Ghoscher's Blog

Hello, welcome to the 6th part of my tutorial on how to create an offline? map app without using Google™ API, okay this time it’s no longer offline :D! now the app will be able to download tiles from the internet :D!.

Series outline:

• Series aim.
• Theory you need to know.
• App design.
• Writing a TilesManager.
• Writing a TilesProvider.
• Seeing results with MapView.
• Adding web support
• Creating MapView in XML.
• Extra: Fuchs Maps.

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At first I planned on writing some basic tips about web tiles server support but then I realized that implementing web support needs some work and code organizing since multi threading becomes a must!. You cannot wait for the tiles to be downloaded in the UI thread!.

We will assume that you have a running tiles server that provides you with tiles you specify using a custom url like this:

http://myserver.com/?x=1&y=3&z=0
or

When we want a specific tile we first search if we have it in our memory cache, if we don’t then we search in the database, if we fail to find it then we download it from the internet.
It’s kind of similar to multi cache levels in a cpu, if one level fails it gets the requested data from the next level, when data found in a certain level all lower levels save a copy of the data since it’s highly likely to be asked for the same data in the near future.
In our case when you don’t have a tile in memory or in database you get it from the web server and then save it in the database and keep it in memory for later use.

We’ve already implemented the first two levels (Memory & database) in the tutorial part 4, the TilesProvider class covered them both. but we didn’t have any threading manipulation, this time we have to be very careful about threading stuff. We will implement new classes and modify some of the old ones, the changed lines in existing classes will be highlighted in grey. Also note that if you’re using Eclipse as your IDE you can always count on the keyboard shortcut Ctrl+Shift+O to automatically import the right packages.


We will add three source files to our project in a new package called com.mapapp.tileManagement.web

Let’s first implement the class TileDownloadTask, this class handles Read more…