文件读写

获取文件地址并返回该文件（获取路径是anync await异步操作）：

Future<File> _getLocalFile() async {
    String dir = (await getApplicationDocumentsDirectory()).path;   // get the path to the document directory.
return File('$dir/counter.txt'); }

通过指定的文件地址获取指定文件后，将该文件读出，并解码为整形数据；

Future<int> _readCounter() async {
    try {
      File file = await _getLocalFile();
　　  String contents = await file.readAsString();// 读取文件是异步操作.

　　  return int.parse(contents); } on FileSystemException { return 0; } }//用int.parse(contents);把String转化为int.

向指定的文件路径写入文件，这是异步操作，注意两个await，都不落下；

Future<Null> _incrementCounter() async {
    setState(() {
      _counter++;
    });
    // write the variable as a string to the file
    await (await _getLocalFile()).writeAsString('$_counter');
  }

下面是一个示例：
直接拷贝到main.dart就可以使用，注意引入依赖：path_provider

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

void main() {
  runApp(
    MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(primarySwatch: Colors.blue),
      home: FlutterDemo(),  
    ),
  );
}

class FlutterDemo extends StatefulWidget {
  FlutterDemo({Key key}) : super(key: key);

  @override
  _FlutterDemoState createState() =>  _FlutterDemoState();
}

class _FlutterDemoState extends State<FlutterDemo> {
  int _counter;

  @override
  void initState() {
    super.initState();
    _readCounter().then((int value) {
      setState(() {
        _counter = value;
      });
    });
  }

  Future<File> _getLocalFile() async {
    // get the path to the document directory.
    String dir = (await getApplicationDocumentsDirectory()).path;
    return File('$dir/counter.txt');
  }

  Future<int> _readCounter() async {
    try {
      File file = await _getLocalFile();
      // read the variable as a string from the file.
      String contents = await file.readAsString();
      return int.parse(contents);
    } on FileSystemException {
      return 0;
    }
  }

  Future<Null> _incrementCounter() async {
    setState(() {
      _counter++;
    });
    // write the variable as a string to the file
    await (await _getLocalFile()).writeAsString('$_counter');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: new Text('Flutter Demo')),
      body: Center(
        child: Text('Button tapped $_counter time${
            _counter == 1 ? '' : 's'
        }.'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}


整个示例看下来可以发现，所有的异步操作要么在initstate里面，要么在onpress里面。所以咱们以后主要使用的异步操作就暂时先放在这两个地方！