如何確保方法等待http響應而不是在Dart中返回null？

Question

我想在Dart中編寫一個小命令行庫來與Facebook API一起工作。我有一個類'fbuser'，它將auth-token和user-id作爲屬性獲取，並有一個方法'groupIds'，它應該返回一個List，其中包含來自用戶的所有組的ID。

當我調用該方法時，它將返回null，儘管在http響應之後調用了兩個possbile返回值。我做錯了什麼？

這裏是我的代碼：

import 'dart:convert'; //used to convert json 
import 'package:http/http.dart' as http; //for the http requests 

//config 
final fbBaseUri = "https://graph.facebook.com/v2.1"; 
final fbAppID = "XXX"; 
final fbAppSecret = "XXY"; 

class fbuser { 
    int fbid; 
    String accessToken; 

    fbuser(this.fbid, this.accessToken); //constructor for the object 

    groupIds(){ //method to get a list of group IDs 
    var url = "$fbBaseUri/me/groups?access_token=$accessToken"; //URL for the API request 
    http.get(url).then((response) { 
     //once the response is here either process it or return an error 
     print ('response received'); 
     if (response.statusCode == 200) { 
     var json = JSON.decode(response.body); 
     List groups=[]; 
     for (int i = 0; i<json['data'].length; i++) { 
      groups.add(json['data'][i]['id']); 
     } 
     print(groups.length.toString()+ " Gruppen gefunden"); 
     return groups; //return the list of IDs 
     } else { 
     print("Response status: ${response.statusCode}"); 
     return (['error']); //return a list with an error element 
     } 
    }); 
    } 
} 

void main() { 
    var usr = new fbuser(123, 'XYY'); //construct user 
    print(usr.groupIds()); //call method to get the IDs 
} 

目前的輸出是：

Observatory listening on http://127.0.0.1:56918 
null 
response received 
174 Gruppen gefunden 

的方法運行的http請求，但空立即返回。

（我開始這個夏天的編程。感謝您的幫助。）

Answer 1

return http.get(url) // add return 

void main() { 
    var usr = new fbuser(123, 'XYY'); //construct user 
    usr.groupIds().then((x) => print(x)); //call method to get the IDs 
    // or 
    usr.groupIds().then(print); //call method to get the IDs 
}