fix(database): retrieve initial list content once - #820

Your approach is much simpler! Do know though, that the data is still only retrieved once over the network. The second set of listeners retrieve from the cache.

I agree that it would make sense for the second set of listeners to receive cached data, but why does it show up in the WebSocket traffic in the Dev Tools? Is this an SDK issue? Is it not using the cache it ought to be?

Actually, I suspect the second set of listeners would receive cached data if the first listener was attached with on, but it's attached with once. So perhaps the first listener is already 'disconnected' (so to speak) by the time the promise resolves and the second set of listeners establish a new 'connection' - which sees them retrieve data via the WebSocket?

@davideast I've had a bit of a poke around with this and there's another plunk here that demonstrates the differences in behaviour between overlapping and non-overlapping listeners.

It seems that if the listeners don't overlap, data for the second listener is re-retrieved via the WebSocket - which is the behaviour that I see with the current FirebaseListObservable and its initial content.

@cartant I went and spoke with a member on the Database team and your suspicions are correct. A once() will force a network call each and every time because internally it calls .off() which "disconnects". While on() calls will attempt to read from the cache if available because it is an active listener.

This should PR should fix these issues. Thank you :)

@davideast Thanks for looking further into it. What you say makes sense and fits with the behaviour I see. I'm looking forward to the performance improvements that should come with the next release.