offline ios maps application

See the RouteMe library, which is the go-to open source library for raster OSM mapping on iOS. This includes offline capabilities. Documentation is fairly sketchy but you'll find plenty of discussion of the offline capabilities in theIr Google Group.

There are many applications like that already, sadly the idea of Open Source seems to be virtually unheard of in the IOS world so you'll probably have to re-invent the wheel and code what lots of others have coded before you.

Read this article for a basic analysis of the options available to you. On our Wiki there is a list of IOS applications that use OSM, and the list also tells you whether these opted for tile or vector storage. A few of the programs mentioned make their source code available so you can look there for inspiration.

If your needs are relatively simple then you might get away with using the JavaScript library OpenLayers that we use on web pages and run that locally on the browser. Note however that any solution based on raster tiles, whether OpenLayers or other, requires that the tiles for the area of interest have to be created and downloaded onto the device first, and that you cannot download them from OpenStreetMap's tile server due to its Tile Usage Policy.

MapBox ios sdk (which is also based on Route-me, more specifically, the Alpstein fork) has a fairly good implementation of offline cache. You can modify it to make it store permanently. However, the license prohibit you from re-distribute the tiles you cache from MapBox for yourself and transfer them to users. So you would have to change the tile source if you wish to offer offline maps directly to users.