Create lists with different types of items

有時候可能需要建立顯示不同類型內容的list，例如標題和其顯示的內容是不同類型的value，我們可以用以下兩步建立出不同類型的list

1. 創建具有不同類型項目的data source
2. 將各種data source轉換為widget list

1. Create a data source with different types of items

為每種類型的項目定義一個class表示列表中不同類型的item，在這個範例中，創建一個標題後跟著五條訊息的App。因此我們需要創建三個類：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.headline5,
    );
  }

  @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);
}

然後產生或取得list裡面需要的內容

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

2. Convert the data source into a list of widgets

要將每個項目轉換為widget，可以用 ListView.builder() 構造函數，一般我們會用一個builder()函數來檢查正在處理的項目類型，並為該類型的項目return適合的widget

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),
    );
  },
)

example

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.headline5,
    );
  }

  @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);
}

output

參考資料:
https://docs.flutter.dev/cookbook/lists/mixed-list