Calling an async method from component constructor

问题:

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

回答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.