Provide notification mechanism for failed TileJSON requests about mbxmapkit

[boundsj]

Hi @wsnook fwiw; using singletons like NSNotificationCenter can make things a little harder to test compared to the much cleaner delegation pattern (or even a block callback). If there is a clear need to notify many disparate listeners about the failure of an MBXMapView init call then notification center or a well documented KVO scheme may be a good option... But, this seems more like something the delegate of the MBXMapView should be responsible for if the app developer wants to broadcast error conditions in notification center.

In mapbox/DEPRECATED-mapbox-ios-sdk#397 I provided a solution for a somewhat similar tile loading dilemma where the init method ends up making a synchronous networking call. The refactor I started there could be extended to also offer an error case delegate callback. A similar API in mbxmapkit would be awesome. Glad you guys are working on this!

Just my two cents!

@boundsj, Thanks for the perspective. I'll check that out.

I'm working a branch over at #73 which addresses this issue like so:

#pragma mark - Constants for the MBXMapKit error domain

extern NSString *const MBXMapKitErrorDomain;
extern NSInteger const MBXMapKitErrorCodeHTTPStatus;
extern NSInteger const MBXMapKitErrorCodeDictionaryMissingKeys;

#pragma mark - Delegate callbacks for asynchronous loading of map metadata and markers

@protocol MBXRasterTileOverlayDelegate <NSObject>
- (void)tileOverlay:(MBXRasterTileOverlay *)overlay didLoadMetadata:(NSDictionary *)metadata withError:(NSError *)error;
- (void)tileOverlay:(MBXRasterTileOverlay *)overlay didLoadMarkers:(NSArray *)markers withError:(NSError *)error;
- (void)tileOverlayDidFinishLoadingMetadataAndMarkersForOverlay:(MBXRasterTileOverlay *)overlay;
@end

#pragma mark -

@interface MBXRasterTileOverlay : MKTileOverlay

#pragma mark - Map tile overlay layer initialization and configuration

- (id)initWithMapID:(NSString *)mapID;
- (id)initWithMapID:(NSString *)mapID metadata:(BOOL)metadata markers:(BOOL)markers;
- (id)initWithMapID:(NSString *)mapID metadata:(BOOL)metadata markers:(BOOL)markers imageQuality:(MBXRasterImageQuality)imageQuality;

@property (weak,nonatomic) id<MBXRasterTileOverlayDelegate> delegate;

For the implementation in the issue79 branch and PR #81, HTTP requests that yield a status code != 200 will trigger an error message getting passed back through the delegate callbacks. That should take care of it. The callbacks look like:

@protocol MBXRasterTileOverlayDelegate <NSObject>
- (void)tileOverlay:(MBXRasterTileOverlay *)overlay didLoadMetadata:(NSDictionary *)metadata withError:(NSError *)error;
- (void)tileOverlay:(MBXRasterTileOverlay *)overlay didLoadMarkers:(NSArray *)markers withError:(NSError *)error;
...
@end

and

@protocol MBXOfflineMapDownloaderDelegate <NSObject>
...
- (void)offlineMapDownloader:(MBXOfflineMapDownloader *)offlineMapDownloader didEncounterRecoverableError:(NSError *)error;
- (void)offlineMapDownloader:(MBXOfflineMapDownloader *)offlineMapDownloader didCompleteOfflineMapDatabase:(MBXOfflineMapDatabase *)offlineMapDatabase withError:(NSError *)error;
@end

[incanus]

#81 merged.