ViewModelBuilder返回的变量为null。未检索状态

Question

我正在重构应用以使用堆叠式应用，但有问题。状态似乎在小部件之间传递不正确。

我在要影响的小部件树顶部添加了ViewModelBuilder。

  return ViewModelBuilder<vehicleData>.reactive(
                    builder: (context, model, child){
                      return Container(
                        padding: EdgeInsets.symmetric(horizontal: 20.0),
                        child: InkWell(
                          splashColor: Colors.blueGrey,
                          child: Card(
                            //color: Colors.blueGrey,
                            child: ListTile(
                              leading: AspectRatio(
                                  aspectRatio: 0.8,
                                  child: Image.network(
                                    //data.images,
                                    snapshot.data.documents[index]['icon'],
                                    height: 500,
                                    //width: 300,
                                    fit: BoxFit.contain,
                                  )
                              ),
                              title: Text(
                                //data.make,
                                snapshot.data.documents[index]['make'],
                                textAlign: TextAlign.start,
                                style: TextStyle(fontSize: 18.0,
                                  fontWeight: FontWeight.bold,),
                              ),
                              onTap: (){
                                Navigator.push(context, MaterialPageRoute(builder: (context) =>
                                //ProformaScreen(data.make),
                                ProformaScreen(snapshot.data.documents[index]['make']),
                                  ),
                                );
                                model.setModel(snapshot.data.documents[index]['model']);
                              } ,
                            ),
                            shape: RoundedRectangleBorder(
                              side: BorderSide(color: Colors.blueGrey),
                              borderRadius: BorderRadius.circular(10),
                            ),
                          ),
                        ),
                      );
                    },
                      viewModelBuilder: ()=> vehicleData(),
                    );

这将调用model.setModel并根据所选汽车分配模型。

import 'package:cloud_firestore/cloud_firestore.dart';
import 'dart:core';
import 'package:stacked/stacked.dart';

class vehicleData extends BaseViewModel{
  String _make;
  String _images;
  var _model;

   setModel(var model){
    _model = model;
    print ('This is the Model of the Car \n');
    print (_model);
    notifyListeners();
  }

   setMake(String make){
    _make = make;
    notifyListeners();
  }
   setImages(String images){
    _images = images;
    notifyListeners();
  }
  get getModel => print('This is within the Get ' + _model);
  get getMake => _make;
  get getImage => _images;
}
  

两个print语句都返回正确的模型，我仍然可以看到它们。 但是当我尝试在另一个页面模型中使用ViewModelBuilder调用时，返回null：

import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:gerente_loja/core/services/vehicle_data.dart';
import 'package:stacked/stacked.dart';

class menuModel extends StatefulWidget {
  @override
  _menuModelState createState() => _menuModelState();
}
class _menuModelState extends State<menuModel> {

  var carModel;
  @override
  Widget build(BuildContext context) {

   String selectModel;

    return ViewModelBuilder<vehicleData>.reactive(
        builder: (context, modelView, child){
      return Padding(
        padding: const EdgeInsets.only(left: 16.0, right: 16.0),
        child: Container(
          decoration: BoxDecoration(border: Border.all(color: Colors.black),
              borderRadius: BorderRadius.circular(25.0)
          ),
          padding: EdgeInsets.only(left: 50.0, right: 16.0),
          child: DropdownButton(
            hint: Text('Ano'),
            dropdownColor: Colors.transparent,
            elevation: 5,
            icon: Icon(Icons.arrow_drop_down),
            isExpanded: true,
            iconSize: 36.0,
            value: selectModel,
            onChanged: (value){
              setState(() {
                print (modelView.getModel);

                selectModel = value;
              });
            },
            items: carModel.map((value) {
              return DropdownMenuItem (
                  value: value,
                  child: Text(value));
            }).toList(),
          ),
        ),
      );
     },
      viewModelBuilder: ()=> vehicleData(),
    );
  }

}

打印（modelView.getModel）;一片空白。 有人知道为什么模型返回null吗？我看起来很难解决，但不知道为什么。