Flutter 下拉值如果没有选中，则分配初始值

Question

我有一个DropDownButtonFormField 我需要检查一下。如果用户没有选择下拉值，则在提交时分配初始值。

自定义下拉菜单

 Container myDropDownContainer(String initialVal, List<String> listItems,
      String text, Function myFunc, Function validate) {
    return Container(
      margin: const EdgeInsets.all(8),
      child: Row(
        mainAxisSize: MainAxisSize.min,
        mainAxisAlignment: MainAxisAlignment.start,
        children: [
          SizedBox(
            width: 120,
            child: Text(
              text,
              style: kTextStyle,
            ),
          ),
          const SizedBox(
            width: 20,
          ),
          Expanded(
            child: Container(
              height: 50,
              decoration: BoxDecoration(
                  color: Colors.orangeAccent,
                  borderRadius: BorderRadius.circular(5)),
              child: DropdownButtonFormField<String>(
                autovalidateMode: AutovalidateMode.always,
                //menuMaxHeight: 300,
                validator: (value) {
                  if(value!.isEmpty) {
                    return "485s4a8sd4as85";
                  }
                } ,
                decoration: const InputDecoration(border: InputBorder.none),
                isExpanded: true,
                onTap: () => myFunc,
                //borderRadius: BorderRadius.circular(5),
                value: initialVal,
                icon: const Icon(
                  Icons.arrow_downward,
                  color: Colors.black38,
                ),
                iconSize: 24,
                elevation: 16,
                dropdownColor: Colors.deepOrange,
                style: kTextStyle.copyWith(color: Colors.black),
                onChanged: (val) => myFunc(val),
                items: listItems.map<DropdownMenuItem<String>>((String? val) {
                  return DropdownMenuItem(
                    //TODO: Set default values
                    value: val,
                    child: Text(
                      val,
                      style: kTextStyle.copyWith(color: Colors.black),
                    ),
                  );
                }).toList(),
              ),
            ),
          )
        ],
      ),
    );
  }

这是我的onChanged 属性，由用户分配选定的值。我添加了一些关于我想要做什么的解释。

 String _valueCinsiyet = "Diğer"; // initial value
  void onChangedCinsiyet(String? newVal) {
    setState(() {
      if(newVal==null) {
        _formData.setCinsiyet(_valueCinsiyet);
        /*
        'if newVal is null' means that if the value is not selected by the user
        then set the initialValue( _valueCinsiyet)
         */

      } else {
        /*
        if newVal is not null then assign the newVal( which means the selected value)
        into my initialValue, then set the data to use it on different pages. What is missing?
         */
        _valueCinsiyet = newVal;
        _formData.setCinsiyet(_valueCinsiyet);
      }

    });
  }

Answer 1

您可以使用可为空的数据来跟踪DropdownButtonFormField 的更改。可以为空，您可以检查它是否为空，无需在onChanged: 上进行任何额外操作，只需按常规方式分配新值即可。

构建方法之前的开启状态： String? value; // value to keep track

child: DropdownButtonFormField<String>(
  value: value,
  onChanged: (val) {
    setState(() {
      value = val;
    });
  },

现在onSaved/submit 按钮可以通过检查null来传递值，简单的方法是

value?? "default Value"。在你的情况下，它是value??Diğer