长列表的处理

标准的 ListView 构造函数适用于短列表，对于具有大量列表项的长列表，需要用 ListView.builder 构造函数来创建。

The standard ListView constructor works well for small lists. To work with lists that contain a large number of items, it’s best to use the ListView.builder constructor.

与标准的 ListView 构造函数需要一次性创建所有列表项不同的是，ListView.builder 构造函数只在列表项从屏幕外滑入屏幕时才去创建列表项。

In contrast to the default ListView constructor, which requires creating all items at once, the ListView.builder() constructor creates items as they’re scrolled onto the screen.

1. 创建数据源

1. Create a data source

首先，需要获取列表的数据源。例如，数据源可以是消息集、搜索结果集或者商店商品集。大部分情况下，这些数据来自于网络请求或者数据库获取。

First, you need a data source. For example, your data source might be a list of messages, search results, or products in a store. Most of the time, this data comes from the internet or a database.

在下面的例子，使用 List.generate 构造函数生成包含 10,000 个字符串的集合。

For this example, generate a list of 10,000 Strings using the List.generate constructor.

final items = List<String>.generate(10000, (i) => "Item $i");

2. 将数据源渲染成组件

2. Convert the data source into widgets

为了将字符串集合展示出来，需要把集合中的每个字符串都渲染成组件。

To display the list of strings, render each String as a widget using ListView.builder().

这个渲染过程正是 ListView.builder 的作用所在。在下面的例子中，将会把每个字符串用单行列表项显示在列表中。

In this example, display each String on its own line.

ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) {
    return ListTile(
      title: Text('${items[index]}'),
    );
  },
);

完整样例

Complete example

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp(
    items: List<String>.generate(10000, (i) => "Item $i"),
  ));
}

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

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

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

    return MaterialApp(
      title: title,
      home: Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: ListView.builder(
          itemCount: items.length,
          itemBuilder: (context, index) {
            return ListTile(
              title: Text('${items[index]}'),
            );
          },
        ),
      ),
    );
  }
}