mobile - 将项目动态附加到 ListView

Question

我是 Dart 和 Flutter 的新手，并尝试将新项目附加到我的 ListView。我创建了一个递增 this.value 的按钮，但没有任何反应。我是否错过了对 UI 的更新调用，这是否是正确的方法？ 由于我将 ListView.builder 的值直接返回给 build 的调用者，因此我不确定如何获取列表以添加更多项目。非常感谢!

class MyList extends State<MyList> {

... 
int value = 2;

@override
Widget build(BuildContext context) {
return ListView.builder(
    itemCount: this.value,
    itemBuilder: (context, index) => this._buildRow(index)
);

TL;DR:调用 setState 是触发 UI 更新的正确方法吗？

Answer 1

调用 setState 是触发 UI 更新的正确方法。

来自docs :

Calling setState notifies the framework that the internal state of this object has changed in a way that might impact the user interface in this subtree, which causes the framework to schedule a build for this State object.

If you just change the state directly without calling setState, the framework might not schedule a build and the user interface for this subtree might not be updated to reflect the new state.

这里是一个 ListView 的小例子，它带有一个 Button，可以向其附加项目。

import 'package:flutter/material.dart';

void main() => runApp(new MaterialApp(home: MyList()));

class MyList extends StatefulWidget {
  @override
  _MyListState createState() => _MyListState();
}

class _MyListState extends State<MyList> {
  int value = 2;

  _addItem() {
    setState(() {
      value = value + 1;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text("MyApp"),
      ),
      body: ListView.builder(
          itemCount: this.value,
          itemBuilder: (context, index) => this._buildRow(index)),
      floatingActionButton: FloatingActionButton(
        onPressed: _addItem,
        child: Icon(Icons.add),
      ),
    );
  }

  _buildRow(int index) {
    return Text("Item " + index.toString());
  }
}