Create lists with different types of items

You might need to create lists that display different types of content. For example, you might be working on a list that shows a heading followed by a few items related to the heading, followed by another heading, and so on.

Here’s how you can create such a structure with Flutter:

  1. Create a data source with different types of items.
  2. Convert the data source into a list of widgets.

1. Create a data source with different types of items

Types of items

To represent different types of items in a list, define a class for each type of item.

In this example, create an app that shows a header followed by five messages. Therefore, create three classes: ListItem, HeadingItem, and 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);

  Widget buildTitle(BuildContext context) {
    return Text(
      heading,
      style: Theme.of(context).textTheme.headline5,
    );
  }

  Widget buildSubtitle(BuildContext context) => null;
}

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

  MessageItem(this.sender, this.body);

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

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

Create a list of items

Most of the time, you would fetch data from the internet or a local database and convert that data into a list of items.

For this example, generate a list of items to work with. The list contains a header followed by five messages. Each message has one of 3 types: ListItem, HeadingItem, or MessageItem.

final items = List<ListItem>.generate(
  1200,
  (i) => i % 6 == 0
      ? HeadingItem("Heading $i")
      : MessageItem("Sender $i", "Message body $i"),
);

2. Convert the data source into a list of widgets

To convert each item into a widget, use the ListView.builder() constructor.

In general, provide a builder function that checks for what type of item you’re dealing with, and returns the appropriate widget for that type of item.

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),
    );
  },
);

Interactive 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;

  MyApp({Key key, @required this.items}) : super(key: key);

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

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: 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);

  Widget buildTitle(BuildContext context) {
    return Text(
      heading,
      style: Theme.of(context).textTheme.headline5,
    );
  }

  Widget buildSubtitle(BuildContext context) => null;
}

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

  MessageItem(this.sender, this.body);

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

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