Question

此下拉列表将从将来的构建器中获取其项目但是从列表中选择一个项目后，它不会更改所选项目，我相信这是因为我没有使用常规字符串或int项目

FutureBuilder(
  future: getCategories(http.Client()),
  builder: (BuildContext context, AsyncSnapshot<List<Categories>> snapshot) {
    if (!snapshot.hasData) return CircularProgressIndicator();

    selectedCategory = snapshot.data[0].id; 

    if (snapshot.hasData) {
      this.categoriesList = snapshot.data;
      if (this.categoriesList.length > 0) {
        return DropdownButton<int>(
          value:  selectedCategory,
          onChanged: (int newValue) {
            setState(() {
              selectedCategory = newValue;
              this.eventOpj.categoryId = newValue;
            });
          },
          items: snapshot.data
                         .map<DropdownMenuItem<int>>((Categories catg) {
                           return DropdownMenuItem<int>(
                             value: catg.id,
                             child: Text(catg.nameEn),
                           );
                         }).toList(),
        );
      } else {
        return Text('data');
      }
    }
  },
),

Answer 1

我认为这是因为您在构建方法中加入了逻辑。

def myview(request):
   # if you have variables that you want to always
   # be in the context, the safest way is to define 
   # them right from the start (with a dummy value):

   name = None

   # now FIRST test the request method:
   if request.method == "POST":
       # and ONLY then build a bound form:
       form = MyForm(request.POST)
       if form.is_valid():
           # use sanitized data, not raw POST data:
           name = form.cleaned_data["name"]

   else:
      # build an unbound form
      form = MyForm()

   # here you know that both variables have been defined whatever
   return render(request, "mytemplate.html", {"form": form, "name": name}

使逻辑脱离构建方法，如果需要在接收数据时应用逻辑，则应在有状态小部件的initState中调用此方法。

Answer 2

您可以仅在initState()中进行一次future调用，将future设置为_future变量，然后在FutureBuilder上使用该变量，如下所示：

@override
void initState() {
  super.initState();
  _future = getCategories(http.Client());
}

FutureBuilder(
  future: _future,
  ...

此外，selectedCategory = snapshot.data[0].id;行仅应在第一次加载结果时执行，因为这只是选择DropdownButton的第一个元素。您可以将该行放在if (selectedCategory == null)

中

我如何获得下拉列表以在setState中显示所选项目

2 个答案: