Flutter 忽略“if”分支内的 return 语句

Question

我正在尝试使用 PopupMenuButton 小部件从对象列表中创建过滤器。我认为逻辑是合理的，但 Flutter/Dart 似乎忽略了下面代码块中名为“fliterFavs”的函数中“if 分支”中的 return 语句。

当我选择“Favs”时，它会按预期进行识别，并且返回之前的打印语句工作正常。

当我使用相同的 return 语句代替“if 语句”之外的那个时，它可以正常工作。我得到了我期望的过滤数据集。

我什至尝试将它作为两个背靠背的 if 语句，而不是您在下面看到的嵌套语句。还是不行。

有什么想法吗？

final List<Product> favProducts = [];

  Widget filterFavs(value) {
    loadedProducts.forEach((i) {
      if (i.isFavorite == true) {
        favProducts.add(i);
        if (value == "Favs") {
          print(value);
          return buildGrid(favProducts);
        }
      }
    });
    return buildGrid(loadedProducts);
  }

  Widget buildGrid(List newList) {
    return GridView.builder(
      padding: const EdgeInsets.all(10.0),
      itemCount: newList.length,
      itemBuilder: (ctx, i) => ProductItem(
        newList[i].id,
        newList[i].title,
        newList[i].imageUrl,
      ),
      gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
        crossAxisCount: 2,
        childAspectRatio: 3 / 2,
        crossAxisSpacing: 10,
        mainAxisSpacing: 10,
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('MyShop'),
          actions: <Widget>[
            PopupMenuButton(
              onSelected: (val) {
                filterFavs(val);
              },
              icon: Icon(
                Icons.more_vert,
              ),
              itemBuilder: (context) => [
                PopupMenuItem(
                  child: Text("Favs"),
                  value: "Favs",
                ),
                PopupMenuItem(
                  child: Text("All"),
                  value: "All",
                ),
              ],
            ),
          ],
        ),
        //use a filter on the item builder
        body: filterFavs(context));
  }
}

Answer 1

想想你所指的 return 是从哪个函数返回的。它返回到传递给forEach 方法的匿名函数，而不是像您期望的那样传递给filterFavs。这不是颤振/飞镖的问题，而是您的期望的问题。

您可以使用替代的 for-each 语法来解决这个问题：

Widget filterFavs(value) {
  for(var i in loadedProducts) {
    if (i.isFavorite == true) {
      favProducts.add(i);
      if (value == "Favs") {
        print(value);
        return buildGrid(favProducts);
      }
    }
  }
  return buildGrid(loadedProducts);
}

Answer 2

Christopher Moore 感谢您的帮助，这是现在可行的最终解决方案。

import 'package:flutter/material.dart';

import '../models/product.dart';
import '../widgets/product_item.dart';

class ProductsOverviewScreen extends StatefulWidget {
  @override
  _ProductsOverviewScreenState createState() => _ProductsOverviewScreenState();
}

class _ProductsOverviewScreenState extends State<ProductsOverviewScreen> {
  List choices = ["Favs", "All"];
  String _choice = '';

  void _select(userChoice) {
    setState(() {
      _choice = userChoice;
      print(_choice);
    });
  }

  final List<Product> loadedProducts = [
    Product(
      id: 'p1',
      title: 'Red Shirt',
      description: 'A red shirt - it is pretty red!',
      price: 29.99,
      isFavorite: true,
      imageUrl:
          'https://cdn.pixabay.com/photo/2016/10/02/22/17/red-t-shirt-1710578_1280.jpg',
    ),
    Product(
      id: 'p2',
      title: 'Trousers',
      description: 'A nice pair of trousers.',
      price: 59.99,
      isFavorite: true,
      imageUrl:
          'https://upload.wikimedia.org/wikipedia/commons/thumb/e/e8/Trousers%2C_dress_%28AM_1960.022-8%29.jpg/512px-Trousers%2C_dress_%28AM_1960.022-8%29.jpg',
    ),
    Product(
      id: 'p3',
      title: 'Yellow Scarf',
      description: 'Warm and cozy - exactly what you need for the winter.',
      price: 19.99,
      isFavorite: false,
      imageUrl:
          'https://live.staticflickr.com/4043/4438260868_cc79b3369d_z.jpg',
    ),
    Product(
      id: 'p4',
      title: 'A Pan',
      description: 'Prepare any meal you want.',
      price: 49.99,
      isFavorite: false,
      imageUrl:
          'https://upload.wikimedia.org/wikipedia/commons/thumb/1/14/Cast-Iron-Pan.jpg/1024px-Cast-Iron-Pan.jpg',
    ),
  ];

  List<Product> favProducts = [];

  Widget filterFavs(value) {
    if (favProducts.isEmpty) {
      // this is to prevent items from being infinitely added to favProducts
      //if user selects Favs more than once
      for (var i in loadedProducts) {
        if (i.isFavorite == true) {
          favProducts.add(i);
        }
      }
    }
    if (value == "Favs") {
      print(value);
      return buildGrid(favProducts);
    }
    favProducts = [];
    return buildGrid(loadedProducts);
  }

  Widget buildGrid(List newList) {
    return GridView.builder(
      padding: const EdgeInsets.all(10.0),
      itemCount: newList.length,
      itemBuilder: (ctx, i) => ProductItem(
        newList[i].id,
        newList[i].title,
        newList[i].imageUrl,
      ),
      gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
        crossAxisCount: 2,
        childAspectRatio: 3 / 2,
        crossAxisSpacing: 10,
        mainAxisSpacing: 10,
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('MyShop'),
          actions: <Widget>[
            PopupMenuButton(
                elevation: 3.2,
                initialValue: choices[1],
                onSelected: _select,
                icon: Icon(
                  Icons.more_vert,
                ),
                itemBuilder: (context) => [
                      PopupMenuItem(
                        child: Text(choices[0]),
                        value: choices[0],
                      ),
                      PopupMenuItem(
                        child: Text(choices[1]),
                        value: choices[1],
                      ),
                    ]),
          ],
        ),
        //use a filter on the item builder
        body: filterFavs(_choice));
  }
}