Calling an async method from component constructor in Dart

Question

Let's assume that an initialization of MyComponent in Dart requires sending an HttpRequest to the server. Is it possible to construct an object synchronously and defer a 'real' initialization till the response come back?

In the example below, the _init() function is not called until "done" is printed. Is it possible to fix this?

import 'dart:async'; import 'dart:io';  class MyComponent{   MyComponent() {     _init();   }    Future _init() async {     print("init");   } }  void main() {   var c = new MyComponent();   sleep(const Duration(seconds: 1));   print("done"); } 

Output:

done init 

Answer 1

A constructor can only return an instance of the class it is a constructor of (MyComponent). Your requirement would require a constructor to return Future<MyComponent> which is not supported.

You either need to make an explicit initialization method that needs to be called by the user of your class like:

class MyComponent{   MyComponent();    Future init() async {     print("init");   } }  void main() async {   var c = new MyComponent();   await c.init();   print("done"); } 

or you start initialization in the consturctor and allow the user of the component to wait for initialization to be done.

class MyComponent{   Future _doneFuture;    MyComponent() {     _doneFuture = _init();   }    Future _init() async {     print("init");   }    Future get initializationDone => _doneFuture }  void main() async {   var c = new MyComponent();   await c.initializationDone;   print("done"); } 

When _doneFuture was already completed await c.initializationDone returns immediately otherwise it waits for the future to complete first.