dart - 如何在 Dart 语言中将 Future 转换为 List？

Question

我对使用 dart 语言进行编码有疑问。如何将 Future 转换为普通 List？在我的程序中，我需要来自网络的结果才能继续下一步，但我不知道该怎么做。就我而言，我需要将变量“data”的值作为列表返回。

class _MyAppState extends State<MyApp> {
    static List getList() {
    var url = "http://watcherman.cn/web/json.php";
    var data;
    HttpClient client = new HttpClient();
    client.getUrl(Uri.parse(url))
      .then((HttpClientRequest request) => request.close())
    .then((HttpClientResponse response) {
      response.transform(UTF8.decoder).listen((contents) {
        String jsonContent = contents.toString();
        data = JSON.decode(jsonContent);
      });
    });
    return data;
  }
}

但似乎没有执行网络部分。 flutter 的代码部分似乎无法接受 Future。

class _MyAppState extends State<MyApp> {
  static List content = getList();
  @override
  Widget build(BuildContext context) {
    return new Container(
        child: new ListView.builder(
            scrollDirection: Axis.vertical,
            padding: new EdgeInsets.all(6.0),
            itemCount: content.length,
            itemBuilder: (BuildContext context, int index) {
              return new Container(
                  alignment: FractionalOffset.center,
                  margin: new EdgeInsets.only(bottom: 6.0),
                  padding: new EdgeInsets.all(6.0),
                  color: Colors.blueGrey,
                  child: new Text(content[index]["title"])
              );
            }
        )
    );
  }
}

Answer 1

你应该重写你的getList返回 Future<List> 的方法.可以使用 then() 的链来执行此操作调用，但 IMO 更易读 async / await .

一旦你有一个返回 Future<List> 的方法, 你可以使用 FutureBuilder 构建一个依赖于异步计算结果的树。

import 'package:flutter/material.dart';
import 'dart:io';
import 'dart:convert';
import 'dart:async';

void main() {
  runApp(new MaterialApp(
    home: new MyApp(),
  ));
}

class MyApp extends StatefulWidget {
  State createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {

  static Future<List> getList() async {
    var url = "http://watcherman.cn/web/json.php";
    HttpClient client = new HttpClient();
    HttpClientRequest request = await client.getUrl(Uri.parse(url));
    HttpClientResponse response = await request.close();
    return response.transform(UTF8.decoder).transform(JSON.decoder).toList();
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Futures Demo'),
      ),
      body: new FutureBuilder(
        future: getList(),
        builder: (BuildContext context, AsyncSnapshot<List> snapshot) {
          if (!snapshot.hasData)
            return new Container();
          List content = snapshot.data[0];
          return new ListView.builder(
            scrollDirection: Axis.vertical,
            padding: new EdgeInsets.all(6.0),
            itemCount: content.length,
            itemBuilder: (BuildContext context, int index) {
              return new Container(
                alignment: FractionalOffset.center,
                margin: new EdgeInsets.only(bottom: 6.0),
                padding: new EdgeInsets.all(6.0),
                color: Colors.blueGrey,
                child: new Text('${content[index]['title']}'),
              );
            },
          );
        }
      )
    );
  }
}