Added a LoadMode property to tell MapCache how to load the tiles

[kkieffer]

Following up on discussion from #17 I did find that using two closures did not seem to work properly. So since you can only select from either server or cache I made this modification.

I was able to make this work for my local fork. I added a property called LoadMode that determines how loadTile(at path: MKTileOverlayPath... loads tiles. The user could change this mode dynamically depending on their use case, their network connection status, etc.

• case always_server always fetches from server, updating cache. Useful when on unmetered and high bandwidth (eg. Wifi) connection. User always receives most recent data.

• case cache_then_server is the current paradigm. Use the cache, unless not found, then fetch from server and update. Minimum amount of network access.

• case cache_if_expired_then_server returns from cache if the cache file is new enough, otherwise fetch from server and update. This I left as a TODO because a method is needed to first retrieve the file date and compare it to .now minus some user supplied delta. This is useful if the user cares mostly about limiting network access, but will accept some loading of tiles from the server if the cache is too old.

• case cache_only Only fetch from cache, never go out to server. Useful to completely eliminate any bandwidth usage (perhaps user is on a limited or expensive data connection - they want to just use their cache which they've presumably loaded up prior).

[merlos]

Thanks @kkeiffer 👍

I am looking at it. I´ll merge it ASAP. I may refactor part of your code (for instance extract LoadMode to a file and use camelCase naming in the enum...

I think we can skip cache_if_expired_then_server and use only cache_then_server. If the cache supports expiration, then cache_then_server is the same but with an infinite expiration time.
I added #24 as a separate enhancement that should provide support for this scenario.

Also, I am thinking that to download a map region with RegionDownloader, to have a only_server mode that performs the download and then update the cache, but does not return the tile in case of failure when it is already in the cache. This mode may be useful if you want to download/refresh a whole map. In this case, if the download of the tile fails you want to know that the tile failed to potentially do some kind of retry. I added #23 as I noticed that RegionDownloaderdoes not provide the list of failed tiles.

[merlos]

I added some changes. It still needs testing.