Capture events from a state variable which is a contract

Question

I developed a contract which uses another contract in it state variable, but looks like the events from the contract in the state variable aren’t dispatched.

contract A {
  event Hello(uint num);
  function doSomething() {
   Hello(1);
  }
}

contract B {
  A public a;

  function doAnotherThing() public {
    a = new A();
    a.doSomething();
  }
}

What is expected: To be possible to capture the events from the contract A which is a state variable in the contract B. So when call a.doSomething() in the contract B should be dispatched the event Hello(1) and be possible to capture in the watch.

Answer 1

It looks like you're confusing inheritance with deployment.

Your contract B deploys a new instance of contract A. contract A has an event emitter and a function doSomething() and you can invoke it directly with addressA.dosomething() or indirectly with addressB.doSomethingElse(). In either case, you can listen to events from contractA but contractB will not emit events.

This would be an entirely different picture in the case of inheritance.

You could say: contract B is A { ...}

In this case, you would be able to invoke addressB.doSomething() and listen for the event at addressB because all of A would become part of B. You would drop the references to A as an external thing in that case.

contract B is A {
  // A public a;

  function doAnotherThing() public {
    // a = new A();
    doSomething();
  }
}

Hope it helps.

Answer 2

The events are triggered and recorded in the transaction receipt. In your case what is failing is that the source of the events is not the contract B, but contract A.

A new instance of contrac A is created before calling a.doSomething(), that will be the source of the event.