FutureBuilder snapshot.data 在 http 调用之前被设置为 null答案

【问题标题】：FutureBuilder snapshot.data getting set to null before the http callFutureBuilder snapshot.data 在 http 调用之前被设置为 null
【发布时间】：2021-08-29 14:05:54
【问题描述】：

我正在尝试修改在未来调用期间获取的列表并且不再调用未来函数，因此将变量分配给FutureBuilder 的future 选项并在initState() 中调用实际函数，但现在build 函数直接呈现snapshot.error 条件，并且构建甚至没有进入ConnectionState.loading 状态。下面是相关代码sn-p：

class _BusinessDetailScreenState extends State<BusinessDetailScreen> {
  late List<ProductCategory> _uniqueProductCategoryList;
  late Future<List<BusinessProduct>> _productsFetch;

  @override
  void initState() {
    super.initState();
    _productsFetch = _fetchDetails();
  }

  List<ProductCategory> fetchUniqueCategory (productList) {
    // function body
    return List<ProductCategory>.generate(result.length, (index) {
        return ProductCategory(
          categoryName: result[index]["productCategoryName"],alternateName: result[index]["productCategoryAlternateName"]??= "", id:result[index]["id"]
        );
    });
  }

  Future<List<BusinessProduct>> _fetchDetails() async {
    var queryParams = {
     //params
    };
    var response = await http.get(Uri.https(Provider.of(context)!.baseUrl,'endpoint',queryParams));
    if (response.statusCode == 200) {
      List jsonResponse = json.decode(response.body);
      List<BusinessProduct> products = jsonResponse.map((e) => BusinessProduct.fromJson(e)).toList();
      _uniqueProductCategoryList = fetchUniqueCategory(products);
      return products;
    }else {
      throw Exception('Failed to load Details');
    }
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<List<dynamic>>(
      future: _productsFetch,
      builder: (context,snapshot){
      print(snapshot); //AsyncSnapshot<List<dynamic>>(ConnectionState.done, null, dependOnInheritedWidgetOfExactType<Provider>() or dependOnInheritedElement() was called before _BusinessDetailScreenState.initState() completed.
        if(snapshot.hasData){
          return Scaffold(//for data)
        } else if(snapshot.hasError){
            print(snapshot.error);
            // I/flutter ( 4211): dependOnInheritedWidgetOfExactType<Provider>() or dependOnInheritedElement() was called before _BusinessDetailScreenState.initState() completed.

            return Scaffold( // for error)
        } return Scaffold(
            backgroundColor: palletWhite,            
            body: Center(
              child: LoadingComponent(loadingText: 'Fetching data, Please wait',)
            )
        );
      },
    );
  )

现在被困了一段时间，有什么我做错了，或者我应该尝试一些不同的方法，请告诉我

【问题讨论】：

snapshot.error 中有什么内容？
````snapshot.data``` 正在获得价值null，这就是为什么它只是去snapshot.hasError
我问的是snapshot.error，而不是snapshot.data - 只需在if(snapshot.hasData){ 之前添加print(snapshot);，您在日志中看到了什么？
是的，对不起，我的错，用 snapshot.error 的内容编辑问题，得到这个：I/flutter ( 4211): dependOnInheritedWidgetOfExactType<Provider>() or dependOnInheritedElement() was called before _BusinessDetailScreenState.initState() completed.
E/flutter ( 3677): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: dependOnInheritedWidgetOfExactType<Provider>() or dependOnInheritedElement() was called before _BusinessDetailScreenState.initState() completed. E/flutter ( 3677): When an inherited widget changes, for example if the value of Theme.of() changes, its dependent widgets are rebuilt. If the dependent widget's reference to the inherited widget is in a constructor or an initState() method, then the rebuilt dependent widget will not reflect the changes in the inherited widget.

标签： android flutter dart flutter-futurebuilder

【解决方案1】：

参考link 上的答案 - Provider.of(context) 应该在小部件树内使用，而 build() 方法之外的任何东西都不在小部件树中.但是如果你还想使用它，那么你需要将listen参数设置为false 因此，在_fetchDetails() 中替换Provider.of(context)!.baseUrl 有助于解决问题。感谢@musausman.com 和@pskink

【讨论】：