android - flutter 错误 : The getter 'iterator' was called on null

Question

我正在尝试按 id 单一输出产品库。但是我得到一个错误 Map is not a subtype of type iterable。我还是新手，我有点卡在这里。

这是我的 API，我认为错误来自这里。提前谢谢你的帮助。

Future<List<Product>> fetchProduct(String productID) async{
String productPost = ApiUtil.productPost(productID);
Map<String, String> headers = {
  'Accept' : 'application/json'
};
var response = await http.get(productPost, headers: headers);
List<Product> productpost = [];
if( response.statusCode == 200 ) {
  Map<String, dynamic> body = json.decode(response.body);
  for ( var item in body['data'] ){
    Product product = Product.fromJson(item);
    productpost.add(product);
  }
}
return productpost;

}

这是我的模型。

class Product {

     String id, name, description;

     Product(this.id, this.name, this.description);

     Product.fromJson( Map<String,dynamic> jsonObject ){
        this.id = jsonObject['id'].toString();
        this.name = jsonObject['name'];
        this.subname = jsonObject['description'];
     }
}

这是我的详细类(class)。

class ProductDetails extends StatefulWidget{

   final String productID;
   ProductDetails(this.productID);

   @override
   ProductDetailsState createState() => new ProductDetailsState();

   class ProductDetailsState extends State<ProductDetails> {

     ProductApi productapi = ProductApi();

     @override
     void initState() {
      super.initState();
     }

.... return Scaffold ...
Container(
   child: FutureBuilder(
      future:  productapi.fetchProduct(widget.productID),
      builder: (BuildContext context, AsyncSnapshot snapshot) {

         if( snapshot.hasError ) {
            return Text ('NO DATA');
         }
         if( snapshot.hasData) {
            return _productDetail( snapshot.data );
         }
         return null;
      },
    )
  ) 

_productDetail( List<Product> product ){
   return PageView.builder(
      itemCount: product.length,
      itemBuilder: ( BuildContext context, int index ){
         return new Padding(
         padding: EdgeInsets.all(10.0),
         child: Column(
            mainAxisSize: MainAxisSize.max,
            crossAxisAlignment: CrossAxisAlignment.stretch,
            children: <Widget>[
               Text(product[index].name, textAlign: TextAlign.center, style: 
               TextStyle(fontSize: 28, color: Colors.black, fontWeight: FontWeight.w700)),
            ]
         )
      );
    }
  );
}

我试图通过在线搜索引用资料来学习和理解，但有些问题我仍然不知道有什么问题。

这是我从控制台得到的错误

[VERBOSE-2:ui_dart_state.cc(148)] Unhandled Exception: NoSuchMethodError: The getter 'iterator' was called on null.
Receiver: null
Tried calling: iterator
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      ProductApi.fetchProduct (package:tampr/api/product_api.dart:61:29)
<asynchronous suspension>
#2      ProductDetailsState.initState (package:tampr/pages/product_details.dart:26:16)
#3      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4068:58)
#4      ComponentElement.mount (package:flutter/src/widgets/framework.dart:3919:5)
#5      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3101:14)
#6      Element.updateChild (package:flutter/src/widgets/framework.dart:2904:12)
#7      SingleChildRenderObjectElement.mount (package:flutter/src/widgets/framework.dart:5127:14)
#8      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3101:14)
#9      Element.updateChild (package:flutter/src/widgets/framework.dart:2904:12)
#10  <…>

JSON数据

{"data":[{
   "id": 1,
   "name": "Product Name",
   "description": "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.",
   "productimage": "product_img.jpg",
   "logo": "logo_img.png",
   "user_id": 1,
   "created_at": "2019-11-18 05:25:49",
   "updated_at": "2019-11-18 05:25:49",
   "options": [
      {
            "id": 1,
            "name": "option name",
            "image": "optionimage.jpg",
            "pivot": {
                "product_id": 1,
                "option_id": 1
            }
        },
   ]
}]}

Answer 1

我也遇到了同样的问题，就我而言，这是因为:

Product product = Product.fromJson(item);[item] structure coming from Firestore doesn't match [fromJson] decoding structureor maybe one of them is null or typo error in the field naming.

我的错误在于 flutter ，但这没关系。