Although thats an awfully complex title to say fast 10 times its also my solution to a problem that I’ve been having with SQLite databases on the iPhone.

While developing both TEC and Tagger (yes thats my highly unimaginative name for my dissertation app) I keep running into threading issues with the SQLite database I use to store all the data. This is partly because of my use of many threads to keep the UI fast and useable. Because of these many threads there is occasionally a collision between the normal getting the details of the local area from a MAC address and the adding of a new location or a background database update. This is a problem that could be solved in a few ways – first checking that we aren’t running a database update when we request location details or, my preferred option, a queue.

The good thing is in Objective-C there is a built in queue class that can either process things sequentially or in parallel based on the free resources of the computer. In this sense it’s actually possible to think of it as a higher level abstraction of the Grand Central Dispatch found in Snow Leopard but the real beauty of this class is that its available on the iPhone. This means that I can now add all the queries I want to perform to a queue and then when the previous one has finished it’ll automatically start on the next one. Bliss.

Well this is all well and good but to do this you need to make the SQL query into a task that is completed and this is the bit that isn’t so easy. With an insert method that requires no feedback it really is as simple as creating a NSInvocationOperation object with the insert query and data then adding that to the queue but what if you actually want to get data back out? Turns out that you should be able to grab the result of an NSInvocationOperation when it has finished processing but if I’ve called this from another thread it quickly becomes problematic. So to solve this new problem I turned to properties and good old NSNotifications.

Simply put I place the data pulled from the database in a property in the DatabaseManager instance (its a singleton so its always the same…) then send out an NSNotification which is picked up by the class that is expecting it, in this case the Tags class and then it copies from the DatabaseManager property to a local variable. Its not the most elegant way of solving this but it works and after all that’s all that ultimately matters!