Implementing Your Own Cocoa Bindings

on July 31st, 2009 by Tom 19 Comments Cocoa

This post is the result of investigation into a stackoverflow.com question of mine.

So, you’ve created a spiffy NSView of your own, and have decided to make it compatible with bindings. Great! So you go and read the documentation, and you look at mmalc’s GraphicsBindings example. You override bind:toObject:withKeyPath:options: and everything works. But wait! Why isn’t the NSWindowController ever being deallocated anymore?

Now you’ve got a nasty retain cycle on your hands. You do a little research and discover that not only do other people have the same problem, but even Apple’s bindings used to have it a few years ago. How did Apple fix the problem? With the magic, undocumented class NSAutounbinder, which nobody seems to know much about.

Other people will tell you that you don’t need to override bind:toObject:withKeyPath:options: and that bindings work automatically. This is only a half truth. NSObject does provide an implementation of bind:toObject:withKeyPath:options:, but it only half works. Using the default NSObject implementation, changes in the model will update the view, but the reverse is not true. When the bound property of the view changes, nothing happens to the model.

So, what is a Cocoa developer to do? I’ll explain how to implement your own bindings that work exactly like Apple’s, with no retain cycles. I haven’t found this solution anywhere else, so as far as I know, I’m the discoverer. I feel so special. It has been mentioned before at least once. The solution is hard to find, though.

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Coding Tip: Don’t Mix Abstraction Levels

on July 5th, 2009 by Tom Coding Tips

Can you tell what is wrong with the abstraction in this class interface?

@interface EmployeeList
-(void) addObject:(Employee*)anEmployee;
-(void) removeObject:(Employee*)anEmployee;
-(NSUInteger) numberOfEmployees;
@end

Here is the answer:
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