Map Tile Caching for Offline Viewing

Question

I\'m trying to build an application that will use open source maps from Open Street Maps (though the concept should be applicable to any map provider). The application will

Answer 1

If your app is running on WP8 then use the built in maps control in the Windows Phone 8 SDK as this already supports offline maps out of the box. If targeting WP7 it is possible to get offline maps to work but takes a lot of work. I created this for a customer a few years ago and I believe that it took me a little over 3000 lines of code to do. Mind you they wanted to also have a framework for adding tiles from various sources such as downloading over and area and downloading zipped files. They way I managed to get the rendering to work was to a canvas to the map without setting it's position. This will be default make it a child of the map but it will not move. I then made the canvas the same size as the map and used the resize event to resize the canvas should the map be resized. I then used the view change event to trigger a method to render the tiles. When this event fired I first calculated all the tiles in view using the code found here: http://msdn.microsoft.com/en-us/library/bb259689.aspx

I then would pull the tiles from isolated storage and draw them on the canvas. For performance I keep track of which tiles I added to the canvas so that if the tile was still in view I simply changed it's position rather than reloading it from isolated storage. I also removed any images that were no longer in view. Overall this works fine but there were some minor issues such as not having the smooth transition between zoom levels. If you really wanted that it is possible to get that to work but requires a lot more math. Also, if you zoom into an area where there is no tiles you end up with an empty map. You can create a custom map mode to prevent the user from going into areas where you don't have tiles.