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Online resources caching

Online resources caching

To display the map some external resources are needed by the map renderer:
map style and map tiles.

Map style itself (.json file describing map layers display properties and rendering order)
and assets needed by the style are bundled with SDK library.
Style assets include sprites (sets of icons) displayed on the map and fonts.

When the map viewer is focused at particular spot, map tiles provided by
remote online server are needed to display the map. When a part of the map
is displayed for the first time, needed tiles are downloaded from the server.
Resources downloaded from the server are stored (cached) in an offline
database for further use. The server communicates maximum tile resource lifetime
by means of "Cache-Control" HTTP header. Expiration time
is stored in the offline database together with the tile resource.
If a tile resides in the cache it can be displayed even if internet
connection is not available. When the object in the cache expires, the server is
asked again about the object. If a newer version of the object is available,
it is downloaded and
the cache contents are refreshed. If the old resource is still valid, its lifetime
is prolonged.
Cache size is limited to 50MB of disk space. When the limit is reached,
least recently used tile resources are discarded.

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