利用gestudetector小部件DropdownButtonFormField

Question

总体目标是使用异步getpays函数在下拉菜单中提供项目列表 如何在小部件中使用手势检测器以及如何设置状态下拉菜单？我想用它来调用我的getpays函数，但是当我将其插入到dropdow小部件下时，出现语法错误“发现期望的正电子参数过多0。1。我使用MVC模式。 我的屏幕

Widget build(BuildContext context) {
    return new Scaffold(
        body: Container(
            width: double.infinity,
            height: double.infinity,
            child: new Form(
                child: Padding(
                    padding: const EdgeInsets.only(top: 16.0, left: 28),
                    child: new Column(children: <Widget>[
                      DropdownButtonFormField(
                        GestureDetector(onTap: () async {
                          await moncontrolleurdropdown.getpays();
                          setState(() {});
                        }),
                        hint: Text("Select"),
                        items: moncontrolleurdropdown.myPays
                            ?.map((PaysModel pays) {
                          DropdownMenuItem<String>(
                            child: new Text(pays.nompays),
                            value: pays.codestate,
                          );
                        })?.toList(),
                        onChanged: (value) =>
                            moncontrolleurdropdown.onSelectedState(value),
                        value: moncontrolleurdropdown.selectedState,
                      ),
                      DropdownButtonFormField(
                        items: moncontrolleurdropdown.tempvillesList
                            ?.map((VilleModel ville) {
                          return new DropdownMenuItem<String>(
                            child: new Text(ville.nomville),
                            value: ville.codeville,
                          );
                        })?.toList(),
                        onChanged: (value) =>
                            moncontrolleurdropdown.onSelectedVille(value),
                        value: moncontrolleurdropdown.selectedVille,
                      ),
                    ])))));
  }
}

Widget build(BuildContext context) { return new Scaffold( body: Container( width: double.infinity, height: double.infinity, child: new Form( child: Padding( padding: const EdgeInsets.only(top: 16.0, left: 28), child: new Column(children: <Widget>[ DropdownButtonFormField( GestureDetector(onTap: () async { await moncontrolleurdropdown.getpays(); setState(() {}); }), hint: Text("Select"), items: moncontrolleurdropdown.myPays ?.map((PaysModel pays) { DropdownMenuItem<String>( child: new Text(pays.nompays), value: pays.codestate, ); })?.toList(), onChanged: (value) => moncontrolleurdropdown.onSelectedState(value), value: moncontrolleurdropdown.selectedState, ), DropdownButtonFormField( items: moncontrolleurdropdown.tempvillesList ?.map((VilleModel ville) { return new DropdownMenuItem<String>( child: new Text(ville.nomville), value: ville.codeville, ); })?.toList(), onChanged: (value) => moncontrolleurdropdown.onSelectedVille(value), value: moncontrolleurdropdown.selectedVille, ), ]))))); } }

我的函数getpays和onselecstate

void getpays() async {

    try {
      var response =
          await http.get("http://..../getpays.php");
      if (200 == response.statusCode) {
        print("testfonction");
        print(response.body);
     //   setState(() {

        _myPays = (json.decode(response.body) as List)
            .map((i) => PaysModel.fromJson(i))
            .toList();
        // print(_myPays.length);
        print(_myPays);
        _myPaystest = _myPays;
        //});
      }
    } catch (e) {
      //return _myPays;
    }
    //return _myPays;
  }
<code>
 void onSelectedState(String value) {
    this.getville();

    print(value);
    print("a");
    print(this.villesList.length);
    print("b");
    //selecteddState=value;
    print("c");

    //print(_selectedState);

    this.tempvillesList.clear();

    for (var x = 0; x == this.villesList.length - 1; x++) {
      print(x);
      if (this.villesList[x].codepays == _selectedState) {
        print(this.villesList[x].codepays);
        this.tempvillesList.add(this.villesList[x]);
        print(this.tempvillesList);
      }
    }
  }