Dart 使用FutureBuilder时，如何根据另一个下拉列表选择更改下拉列表的项目？

我有两个下拉列表，第二个下拉列表中的项目是动态的，根据第一个选择的项目下拉列表如何做？我可以创建两个单独的下拉列表，但如何创建取决于另一个

 FutureBuilder(future: getCities(),builder: (BuildContext context, AsyncSnapshot<Post> snapshot) {
                      return DropdownButton<String>(
                        hint: new Text(city),
                        items: cities.map((String value) {
                          return new DropdownMenuItem<String>(
                            value: value,
                            child: new Text(value),
                          );
                        }).toList(),
                        onChanged: (String value) {
                          setState(() => city = value);
                          setState(() => getAreas(2));
                        },
                      );
                    }),
                    FutureBuilder(future: What to write here ?????,builder: (BuildContext context, AsyncSnapshot<Post> snapshot) {
                      return DropdownButton<String>(
                        hint: new Text(area),
                        items: areas.map((String value) {
                          return new DropdownMenuItem<String>(
                            value: value,
                            child: new Text(value),
                          );
                        }).toList(),
                        onChanged: (String value) {
                          setState(() => area = value);
                        },
                      );
                    })

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我认为您没有正确使用FutureBuilder小部件。您不应该在FutureBuilder声明中直接调用getCities方法，而应该在生成器中使用快照引用。下面的代码将位于您使用此FutureBuilders的state类中：

// I see that you have the AsyncSnapshot be of type Post... is there where you get // the cities list or how exactly you initialize cities list?
Future citiesFuture; // the cities future
Future areasFuture; // the data for the second drop down

// get the Future for the cities
@override void initState() {
   super.initState();
   citiesFuture = getCities();
}

那么您的第一个FutureBuilder将是：

FutureBuilder(future: citiesFuture, builder: (BuildContext context, AsyncSnapshot<Post> snapshot) {
      // here you would initialize the cities list
      List cities = snapshot.hasData ? snapshot.data : [];
                      return DropdownButton<String>(
                        hint: new Text(city),
                        items: cities.map((String value) {
                          return new DropdownMenuItem<String>(
                            value: value,
                            child: new Text(value),
                          );
                        }).toList(),
                        onChanged: (String value) {
                          setState(() {
                             areasFuture = getAreas(2))
                          });
                        },
                      );
                    }),

您应该使用对此方法的方法调用替换第二个FutureBuilder：

Widget provideSecondDropdown() {
    if (areasFuture == null) {
       // the user didn't select anything from the first dropdown so you probably want to show a disabled dropdown
       return DropdownButton<String>(                        
                        items: [],
                        onChanged:null,
    }
    // return the FutureBuilder based on what the user selected
    return FutureBuilder(future: areasFuture, builder: (BuildContext context, AsyncSnapshot<Post> snapshot) {
                      List areas = snapshot.hasData ? snapshot.data : [] 
                      return DropdownButton<String>(
                        hint: new Text(area),
                        items: areas.map((String value) {
                          return new DropdownMenuItem<String>(
                            value: value,
                            child: new Text(value),
                          );
                        }).toList(),
                        onChanged: (String value) {
                          setState(() => area = value);
                        },
                      );
                    })
}

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第二个下拉列表中的项目将立即可用，或者您需要根据第一个下拉列表中的选择从某处获取它们？我的问题是我有一个字符串列表，该列表最初为空，将填充字符串，并且是动态的。我希望更改反映在下拉列表中