There is now only one instance of mbgl::OfflineFileSource, created when the shared MGLOfflineStorage object is initialized. Also create and use the shared MGLOfflineStorage object when initializing an MGLMapView object. The unified cache file is located in a subdirectory of Application Support, where the SDK has control over the file’s lifetime. The subdirectory is already named after the host application’s bundle identifier, ensuring that each Mapbox-powered application has an independent tile limit. If there’s an ambient cache in a subdirectory of Caches, delete it. If there’s an offline cache in a subdirectory of Documents on iOS or Caches on OS X, move it to the unified cache location in a subdirectory of Application Support.
Fixes the iOS/OS X side of #4338.
Due to ARC, MGLOfflinePack can be deallocated at any time following its initialization. -dealloc ensures that the offline region observer is unset, but that step is asynchronous; there is no guarantee that it completes before the first status change update. In fact, the pack is deallocated before the first status change update unless it is captured outside of the MGLOfflinePackAdditionCompletionHandler. The pack nil check is unnecessary because the subsequent code would be a no-op.
Fixed a race condition that could occur if a progress update comes after a call to remove the task but before the removal is complete. Avoid synchronously setting the task’s state inside -resume and -suspend; instead, rely on the observer to set the state asynchronously.
Also categorized offline symbols for jazzy and moved styleURL from MGLOfflineRegion to MGLTilePyramidOfflineRegion, since you never know if there will be a region type not constrained by a style in the future.
Renamed SDK classes related to offline viewing to more closely match the terminology used by mbgl and the Android SDK while remaining consistent with Cocoa naming principles.
The lock is in place to enforce that `async` is not accessed if the request has been, cancelled. Therefore it's not necessary to hold the lock beyond setting cancelled to true, and in fact it's unsafe to so: if this is the last remaining shared reference, `delete this` will destroy the mutex. If the lock was held, it would then be orphaned.
