Future 是在未来某个时间获得想要对象的一种手段.
1、定义一个名为ff的future,返回字符串。

  Future<String> ff = Future(() {
    return 'future test';
  });
  ff.then((value) => print(value));

  网络请求示例
 Future<Map<String,dynamic>> getShopListData() async{
      var data = //耗时的操作
      return data;
 }

2、future其他用法

//1
Future.delayed(Duration(seconds: 2), () {
    print('test future1');
  });
//2
 var future = Future.delayed(Duration(seconds: 2), () {
    return 'test future2';
  });
  future.then((value) => print(value));
//3
  Future<String> testFuture() async {
    var futureResult = await Future.delayed(Duration(seconds: 2), () {
      return 'test future3';
    });
    return futureResult;
  }
  testFuture().then((value) => print(value)).catchError((e) {});

  //4
  Future<String> getUserName(String parameter) {
    return Future.value(parameter);
  }

  getUserName('jacky').then((value) => print('my name is' + value));

  //4
  Future<Map<String, dynamic>> getUserInfo(String parameter) async {
    final map = await {'name': 'jacky'};
    return map;
  }

  getUserInfo('jacky').then((value) => print('userinfo is' + value.toString()));

  //5
  Future<Map<String, dynamic>> getUserInfo2(
      String parameter, Function(String name, String email) userFunc) async {
    final map = await {'name': 'jacky'};
    userFunc('Jacky', '123456@qq.com');
    return map;
  }

  getUserInfo2(
      'Jacky',
      (name, email) => {
            print('user name and email is' + name + '??' + email)
          }).then((value) => print(value));

//6
  Future.wait([
    Future.delayed(Duration(seconds: 2), () {
      return 'hello1';
    }),
    Future.delayed(Duration(seconds: 3), () {
      return 'hello2';
    }),
    Future.delayed(Duration(seconds: 4), () {
      return 'hello3';
    }),
  ]).then((value) => print(value));