创建包含不同类型项目的列表

您可能需要创建显示不同类型内容的列表。例如，您可能正在处理一个列表，该列表显示一个标题，然后是与标题相关的几个项目，然后是另一个标题，等等。

以下是如何使用 Flutter 创建这种结构。

1. 创建一个包含不同类型项目的數據源。
2. 将数据源转换为小部件列表。

要表示列表中不同类型的项目，请为每种类型的项目定义一个类。

在本例中，创建一个显示标题后跟五个消息的应用程序。因此，创建三个类：ListItem、HeadingItem 和 MessageItem。

/// The base class for the different types of items the list can contain.
abstract class ListItem {
  /// The title line to show in a list item.
  Widget buildTitle(BuildContext context);

  /// The subtitle line, if any, to show in a list item.
  Widget buildSubtitle(BuildContext context);
}

/// A ListItem that contains data to display a heading.
class HeadingItem implements ListItem {
  final String heading;

  HeadingItem(this.heading);

  @override
  Widget buildTitle(BuildContext context) {
    return Text(
      heading,
      style: Theme.of(context).textTheme.headlineSmall,
    );
  }

  @override
  Widget buildSubtitle(BuildContext context) => const SizedBox.shrink();
}

/// A ListItem that contains data to display a message.
class MessageItem implements ListItem {
  final String sender;
  final String body;

  MessageItem(this.sender, this.body);

  @override
  Widget buildTitle(BuildContext context) => Text(sender);

  @override
  Widget buildSubtitle(BuildContext context) => Text(body);
}

大多数情况下，您会从互联网或本地数据库中获取数据，并将这些数据转换为项目列表。

在本例中，生成一个要使用的项目列表。该列表包含一个标题，后面跟着五个消息。每条消息都属于三种类型之一：ListItem、HeadingItem 或 MessageItem。

final items = List<ListItem>.generate(
  1000,
  (i) => i % 6 == 0
      ? HeadingItem('Heading $i')
      : MessageItem('Sender $i', 'Message body $i'),
);

要将每个项目转换为小部件，请使用 ListView.builder() 构造函数。

通常，提供一个构建器函数，该函数检查您正在处理的项目类型，并返回该类型项目的相应小部件。

ListView.builder(
  // Let the ListView know how many items it needs to build.
  itemCount: items.length,
  // Provide a builder function. This is where the magic happens.
  // Convert each item into a widget based on the type of item it is.
  itemBuilder: (context, index) {
    final item = items[index];

    return ListTile(
      title: item.buildTitle(context),
      subtitle: item.buildSubtitle(context),
    );
  },
)

import 'package:flutter/material.dart';

void main() {
  runApp(
    MyApp(
      items: List<ListItem>.generate(
        1000,
        (i) => i % 6 == 0
            ? HeadingItem('Heading $i')
            : MessageItem('Sender $i', 'Message body $i'),
      ),
    ),
  );
}

class MyApp extends StatelessWidget {
  final List<ListItem> items;

  const MyApp({super.key, required this.items});

  @override
  Widget build(BuildContext context) {
    const title = 'Mixed List';

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: const Text(title),
        ),
        body: ListView.builder(
          // Let the ListView know how many items it needs to build.
          itemCount: items.length,
          // Provide a builder function. This is where the magic happens.
          // Convert each item into a widget based on the type of item it is.
          itemBuilder: (context, index) {
            final item = items[index];

            return ListTile(
              title: item.buildTitle(context),
              subtitle: item.buildSubtitle(context),
            );
          },
        ),
      ),
    );
  }
}

/// The base class for the different types of items the list can contain.
abstract class ListItem {
  /// The title line to show in a list item.
  Widget buildTitle(BuildContext context);

  /// The subtitle line, if any, to show in a list item.
  Widget buildSubtitle(BuildContext context);
}

/// A ListItem that contains data to display a heading.
class HeadingItem implements ListItem {
  final String heading;

  HeadingItem(this.heading);

  @override
  Widget buildTitle(BuildContext context) {
    return Text(
      heading,
      style: Theme.of(context).textTheme.headlineSmall,
    );
  }

  @override
  Widget buildSubtitle(BuildContext context) => const SizedBox.shrink();
}

/// A ListItem that contains data to display a message.
class MessageItem implements ListItem {
  final String sender;
  final String body;

  MessageItem(this.sender, this.body);

  @override
  Widget buildTitle(BuildContext context) => Text(sender);

  @override
  Widget buildSubtitle(BuildContext context) => Text(body);
}