Flutter实现网络请求的方法示例

 environment:
 sdk: ">=2.1.0 <3.0.0"

 dependencies:
  flutter:
   sdk: flutter
 
  json_annotation: ^2.0.0
  connectivity: ^0.4.2
  dio: ^2.0.15
  event_bus: ^1.0.3
  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2
 
 dev_dependencies:
  flutter_test:
   sdk: flutter

  static netFetch(url,params,Map header,Options option,{noTip = false}) async {
   //获取网络的连接状态，如果没有连接网络，返回NETWORK_ERROR
   var connectivityResult = await (new Connectivity().checkConnectivity());
   if(connectivityResult == ConnectivityResult.none) {
    return new ResultData(Code.errorHandleFunction(Code.NETWORK_ERROR, "", noTip),false,Code.NETWORK_ERROR);
   }
   //请求头存放集合
   Map headers = new HashMap();
   if(header != null) {
    headers.addAll(header);
   }
   //option存放请求的一些配置信息
   if(option != null) {
    option.headers = headers;
   }else {
    //get请求
    option = new Options(method:'get');
    option.headers = headers;
   }
   //超时时间15000ms
   option.connectTimeout = 15000;
   //创建dio对象
   Dio dio = new Dio();
   Response response;
   try{
    //执行网络请求，await和async配合使用，表示这是一个异步耗时操作
    //执行结果不会立马返回。
    response = await dio.request(url,data: params,options: option);
   }on DioError catch(e) {
    //异常处理
    ....
    return new ResultData(Code.errorHandleFunction(errorResponse.statusCode, e.message, noTip), false, errorResponse.statusCode);
   }
   try{
     if(response.statusCode == 200 || response.statusCode == 201) {
      //网络请求成功
      return await new ResultData(response.data, true, Code.SUCCESS,headers: response.headers);
     }
   }catch(e) {
    //异常处理
    print('返回参数' + e.toString() + url);
    return new ResultData(response.data, false, response.statusCode,headers: response.headers);
   }
   return new ResultData(Code.errorHandleFunction(response.statusCode, "", noTip), false, response.statusCode);
  }

  //如果这个是耗时方法，就会用async标记，耗时操作用await标记，表示是一个异步操作。
  static getTodayFilmListDao() async {
   //获取请求的url
   String url = Address.getTodayFilmList();
   //调用上面封装的网络请求方法进行网络请求
   var res = await HttpManager.netFetch(url, null, null, null);
   if(res != null && res.result) {
    var data = res.data;
    if(data == null || data.length == 0) {
     return await new DataResult([],true);
    }
    //网络请求成功，进行数据解析
    var response = TodayFilmResponse.fromJson(data);
    //返回数据
    return await new DataResult(response.result,true);
   }else {
    return await new DataResult(null,false);
   }
  }

 static getTodayFilmList() {
   return "${host}movie/movies.today?cityid=1&dtype=&key=713a408e855352d913806ef1e3ce3c33";
  }

{
        "movieId":"135808",
        "movieName":"新喜剧之王",
        "pic_url":"http:\/\/img5.mtime.cn\/mt\/2019\/02\/02\/113216.53857992_182X243X4.jpg"
      },
  
TodayFilmBean类

    //TodayFilmBean.g.dart将在我们运行生成命令之后自动生成
    part 'TodayFilmBean.g.dart';
    
    //这个标注是告诉生成器，这个类是要生成的Model类
    @JsonSerializable()
    class TodayFilmBean{
     String movieId;
     String movieName;
     String pic_url;
     
      //构造函数
     TodayFilmBean(this.movieId,this.movieName,this.pic_url);
     
     //json转换为bean对象
     factory TodayFilmBean.fromJson(Map json) => _$todayFilmBeanFromJson(json);
     
     //bean对象转换为json
     Map toJson() => _$todayFilmBeanToJson(this);
    }
生成的TodayFilmBean.g.dart类是这样的

    part of 'TodayFilmBean.dart';
    
    //json转换为bean对象
    TodayFilmBean _$todayFilmBeanFromJson(Map json) {
     return TodayFilmBean(json['movieId'] as String,json['movieName'] as String,
       json['pic_url'] as String);
    }
    
    //bean对象转换为json
    Map _$todayFilmBeanToJson(TodayFilmBean instance) =>
       {
       'movieId': instance.movieId,
       'movieName': instance.movieName,
       'pic_url':instance.pic_url
    };

//把json数据转化为了bean对象
var filmBean = TodayFilmBean.fromJson(json);

  DataResult dataResult;
  List mData = [];
  
  //当StatefulWiget被嵌入此view树中，就会为此widget创建State对象
  //当State对象被创建了，frameWork就会调用initState()方法
  @override
  void initState() {
   //初始化数据
   getTodayFilm();
   super.initState();
  }
  
  
  void getTodayFilm() async {
   //这是一个异步操作，结果返回有一定延迟
   dataResult = await TodayDao.getTodayFilmListDao();
   //调用setState方法会通知framework控件状态有变化，它会立马触发
   //State的build方法更新widget状态
   setState(() {
    mData = dataResult.data;
   });
  }

   //State的build方法，调用setState方法后，此方法就会被触发
   //用来刷新UI
   @override
   Widget build(BuildContext context) {
  
    return Scaffold(
     appBar: AppBar(
  
      title: Text(widget.title),
     ),
     
     //如果mData.length == 0,展示一个loading框，否则展示数据
     body: mData.length == 0
       ? new Center(child: new CircularProgressIndicator()):
       //创建GridView对象
       new GridView.builder(
  
       gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
         crossAxisCount: 3, //每行2个
         mainAxisSpacing: 1.0, //主轴(竖直)方向间距
         crossAxisSpacing: 1.0, //纵轴(水平)方向间距
         childAspectRatio: 0.7 //纵轴缩放比例
       ),
       //item数量
       itemCount:mData.length,

       //创建每个item
       itemBuilder: (BuildContext context,int index) {
        return _getWidget(index);
       }),
    );
   }

 _getWidget(int index) {
  //添加要展示的item内容
   return new Column(
    children: [
     new Expanded(child: new CardItem(color: Colors.black12,child: _getChild(index)),flex: 8,),
     //显示网络请求文本
     new Expanded(child: new Text(mData[index].movieName,
      textAlign: TextAlign.end,
      maxLines: 1,
     ),
       flex:1)
    ]);
 }

 _getChild(int i) {
  return new Padding(padding: new EdgeInsets.all(1.0),
    //显示网络请求的图片
    child: new Image(image: NetworkImage(mData[i].pic_url)));
 }

class CardItem extends StatelessWidget{
 final Widget child;
 final EdgeInsets margin;
 final Color color;
 final RoundedRectangleBorder shape;
 final double elevation;

 CardItem({@required this.color,this.child,this.elevation = 5.0,this.shape,this.margin});

 @override
 Widget build(BuildContext context) {
  EdgeInsets margin = this.margin;
  RoundedRectangleBorder shape = this.shape;
  Color color = this.color;
  margin ??= EdgeInsets.only(left: 2.0,top: 2.0,right: 2.0,bottom: 2.0);
  shape ??= new RoundedRectangleBorder(borderRadius: new BorderRadius.all(Radius.circular(4.0)));
  color ??= new Color(0xffeeff);
  return new Card(elevation: elevation,shape: shape,color: color,margin: margin,child: child,);
 }
}