Offline first for tile download

Issue Type

• Improvement
• Performance

Description and/or steps/code to reproduce the problem

The process in order to get the tiles works more or less that way:

• we have an ordered list of providers
• some are embedded (zip files, SQLite)
• some are on the internet
• at least one provider is an approximation (MapTileApproximater)
• and we try to find the tile bitmap from this ordered list of providers - and when we find it we don’t check the following providers

Problem: assuming we don’t have the tile in our embedded providers and assuming we have a low connectivity, we will be waiting during the tile downloading, and in the meanwhile we display nothing. If we had no connectivity at all, then we would immediately skip the download provider and display the tile provided by the approximation provider.

The solution would be something like:

• putting the embedded and approximation providers first in MapTileProviderBasic (as “offline”)
• when we find the tile in those providers, if the tile is up to date, this is it
• if we found a downgraded version of the tile (expired or approximated), we display it but we keep looking for it in the next providers of the list (presumably “online”)
• doing so we display immediately an approximation of the tile, while searching for a better version online

Environment

Version of osmdroid the issue relates to:

6.0.2

[monsieurtanuki]

@InI4 I’ll work on that after the next release (6.1.0). Btw:

• it’s already potentially a patchwork if you have several providers
• it’s already async
• I think the tile downloads are already on FIFO or LIFO
• I note your idea of download cancel - I’m not sure it’s implemented now

What I had in mind is:

• either we respect the user’s provider order
• or the user asks us to reorder relevancy-wise
• or the user asks us to reorder speed-wise

And regardless of that initial order or reorder aspect, the process will be:

• to loop on the provider list
• to display the first available tile, regardless of its status
• to keep looking until we find an up-to-date version of the tile

[monsieurtanuki]

@InI4 More or less, but it don’t know if we can be specific about “in database” or not: we’ll let the user set its own tile providers list order. The order is currently “best providers” first; the goal is “fastest providers first”, while still looking for an up-to-date tile. Can we enforce the provider list order? To do that we’ll need to know for each provider its tile relevancy (expired / scaled / up-to-date) and its access speed (local database / local file / network / notoriously slow server). Then we can add a method in the provider array, something like “reorder according to relevancy or speed”.