Dart MappedListIterable 不是子类型

我还是个新手，try 以流的形式从FireStore获取数据并馈送到我的ListView，但我总是收到这个错误:

type 'MappedListIterable<DocumentSnapshot, Product>' is not a subtype
of type 'List<Product>'

我在stackoverflow上看到过其他几个类似的帖子，但它们要么对我没有帮助，要么不适用于我的情况.

这是我的产品页面小部件:

import 'package:xxx/models/Product.dart';
import 'package:agrogator/screens/products/widgets/products_list.dart';
import 'package:xxx/services/product.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class ProductsScreen extends StatelessWidget {
  ProductsScreen({Key key}) : super(key: key);

  final product = ProductService();

  // This widget is the productsucts page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  @override
  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return StreamProvider<List<Product>>.value(
      value: product.streamProducts(),
      child: new Scaffold(
        appBar: new AppBar(
          // Here we take the value from the MyHomePage object that was created by
          // the App.build method, and use it to set our appbar title.
          title: new Text("xxx"),
        ),
        body: new ProductsList(),
        floatingActionButton: new FloatingActionButton(
          onPressed: () {},
          tooltip: 'Increment',
          child: new Icon(Icons.add),
        ), // This trailing comma makes auto-formatting nicer for build methods.
      ),
    );
  }
}

这是我的ProductsList小工具:

import 'package:xxx/models/Product.dart';
import 'package:xxx/screens/products/widgets/product_item.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class ProductsList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var products = Provider.of<List<Product>>(context);

    return Container(
      height: 100,
      child: ListView(
        children: products.map((product) {
          return new ProductItem(product: product);
        }).toList(),
      ),
    );
  }
}

这是我的ProductItem小部件:

import 'package:xxx/models/Product.dart';
import 'package:flutter/material.dart';

class ProductItem extends StatelessWidget {
  final Product product;

  ProductItem({this.product});

  @override
  Widget build(BuildContext context) {
    return Text(product.name, style: TextStyle(color: Colors.black));
  }
}

这是我的产品模型:

import 'package:cloud_firestore/cloud_firestore.dart';

class Product {
  String uid;
  String name;
  String unit;
  int avgQuantity;
  double avgWeight;
  double previousAvgPrice;
  double currentAvgPrice;
  String lastUpdatedBy;
  String lastUpdatedAt;
  String remarks;

  Product(
      {this.uid,
      this.name,
      this.unit,
      this.avgQuantity,
      this.avgWeight,
      this.previousAvgPrice,
      this.currentAvgPrice,
      this.lastUpdatedBy,
      this.lastUpdatedAt,
      this.remarks});

  factory Product.fromFirestore(DocumentSnapshot doc) {
    Map data = doc.data;

    return Product(
        uid: doc.documentID,
        name: data["name"],
        unit: data["unit"],
        avgQuantity: data["avgQuantity"],
        avgWeight: data["avgWeight"],
        previousAvgPrice: data["previousAvgPrice"],
        currentAvgPrice: data["ccurrentAvgPrice"],
        lastUpdatedBy: data["lastUpdatedBy"],
        lastUpdatedAt: data["lastUpdatedAt"],
        remarks: data["remarks"]);
  }
}

我的服务是:

import 'package:xxx/models/Product.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

class ProductService {
  final Firestore _db = Firestore.instance;

  Stream<List<Product>> streamProducts() {
    var ref = _db.collection("products");

    return ref
        .snapshots()
        .map((list) => list.documents.map((doc) => Product.fromFirestore(doc)));
  }
}