Apple WWDC 2012 Session Videos Live

This is a heads up for anyone who does iPad or iPhone development; Apple have made available their awesome WWDC 2012 sessions videos to developers! iOS 6 features such as Passkit, maps and Facebook looks interesting as does the new Game Center functionality such as “Challenges” along with a whole host of new APIs and technologies. More information can be found on the Apple iOS 6 site and the developers’ iOS 6 overview site.

You should also look at the new OS X technologies in the soon-to-be-launched Mountain Lion release. Notification Center seems like such a natural addition that I’m surprised Apple didn’t add it in back in the early days of Mac OS X and Game Center will be exciting to play with on the Mac. I just hope it doesn’t distract me too much from work!

Of course most of it is still under a non-disclosire agreement so only blog and comment about the publicly announced WWDC stuff but this will good to play with over the summer when it’s too hot to be outside.

Core Animation stops animation on app relaunch

On one of my projects I discovered a bug in a never-ending animation I had set up. Whenever the app was suspended (such as when you multitask and open another app), on relaunching the app the animation was frozen. After some investigating, I discovered that with Core Animation you need to set a flag on the CABasicAnimation class called removedOnCompletion to NO otherwise the animation will get cleaned up when it gets suspended.

Does this seem like a bug or a feature?

WWDC Update: 5 key points for iPhone apps

It’s now been just over 3 weeks since the torrent of information unleashed at Apple’s World Wide Developer Conference refreshingly drenched the brains of designers, developers and engineers. I’ve resisted blogging about the public announcements to fully let the impact soak in and gage everyone’s reactions but now feels like a good time to talk about where the future of computing is heading.
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Core Data Objects in Wrong Sections

NSFetchedResultsController is a really handy class. Use one of the default Core Data templates in Xcode and you’ll very quickly have a nice list of managed objects in a table view. With a few more lines of code you can get the NSFetchedResultsController to group your objects by sections. You do this by specifying a key-path in the class’s constructor method but there is another step that if overlooked will cause some confusion.

In a sample app I’ve created a food table that lists food in categories.

FetchedResultsController method grouping sections using a key-path:

Screenshot of Food sample app in wrong order.Save and quit the app a few times and you’ll see the objects seem to be in the wrong sections. If you look closer you’ll see that the objects are actually sorted in ascending name order. On looking at the code, it seems this is exactly what we asked the program to do! After some testing it also seems to show up more often if the table is a grouped one.

As per the docs, after you specify a key-path to group each section with you also need to make sure the first sort descriptor is sorting this key-path. Add a sort descriptor and everything will work as expected.

Revised fetchedResultsController method with missing sort descriptor:

Re-ordering Core Data Objects on iOS4

The Core Data framework on the iPhone is incredibly powerful. It’s also incredibly efficient and part of that is because a collection of objects only have the order that you implicitly give them. For example you typically might give an Event object a timestamp and when you pull out all the events you might sort on that timestamp.

The NSFetchedResultsController is the main power house when dealing with such a scenario and is great for the master part of a master/detail data relationship. It’s main purpose is to manage the results returned from a fetch request similar to the above and provide data for a UITableView via delegate methods. It reacts on the model level so if you delete an Event object, the NSFetchedResultsController informs it’s delegate and so updates the UITableView automatically. It’s very clever indeed!

As I found out earlier yesterday, the problem comes when you want to re-order the objects in a user-defined way. Instead of sorting on a timestamp, I wanted the user to be able to specify that one object should occur before the other… I’m working on an app that lets you place waypoints down on a map. Timestamps in waypoints aren’t much use. It’s much more critical that they have a specific order.

After some Googling I came across a useful article on CocoaIsMyGirlfriend. This helped me 90% of the way but I had problems when re-ordering. When you re-order the objects, the UITableView would move cells about seemingly at random. This is because NSFetchedResultsController is model-driven. When you re-order something using the tableview methods the view is already correct (because you’ve dragged and dropped the cell there – it’s a user-driven change) and so when the delegate detects your index changes, it walks all over your view believing the cells to be in their original position. The trick is to ‘disable’ the UI updates with a boolean in your delegate methods. Set the boolean just before your re-indexing and unset it afterwards.

For an example, see this stack overflow link on re-ordering.

Also have a look at what the Apple docs say on user-driven updates on the NSFetchedResultsController.