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


solution1 ,

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.

solution2 

Probably the best way to handle this is with a factory function, which calls a private constructor.

In Dart, private methods start with an underscore, and "additional" constructors require a name in the form ClassName.constructorName, since Dart doesn't support function overloading. This means that private constructors require a name, which starts with an underscore (MyComponent._create in the below example).

import 'dart:async';
import 'dart:io';

class MyComponent{
  /// Private constructor
  MyComponent._create() {
    print("_create() (private constructor)");

    // Do most of your initialization here, that's what a constructor is for
    //...
  }

  /// Public factory
  static Future<MyComponent> create() async {
    print("create() (public factory)");

    // Call the private constructor
    var component = MyComponent._create();

    // Do initialization that requires async
    //await component._complexAsyncInit();

    // Return the fully initialized object
    return component;
  }
}

void main() async {
  var c = await MyComponent.create();

  print("done");
}

This way, it's impossible to accidentally create an improperly initialized object out of the class. The only available constructor is private, so the only way to create an object is with the factory, which performs proper initialization.