track value callback completion in firebaseLoads

[joenoon]

use case: no data exists vs data not yet loaded.
scenario: this.bindAsArray(ref, "items"); without firebaseLoads
there is no way to tell why there are no items, which is a problem
if we want to show a "no items" message. if we rely on
items.length == 0 we will see the "no items" message flicker
before the data is loaded.

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[joenoon]

CLA signed

[jwngr]

Apologies for the extreme delay in getting back to you on this one. This PR brings up a really great point / use case which I overlooked. There is an existing workaround in which you can attach your own on('value') listener and wait for that to fire before you get rid of your "Loading..." screen. But it would be nice to have something built into ReactFire for this.

That being said, I'm not sure this is the correct fix for that. It works, but I'm not a huge fan of the API, specifically the extra firebaseLoads variable. I think it should be more straight-forward. What do you think about setting this.data[bindVar] to undefined in both the array and object case before we have any data? Currently, we already leave this.data[bindVar] as undefined in the object case so you can actually tell when it loads. This seems logical to me. Setting to null would be worrisome since null is the value if there is no data there, so you wouldn't be able to distinguish between those two cases in the object case. I think undefined may be the way to go. In which case, we may just need to move this logic into the on('child_added') listener callback since it will fire before on('value'). We need some special logic to see if this.data[bindVar] is undefined and, if so, initialize it to [].

If you are willing to update your PR, that would be awesome. Otherwise, I opened #74 to track this and will hopefully get to it next time I have a ReactFire coding session.