如何在DropdownBelow组件中显示不在选项列表内的hintText提示文本

DropdownBelow组件的hint仅在value为null时才会显示，你之前的操作触发了两个问题：

• 传空字符串""作为value：空字符串不在你的categorylist选项列表里，触发了组件的断言校验报错
• 传列表中存在的"Other"作为value：value不为null，组件直接显示选中值，覆盖hint内容

1. 调整选中变量的默认值 #

将_selectedCategory的默认值改为null，注意变量类型声明为可空：

String? _selectedCategory = null;

2. 调整onChanged回调的空值兼容处理 #

onChanged: (newValue) {
  setState(() {
    // 兼容null的情况，只有选中有效值才赋值
    if (newValue != null) {
      _selectedCategory = newValue as String;
      _newCatgoryFlag = _selectedCategory == "Other";
    }
  });
},

如果后续需要重置回显示hint的状态，直接把_selectedCategory赋值回null即可。

var categorylist = <String>[
  'Other',
];
// 改为可空类型，默认值为null
String? _selectedCategory = null;

Container(
  width: MediaQuery.of(context).size.width * 0.9,
  child: DropdownBelow(
    itemWidth: 370,
    itemTextstyle: TextStyle(
        fontSize: 14,
        fontWeight: FontWeight.w400,
        color: Colors.black),
    boxTextstyle: TextStyle(
        fontWeight: FontWeight.w400,
        color: Colors.black),
    boxPadding: EdgeInsets.fromLTRB(13, 12, 13, 12),
    boxWidth: 370,
    boxHeight: 50,
    boxDecoration: BoxDecoration(
        color: Colors.transparent,
        shape: BoxShape.rectangle,
        borderRadius:
            new BorderRadius.all(new Radius.circular(10.0)),
        border: Border.all(width: 1, color: Colors.grey)),
    icon: Icon(
      Icons.keyboard_arrow_down_outlined,
      color: Colors.black,
    ),
    hint: Text('Category',
        style: GoogleFonts.montserrat(
            color: HexColor("#6e6b7b"), fontSize: 15)),
    // 传入null的value，此时会显示hint
    value: _selectedCategory,
    items:categorylist.map<DropdownMenuItem<String>>((String value) {
      return DropdownMenuItem<String>(
          value: value,
          child: Text(value,
              style: GoogleFonts.montserrat(
                color: HexColor("#6e6b7b"),
              )));
    }).toList(),
    onChanged: (newValue) {
      setState(() {
        if (newValue != null) {
          _selectedCategory = newValue.toString();
          _newCatgoryFlag = _selectedCategory == "Other";
        }
      });
    },
  ),
),

内容的提问来源于stack exchange，提问作者TimeToCode