Set default value for dropdown button in flutter

问题

I have a dropdown button which works fine, but when I try to set a default value it will fail with the following error:

'package:flutter/src/material/dropdown.dart': Failed assertion: line 620 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) => item.value == value).length == 1': is not true.

This is my dropdown button:

 Widget changeWorkspace() {
return StatefulBuilder(
    builder: (BuildContext context, StateSetter setState) {
  return Column(mainAxisSize: MainAxisSize.min, children: <Widget>[
    Padding(
      padding: EdgeInsets.all(8.0),
      child: DropdownButton<AssignedWorkspace>(
          isExpanded: true,
          hint: Text("SELECT WORKSPACE"),
          value: selectedWorkspace,
          onChanged: (dropdownValueSelected) {
            setState(() {
              selectedWorkspace = dropdownValueSelected;
            });
          },
          items: workspaces != null && workspaces.length > 0
              ? workspaces.map((AssignedWorkspace workspace) {
                  return new DropdownMenuItem<AssignedWorkspace>(
                    value: workspace,
                    child: new Text(workspace.name,
                        style: new TextStyle(color: Colors.black)),
                  );
                }).toList()
              : null),
    ),
  ]);
});

}

I've tried to set the value of selectedWorkspace onInit as follows but it fails.

selectedWorkspace = new AssignedWorkspace(
id: userSettings.currentWorkspaceId,
name: userSettings.currentWorkspaceName);

Is there a way of setting a default value in a dropdown button?

回答1:

import 'package:flutter/material.dart';

import '../config/app_theme.dart';

class DropdownWidget extends StatefulWidget {
  final String title;
  final List<String> items;
  final ValueChanged<String> itemCallBack;
  final String currentItem;
  final String hintText;

  DropdownWidget({
    this.title,
    this.items,
    this.itemCallBack,
    this.currentItem,
    this.hintText,
  });

  @override
  State<StatefulWidget> createState() => _DropdownState(currentItem);
}

class _DropdownState extends State<DropdownWidget> {
  List<DropdownMenuItem<String>> dropDownItems = [];
  String currentItem;
  AppTheme appTheme;

  _DropdownState(this.currentItem);

  @override
  void initState() {
    super.initState();
    for (String item in widget.items) {
      dropDownItems.add(DropdownMenuItem(
        value: item,
        child: Text(
          item,
          style: TextStyle(
            fontSize: 16,
          ),
        ),
      ));
    }
  }

  @override
  void didUpdateWidget(DropdownWidget oldWidget) {
    if (this.currentItem != widget.currentItem) {
      setState(() {
        this.currentItem = widget.currentItem;
      });
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  Widget build(BuildContext context) {
    appTheme = AppTheme(Theme.of(context).brightness);

    return Container(
      margin: EdgeInsets.symmetric(vertical: 10),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          Container(
            margin: EdgeInsets.only(left: 6),
            child: Text(
              widget.title,
              style: appTheme.activityAddPageTextStyle,
            ),
          ),
          Container(
            padding: EdgeInsets.symmetric(vertical: 3, horizontal: 15),
            margin: EdgeInsets.only(top: 10),
            decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(6),
              color: Colors.white,
              boxShadow: [
                BoxShadow(
                  offset: Offset(0, 2),
                  blurRadius: 10,
                  color: Color(0x19000000),
                ),
              ],
            ),
            child: DropdownButtonHideUnderline(
              child: DropdownButton(
                icon: appTheme.activityAddPageDownArrowSVG,
                value: currentItem,
                isExpanded: true,
                items: dropDownItems,
                onChanged: (selectedItem) => setState(() {
                  currentItem = selectedItem;
                  widget.itemCallBack(currentItem);
                }),
                hint: Container(
                  child: Text(widget.hintText, style: appTheme.hintStyle),
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

This is my dropDownWidget without optimization. It has currentItem. You could use it like:

           DropdownWidget(
                title: kStatus,
                items: state.customerStepInfo.statusList,
                currentItem: status,
                hintText: kCommonPick,
                itemCallBack: (String status) {
                  this.status = status;
                },
              )

来源：https://stackoverflow.com/questions/57869593/set-default-value-for-dropdown-button-in-flutter