[ios] Log event when offline pack is created

[captainbarbosa]

Part II of #12751.

Integrates the new offline event into the Mapbox Maps SDK framework, logging an event when a new region is added with[MGLSharedOfflineStorage addPackForRegion:withContext:completionHandler:].

This PR also bumps mapbox-events-ios to the current mapbox/mapbox-events-ios@61fca37 commit on master in order to take advantage of the corresponding new event class and constants.

[captainbarbosa]

@1ec5 I just realized I'm not sure if there is a way we can access the source of the style being used here. MGLOfflineRegion doesn't have access to an MGLStyle's sources, only the style URL itself, and there doesn't seem to be a way to initialize a new MGLStyle outside of the context of an MGLMapView.

[1ec5]

It’s true that MGLOfflineStorage doesn’t have access to the runtime styling models, but the first resource that mbgl::OfflineManager downloads for any pack is the style JSON file. That file tells mbgl about all the sources, so at that point it could execute a callback that tells the SDK the URLs of the sources involved.

Note that some sources aren’t backed by TileJSON files, instead being inlined in the style. Inlined sources aren’t as interesting for analytical purposes, since they tend to be one-offs. Either way, there’s an mbgl::TileSet object that the SDK could be told about, and the SDK could then decide what to send as part of the event.

[captainbarbosa]

When logging the attributes of the event type, it seems like resourcesDownloaded and tilesDownloaded don't pass in a correct number (they're both zero). 🤔

All other attributes populate as expected.

[julianrex]

At this point countOfResourcesCompleted etc haven't been updated (https://github.com/mapbox/mapbox-gl-native/pull/13039/files#diff-2e0199077c3e9d88f9c6669352f18d67R220).

I wonder if MGLMapView (or something else) should register for the MGLOfflinePackProgressChangedNotification notification - and push the event there?

[captainbarbosa]

The scope of this PR only includes logging when an MGLOfflinePack has been added. Logging of a pack's success/failure/cancel state will be addressed in another PR.

[julianrex]

Since you already have tileRegion and shapeRegion you could just check if (tileRegion) ... here.

[julianrex]

Similarly here, you should check if (shapeRegion) ...

However, I think it would be better if we added a readonly property (or method) to MGLOfflineRegion_Private, along the lines of:

@property (nonatomic, readonly) NSDictionary *eventAttributes;

which both MGLTilePyramidOfflineRegion and MGLShapeOfflineRegion implement, e.g (tweaking your above attribute dictionary):

-(NSDictionary *)eventAttributes {
    return @{
        @"shapeForOfflineRegion": @"tileregion",
        @"minZoom": @(self.minimumZoomLevel),
        @"maxZoom": @(self.maximumZoomLevel),
        @"styleURL": self.styleURL // assuming non-null
    };
}

Then in this code you'd check for conformance:

NSMutableDictionary *eventAttributes = [NSMutableDictionary dictionaryWithObject:MMEventTypeOfflineDownloadStart forKey:@"event"];

// Add region specific attributes
if ([region conformsToProtocol:@protocol(MGLOfflineRegion_Private)]) {
    NSDictionary *regionAttributes = ((id<MGLOfflineRegion_Private>)region).eventAttributes; 
    [eventAttributes addEntriesFromDictionary:regionAttributes];
}

[MGLMapboxEvents pushEvent:MMEventTypeOfflineDownloadStart withAttributes:attributes];

[julianrex]

Longer term, we should consider whether having the pushEvent:withAttributes call here¹ is the right thing to do. Instead perhaps we delegate or broadcast a notification, that something can listen for and it's that object's responsibility to push the event. But this particular topic shouldn't hold up this PR.

---

¹ This also goes for other places we're currently calling pushEvent... /cc @fabian-guerra

[julianrex]

All the attribute keys should be of the form MMEEventKey...; for example, instead of event use MMEEventKeyEvent.

You'll probably need to add constants for the other keys?

[julianrex]

Instead of [NSNumber numberWithDouble:] you can use the short-hand form @(<double>).

[captainbarbosa]

Waiting for mapbox/mapbox-events-ios#83 to merge so I can bump the library here and address #13039 (comment).

[julianrex]

This needs to be inside the method, otherwise you'll be missing an implementation for offlineStartEventAttributes. You could either wrap the MMEEvents, or just have an else clause where you return a @{}

[julianrex]

Move the closing } here, so that we always push the event regardless of whether a completion block is specified.