Support XIBs and storyboards on iOS

[1ec5]

It’s relatively difficult to add an MGLMapView to a storyboard-based project. As expected, you can drag a UIView into a scene and set its custom class to “MGLMapView”. However, that just gets you a blank map. So you need to call [mapView setAccessToken:@"…"] at launch, and [mapView setStyleJSON:nil] sometime after that, but not too soon or you’ll hit #1014.

Basic support for XIBs and storyboards means:

• Make accessToken KVO-compliant, so it can be set via a user-defined runtime attribute in Interface Builder.
• -setAccessToken: should apply the default style if it’s called for the first time and no style has been applied. It’ll be tricky to fix this and avoid triggering [iOS] Crash using bundled style immediately after instantiation #1014 for the usual -initWithFrame:accessToken: case, so [iOS] Crash using bundled style immediately after instantiation #1014 is probably a blocker.

After we get basic support, we can look at enhancing the IB workflow via #929.

[1ec5]

We plan to distribute Mapbox GL via CocoaPods (#1030), which handles repackaging the library as a dynamic framework when use_frameworks! is specified (required for Swift usage). That unblocks #929, so we could in principle ship with an inspectable and designable MGLMapView today. However, we need accessToken to be KVO-compliant and trigger a refresh (without hitting #1147) in order to provide the right developer experience.

[1ec5]

This is definitely blocked on #1147.