
                      right: 20.0),
                  child: openSubFormItem(
                      widget.field, 0,
                      "+ Add new", null),
                )
              ],
            ),
            body: ListView.builder(
              padding: EdgeInsets.all(8.0),
              //shrinkWrap: true,
              itemCount: subFormItems.length,
              itemBuilder: (context, index) {
                return Container(
                  //width: screenWidth,
                  child: openSubFormItem(
                      widget.field, index, "Press Here",
                      subFormItems[index]),

                );
              },

            )

        );
      }
  );
}

Here is mostly the content of my openSubFormItem widget.

The building of the button text has been removed, but you can get the idea...

Widget openSubFormItem(PendigoFields field, int index, String buttonText, dynamic subFormItem) {

String outputText = "A long string of text, that will surely cause the warning bars if I can't make it wrap properly";

if (buttonText == "+ Add new") {
  outputText = buttonText;
}

return Container(
    //padding: EdgeInsets.all(4),
  //width: screenWidth - 30,
    child:
    TextButton(
      child: Row(
        mainAxisSize: MainAxisSize.max,
        //mainAxisSize: MainAxisSize.min,
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: [
          // Flexible(
          //   fit : FlexFit.loose,
          //   child: Text(outputText, style: TextStyle(color: Colors.black,
          //     fontSize: 15.0,
          //     fontWeight: FontWeight.bold,),),
          // ),

          Expanded(child: Text(outputText, style: TextStyle(color: Colors.black,
            fontSize: 15.0,
            fontWeight: FontWeight.bold,),),
          ),

          // Text(outputText, style: TextStyle(color: Colors.black,
          //   fontSize: 15.0,
          //   fontWeight: FontWeight.bold,),),

          // I also have an IconButton here, removed for clarity

       ]
      ),
    )
    decoration: buttonText == "Press Here" ? BoxDecoration(
      
      border: Border(
          bottom: BorderSide(color: Colors.purple[400]!)),
    )
        : null
);

}

How do I get my AppBar to show the AppBar content (and button) and allow my ListView to be able to wrap the text if the text is too long?

Answer 1

As the error says: unbounded width, this is because the Row parent which is the TextButton does not have a specified width, and when using Expanded, the Row or the Column parent of it must know its constrains.

Give a width to the TextButton.

Also a note, don't create your widgets using methods as this is not recommend from flutter team, use separated widgets instead.