That there might just be wisdom in the crowd when they are using a location vector tag based system to rank documents.
Who knew?
Category Archives: Dissertation
SQLite query queue
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!
As I mention on the about page I’m currently developing things…
As I mention on the about page I’m currently developing things for the iPhone and at the moment I’ve got 2 projects on the go that make use of the iPhone SDK. One is my dissertation which is making use of the iPhone platform to demonstrate my research (location based information using tag vector location techniques) and a more personal application for keeping archery scores during both practice and competitions, the point of the scoring app is to make it easier for people to record their scores because I see a lot of archers mess up the scoring of arrows, the totals for the ends and the running totals, especially beginners who lack experience of scoring.
My current workflow and the tools that I use in this (Xcode, AnalysisTool and Versions) make developing things for the platform a little bit easier and dare I say a little fun. The only stumbling block I occasionally run into when I’m sleepy and lacking caffeine is the lack of managed memory on the iPhone platform. Personally I find the lack of managed memory as a bit of a mixed blessing, a blessing because I’ve found it makes me think a lot more about how I’m actually using variables in my code, what needs doing with them and when I can get rid of them and a curse because when I’m sleepy (and lacking a cup of tea) this is because I revert to my Java ways and start failing to use all the memory management rules and just let the variables sit in memory never to be used again. This of course isn’t helped by the fact I’m still using Java on pretty much a day to day basis to complete uni assignments and so I’m switching constantly between having the memory managed for me and having to manage the memory myself.
This switching between memory-managed and non memory-managed environments is why I use AnalysisTool in my workflow, it would be nice to be able to use built-in version of Clang static analyser but for some reason I can’t fathom it doesn’t seem to work on iPhone projects. The use of AnalysisTool also helps save time when I’m looking for leaks, instead of going though my code with a toothpick and finding all these leaks myself I can run the tool and it’ll find the ones I’ve missed. I can then use the information it gives back to me to reinforce my knowledge and fix that leak.
There are many little lovely things that I’m starting to love about objective-c, properties, message passing, every classing needing an interface (although it does get in the way when prototyping) and named parameters to name but a few. The other great thing is all the “free stuff” that you get when developing for the iPhone. The best example of this is having a tableview. This is pre-made just extend the UITableViewController class and override the methods you want to use. If you want to make a custom cell for that table again, extend the “free” one and change what you want.