Flutter（十）网络请求和文件

文件操作

APP目录
Android 和 iOS 的应用存储目录不同，PathProvider (opens new window)插件提供了一种平台透明的方式来访问设备文件系统上的常用位置。该类当前支持访问两个文件系统位置：

临时目录: getTemporaryDirectory() ,可以使用来获取临时目录；系统可随时清除临时目录的文件。在 Android上，这是getCacheDir() (opens new window)返回的值,在 iOS 上，这对应于NSTemporaryDirectory() (opens new window)返回的值。
文档目录: getApplicationDocumentsDirectory()来获取应用程序的文档目录，该目录用于存储只有自己可以访问的文件。只有当应用程序被卸载时，系统才会清除该目录。
在 Android 上，这是AppData目录,在 iOS 上，这对应于NSDocumentDirectory。。
外部存储目录：getExternalStorageDirectory()来获取外部存储目录，如 SD 卡；
Android 下结果是Android SDK 中getExternalStorageDirectory的返回值。
iOS不支持外部目录，所以在 iOS 下调用该方法会抛出UnsupportedError异常，

使用：
pubspec.yaml中添加引用：path_provider: ^2.0.2
文件读写演示：

import 'dart:io';
import 'dart:async';
import 'package:flutter/material.dart';
import 'package:path_provider/path_provider.dart';class FileOperationRoute extends StatefulWidget {FileOperationRoute({Key? key}) : super(key: key);@override_FileOperationRouteState createState() => _FileOperationRouteState();
}class _FileOperationRouteState extends State<FileOperationRoute> {int _counter = 0;@overridevoid initState() {super.initState();//从文件读取点击次数_readCounter().then((int value) {setState(() {_counter = value;});});}Future<File> _getLocalFile() async {// 获取应用目录String dir = (await getApplicationDocumentsDirectory()).path;return File('$dir/counter.txt');}Future<int> _readCounter() async {try {File file = await _getLocalFile();// 读取点击次数（以字符串）String contents = await file.readAsString();return int.parse(contents);} on FileSystemException {return 0;}}_incrementCounter() async {setState(() {_counter++;});// 将点击次数以字符串类型写到文件中await (await _getLocalFile()).writeAsString('$_counter');}@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text('文件操作')),body: Center(child: Text('点击了 $_counter 次'),),floatingActionButton: FloatingActionButton(onPressed: _incrementCounter,tooltip: 'Increment',child: Icon(Icons.add),),);}
}

在实际开发中，如果要存储一些简单的数据，使用shared_preferences插件会比较简单

网络请求

1.Dio库

发起 GET 请求 :

Response response;
response=await dio.get("/test?id=12&name=wendu")
print(response.data.toString());

对于GET请求我们可以将query参数通过对象来传递，上面的代码等同于：

response=await dio.get("/test",queryParameters:{"id":12,"name":"wendu"})
print(response);

发起一个 POST 请求:

response=await dio.post("/test",data:{"id":12,"name":"wendu"})

发起多个并发请求:

response= await Future.wait([dio.post("/info"),dio.get("/token")]);

下载文件:

response=await dio.download("https://www.google.com/",_savePath);

发送 FormData:

FormData formData = FormData.from({"name": "wendux","age": 25,
});
response = await dio.post("/info", data: formData)

如果发送的数据是FormData，则dio会将请求header的contentType设为“multipart/form-data”。

通过FormData上传多个文件:

FormData formData = FormData.from({"name": "wendux","age": 25,"file1": UploadFileInfo(File("./upload.txt"), "upload1.txt"),"file2": UploadFileInfo(File("./upload.txt"), "upload2.txt"),// 支持文件数组上传"files": [UploadFileInfo(File("./example/upload.txt"), "upload.txt"),UploadFileInfo(File("./example/upload.txt"), "upload.txt")]
});
response = await dio.post("/info", data: formData)

dio内部仍然使用HttpClient发起的请求，所以代理、请求认证、证书校验等和HttpClient是相同的，我们可以在onHttpClientCreate回调中设置，例如：

(dio.httpClientAdapter as DefaultHttpClientAdapter).onHttpClientCreate = (client) {//设置代理 client.findProxy = (uri) {return "PROXY 192.168.1.2:8888";};//校验证书httpClient.badCertificateCallback=(X509Certificate cert, String host, int port){if(cert.pem==PEM){return true; //证书一致，则允许发送数据}return false;};   };

注意，onHttpClientCreate会在当前dio实例内部需要创建HttpClient时调用，所以通过此回调配置HttpClient会对整个dio实例生效，如果应用需要多种代理或证书校验策略，可以创建不同的dio实例来分别实现。

dio主页：https://github.com/flutterchina/dio

2.websocket

演示WebSocket通信过程

import 'package:flutter/material.dart';
import 'package:web_socket_channel/io.dart';class WebSocketRoute extends StatefulWidget {@override_WebSocketRouteState createState() => _WebSocketRouteState();
}class _WebSocketRouteState extends State<WebSocketRoute> {TextEditingController _controller = TextEditingController();IOWebSocketChannel channel;String _text = "";@overridevoid initState() {//创建websocket连接channel = IOWebSocketChannel.connect('wss://echo.websocket.events');}@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text("WebSocket(内容回显)"),),body: Padding(padding: const EdgeInsets.all(20.0),child: Column(crossAxisAlignment: CrossAxisAlignment.start,children: <Widget>[Form(child: TextFormField(controller: _controller,decoration: InputDecoration(labelText: 'Send a message'),),),//当服务器传输的数据是指定为二进制时，StreamBuilder的snapshot.data的类型就是List<int>，是文本时，则为StringStreamBuilder(stream: channel.stream,builder: (context, snapshot) {//网络不通会走到这if (snapshot.hasError) {_text = "网络不通...";} else if (snapshot.hasData) {_text = "echo: "+snapshot.data;}return Padding(padding: const EdgeInsets.symmetric(vertical: 24.0),child: Text(_text),);},)],),),floatingActionButton: FloatingActionButton(onPressed: _sendMessage,tooltip: 'Send message',child: Icon(Icons.send),),);}void _sendMessage() {if (_controller.text.isNotEmpty) {channel.sink.add(_controller.text);}}@overridevoid dispose() {channel.sink.close();super.dispose();}
}

3.JSON转Dart Model

Android Studio - Settings - Plugins - 搜索JsonToDart 插件
文件夹右键New - Json To Dart - 输入json文件自动生成

生成示例如下：

import 'as_t.dart';class HotKeyModel {int? id;String? link;String? name;int? order;int? visible;HotKeyModel({this.id, this.link, this.name, this.order, this.visible});HotKeyModel.fromJson(Map<String, dynamic> json) {id = asT<int?>(json['id']);link = asT<String?>(json['link']);name = asT<String?>(json['name']);order = asT<int?>(json['order']);visible = asT<int?>(json['visible']);}Map<String, dynamic> toJson() {final Map<String, dynamic> data = {};data['id'] = this.id;data['link'] = this.link;data['name'] = this.name;data['order'] = this.order;data['visible'] = this.visible;return data;}
}