读写File

在initstat里面将文件里面存储的数值赋值给要显示的组件，这样可以在App打开时就恢复上次App存储的数据；
每次数值变化就随之存储到文件；
下面是一个完整的示例：
import 'package:flutter/material.dart';
import 'dart:io';
import 'dart:async';
import 'package:path_provider/path_provider.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  int count=0;
  File file;

  Future<File> getAppLocation()async{
    String path=(await getApplicationDocumentsDirectory()).path;
    path='$path/count.txt';
    File file=File(path);
    return file;
  }

  Future writeCountToFile()async{
    await (await getAppLocation()).writeAsString('$count');
  }

  Future<String> readCountFromFile()async{
    String value = await (await getAppLocation()).readAsString();
    return value;
  }
  @override
  void initState() {
    // TODO: implement initState
    readCountFromFile().then((value){
      setState(() {
        count=int.parse(value);
      });
    });
    super.initState();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('勇往直前'),),
      floatingActionButton: FloatingActionButton(
          child: Icon(Icons.add),
          onPressed: (){
        setState(() {
          count++;
        });
        writeCountToFile();
      }),
      body: Center(
        child: ActionChip(
            label: Text('点击了$count次'),
            onPressed: (){
              setState(() {
                count++;
              });
              writeCountToFile();
            },
        ),
      ),
    );
  }
}