main.dart

import "package:flutter/material.dart";
import 'package:app1/demos/increase/Increase.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'app',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: '计数器 demo'),
    );
  }
}

demos/increse/Increse.dart

import "package:flutter/material.dart";
class MyHomePage extends StatefulWidget {
  final String title;
  MyHomePage({key, @required this.title}) : super(key: key);
  @override
  _MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
  // 定义一个状态值，用于记录按钮点击的总次数。
  int _counter = 0; // _counter 为保存屏幕右下角带“+”号按钮点击次数的状态。
  // 数字自增函数
  void _increaseCounter() {
    // 数字自增完成后，需要一个setState函数。如果不使用，那么我们的视图不会刷新。
    setState(() {
      _counter++;
    });
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('${widget.title}'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            Text(
              '$_counter',
              style: TextStyle(
                color: Theme.of(context).primaryColor, // 使用主题色
                fontSize: 30,
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _increaseCounter, //传入自增函数
        tooltip: 'increse button',
        child: Icon(Icons.add),
      ),
    );
  }
}