Fultter中的FutureBuilder和StreamBuilder详解

一、目标：使用StreamBuilder模拟实现每隔一秒返回从网络中获取的数据，并刷新页面。
效果图：

Stream.periodic构造，顾名思义，是用来创建流，在周期间隔反复广播事件。

import 'package:flutter/material.dart';

main()=>runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: const Text("StreamBuilder"),
          centerTitle: true,
        ),
        body: Center(
          child: StreamBuilder<int>(
              stream: _getNetWorkData(),
              builder: (BuildContext context, AsyncSnapshot<int> snapshot){
                if(snapshot.hasError){
                  return const Text("出错了");
                }
                switch(snapshot.connectionState){
                  case ConnectionState.none:
                    return const Text("没有 stream");
                  case ConnectionState.waiting:
                    return const Text("等待数据......");
                  case ConnectionState.active:
                    return Text("active: ${snapshot.data}");
                  case ConnectionState.done:
                    return const Text("Stream 已经关闭");
                }
              }),
        )
      ),
    );
  }
}

 Stream<int> _getNetWorkData() {
  return Stream.periodic(const Duration(seconds: 1),(i){
    return i;
  });
 }

深入理解BuildStream，我们将要构建的应用程序的背景颜色会随着时间而变化。它还在屏幕中央显示递增的数字。我们可以通过按下浮动按钮来阻止这些无情的行为。
代码例子：

import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:math';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      // Hide the debug banner
      debugShowCheckedModeBanner: false,
      title: 'KindaCode.com',
      theme: ThemeData(
        primarySwatch: Colors.indigo,
      ),
      home: const HomeScreen(),
    );
  }
}

class HomeScreen extends StatefulWidget {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  State<HomeScreen> createState() => _HomeScreenState();
}

class _HomeScreenState extends State<HomeScreen> {
  final Stream _myStream =
      Stream.periodic(const Duration(seconds: 1), (int count) {
    return count;
  });

  // The subscription on events from _myStream
  late StreamSubscription _sub;

  // This number will be displayed in the center of the screen
  // It changes over time
  int _computationCount = 0;

  // Background color
  // In the beginning, it's indigo but it will be a random color later
  Color _bgColor = Colors.indigo;

  @override
  void initState() {
    _sub = _myStream.listen((event) {
      setState(() {
        _computationCount = event;

        // Set the background color to a random color
        _bgColor = Colors.primaries[Random().nextInt(Colors.primaries.length)];
      });
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: _bgColor,
      appBar: AppBar(
        title: const Text('Lucklyの博客'),
        backgroundColor: Colors.transparent,
      ),
      body: Center(
        child: Text(
          _computationCount.toString(),
          style: const TextStyle(fontSize: 150, color: Colors.white),
        ),
      ),
      // This button is used to unsubscribe the stream listener
      floatingActionButton: FloatingActionButton(
        child: const Icon(
          Icons.stop,
          size: 30,
        ),
        onPressed: () => _sub.cancel(),
      ),
    );
  }

  // Cancel the stream listener on dispose
  @override
  void dispose() {
    _sub.cancel();
    super.dispose();
  }
}

二、目标：使用FutureBuilder构建，等待网络请求回来的数据，显示到页面，还没有拿到数据前，显示loading
效果图：

import 'package:flutter/material.dart';

main()=>runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        appBar: AppBar(
          title: const Text("FutureBuilder"),
          centerTitle: true,
        ),
        body: Center(
          child: FutureBuilder(
              future: _getNetWorkData(),
              builder: (BuildContext context, AsyncSnapshot snapshot){
                 if(snapshot.connectionState==ConnectionState.done){
                   if(snapshot.hasError){
                     return const Text("请求失败！");
                   }else{
                     return Text("content: ${snapshot.data}");
                   }
                 }else{
                   //请求未结束。显示loading
                   return const CircularProgressIndicator();
                 }
              }),
        )
      ),
    );
  }
}

 Future _getNetWorkData() {
    return Future.delayed(const Duration(seconds: 1),(){
      return "从网络获取到的数据";
    });
 }