Be Able to Avoid Endless Loop When Listening for and Firing Same Event

[philippfromme]

Listening for an event and firing that same event in the callback will result in an endless loop:

const listener = () => {
  eventBus.fire('foo');
};

eventBus.on('foo', listener);

eventBus.fire('foo');

Currently, the only way to prevent this is to stop and start listening again in the callback:

const listener = () => {
  eventBus.off('foo', listener);

  eventBus.fire('foo');

  eventBus.on('foo', listener);
};

eventBus.on('foo', listener);

eventBus.fire('foo');

There should be a more elegant way of avoiding this kind of situation. Sometimes the user might not be aware that the event he is listening for is fired through the callback.

Related #524

[nikku]

I'm not sure if this is something we should solve / support in an elegant way on the EventBus layer.

Solely from the API perspective, this looks like expected behavior to me: If I fire foo inside a listener, listening for foo, surely that is going to result in an endless loop? What else should I expect?

[philippfromme]

I'm not saying we should solve this in the event bus. This issue is simply meant to start the discussion. 😉

[nikku]

What is the concrete instance of that problem? I guess, looking into bpmn-io/align-to-origin#2 it is the fact that you cannot safe the diagram on commandStack.changed when some component changes the command stack on diagram save?

[philippfromme]

Yes, this is the original issue. There is no way of listening to the diagram has changed to export it when using the align-to-origin module.

[nikku]

Let's close this one. Now that #524 is closed a simple way to achieve this is:

function listenAndRepeat() {
  
  eventBus.fire('listenOnlyOnce', ...);

  eventBus.once('listenOnlyOnce', listenAndRepeat);
}

eventBus.once('listenOnlyOnce', listenAndRepeat);

In the above case listenAndRepeat can safely fire listenAndRepeat without looping.