fixes races bug in react-native > 0.52

[taxidermic]

In newer versions of react-native dispatch_async in setCurrentScreen causes mixed order. It's obvious if you make several calls of setCurrentScreen and logEvent in a small amount of time.

See: https://github.com/facebook/react-native/blob/0.54-stable/React/Base/RCTBridgeModule.h#L112

[chrisbianca]

@taxidermic I'm unsure why we need this queue, as the dispatch method is using dispatch_get_main_queue(), not the methodQueue you've defined?

[taxidermic]

The queue that will be used to call all exported methods. If omitted, this will call on a default background queue, which is avoids blocking the main thread.
....
If you don't want to specify the queue yourself, but you need to use it
inside your class (e.g. if you have internal methods that need to dispatch
onto that queue), you can just add `@synthesize methodQueue = _methodQueue;`
and the bridge will populate the methodQueue property for you automatically
when it initializes the module.

Docs I mentioned tells that if methodQueue is not synthesized, methods marked with RCT_EXPORT_METHOD will be called in global concurrent queue, which leads to messing. But! This passage lead me to wrong conclusions. My colleague(@vovkasm) made a big research about react-native/iOS interactions, and in fact it seems that react-native always makes that thing: https://github.com/facebook/react-native/blob/master/React/Base/RCTModuleData.mm#L209
So, it brings us to the point, where we don't need to use @synthesize methodQueue = _methodQueue;. But I would like to test it at first, so I'll do it and answer you tomorrow about the results.

[taxidermic]

@chrisbianca Tested, yes, it works without @synthesize methodQueue = _methodQueue;!
Will you edit it by yourself, or should I send new pull_request? (I'm not sure how it works here ^^")

Btw, it fixes races only inside these structures separately. So, it's still possible, that

firebase.SetCurrentScreen(someProps)
firebase.LogEvent(anotherProps)

may mess up.

Though, several calls SetCurrentScreen will save their order

[taxidermic]

I mean:

firebase.SetCurrentScreen(A)
firebase.LogEvent(E1)
firebase.SetCurrentScreen(B)
firebase.LogEvent(E2)

E1.screen == A и E2.screen == B, otherwords results will be accurate, though in Firebase Console you may see:

E1
screen_view(A)
screen_view(B)
E2

[chrisbianca]

Thanks @taxidermic , this is a good step in the right direction. We'll consider how best to synchronize across all the methods.