我想渲染一个需要 HTTP 调用来收集数据的小部件。
得到以下代码(简体)
import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:convert';
void main() {
runApp(new MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter demo',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new MyHomePage(title: 'async demo'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
var asyncWidget;
@override
initState() {
super.initState();
loadData().then((result) {
print(result);
setState(() {
asyncWidget = result;
});
});
}
loadData() async {
var widget = new AsyncWidget();
return widget.build();
}
@override
Widget build(BuildContext context) {
if(asyncWidget == null) {
return new Scaffold(
appBar: new AppBar(
title: new Text("Loading..."),
),
);
} else {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Center(
child: this.asyncWidget,
),
);
}
}
}
class MyRenderer {
MyRenderer();
Widget render (List data) {
List<Widget> renderedWidgets = new List<Widget>();
data.forEach((element) {
renderedWidgets.add(new ListTile(
title: new Text("one element"),
subtitle: new Text(element.toString()),
));
});
var lv = new ListView(
children: renderedWidgets,
);
return lv;
}
}
class MyCollector {
Future gather() async {
var response = await // do the http request here;
return response.body;
}
}
class AsyncWidget {
MyCollector collector;
MyRenderer renderer;
AsyncWidget() {
this.collector = new MyCollector();
this.renderer = new MyRenderer();
}
Widget build() {
var data = this.collector.gather();
data.then((response) {
var responseObject = JSON.decode(response);
print(response);
return this.renderer.render(responseObject);
});
data.catchError((error) {
return new Text("Oups");
});
}
}
我的代码是这样工作的:使用异步数据的小部件需要一个收集器(进行 http 调用)和一个渲染器,它将使用 http 数据渲染小部件。 我在 initState() 上创建了这个小部件的一个实例,然后进行异步调用。
我发现一些文档说我们应该使用 setState() 方法用新数据更新小部件,但这对我不起作用。
但是,当我放置一些日志时,我看到 HTTP 调用已完成并调用了 setState() 方法,但小部件没有更新。
最佳答案
最好的方法是使用 FutureBuilder 。
来自 FutureBuilder 文档:
new FutureBuilder<String>(
future: _calculation, // a Future<String> or null
builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.none: return new Text('Press button to start');
case ConnectionState.waiting: return new Text('Awaiting result...');
default:
if (snapshot.hasError)
return new Text('Error: ${snapshot.error}');
else
return new Text('Result: ${snapshot.data}');
}
},
)
另一件事是您在 State.build 方法之外构建小部件并保存小部件本身,这是一种反模式。您实际上应该每次都在 build 方法中构建小部件。
你可以在没有 FutureBuilder 的情况下让它工作,但你应该保存 http 调用的结果(经过适当处理),然后在你的构建函数中使用数据。
请参阅此内容,但请注意,使用 FutureBuilder 是执行此操作的更好方法,我只是提供此内容供您学习。
import 'dart:async';
import 'dart:convert';
import 'package:flutter/material.dart';
void main() {
runApp(new MyApp());
}
class MyApp extends StatelessWidget {
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter demo',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
home: new MyHomePage(title: 'async demo'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => new _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
List data;
@override
initState() {
super.initState();
new Future<String>.delayed(new Duration(seconds: 5), () => '["123", "456", "789"]').then((String value) {
setState(() {
data = json.decode(value);
});
});
}
@override
Widget build(BuildContext context) {
if (data == null) {
return new Scaffold(
appBar: new AppBar(
title: new Text("Loading..."),
),
);
} else {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
),
body: new Center(
child: new ListView(
children: data
.map((data) => new ListTile(
title: new Text("one element"),
subtitle: new Text(data),
))
.toList(),
),
),
);
}
}
}
关于async-await - Flutter,异步调用后渲染小部件,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/49930180/