Add single (“simple”) offline pack example [WIP]

[friedbunny]

Fixes #11. After some futzing around with KVO, this example is essentially the current offline doc with a progress bar. I left in the not-strictly necessary MGLOfflinePack error notifications because it seems like the friendly thing to do.

To-do:

• Swift
• Remove debug and example website formatting code (navbar hiding, progress bar coloring/sizing).
• Move startOfflinePackDownload call later Downloading an offline pack blocks the visible map from loading mapbox-gl-native#4414.

Notes:

• Downloads 1661 resources, 89.6 MB

/cc @1ec5

[1ec5]

It isn’t strictly necessary for the example descend to the maximum zoom level. We could simply add a few to the current zoom level and set that as the upper bound.

[friedbunny]

I think you’re right, this isn’t a very realistic way to control the extent of the downloaded region. I’ll look at changing how the min/max zoom are set.

[1ec5]

The most realistic UI would be a range slider. There are many range slider pods on CocoaPods, but I don’t know that we want to introduce that kind of dependency into an example. Maybe just a comment to the effect of “how cool would it be to use a range slider here!”

[friedbunny]

Swift notes from converting https://www.mapbox.com/help/mobile-offline/:

• Observer names needed MGL prepended (OfflinePackProgressChangedNotification → MGLOfflinePackProgressChangedNotification)
• addPackForRegion() pack.resume() needed unwrapping

[1ec5]

Whoops, thanks for catching that copy-pasta!