A more convenient approach to manual KVO notifications

Nov 25, 2010

Key-Value Observing (KVO) in Cocoa programming is for the most part a dance on roses – less code, less confusion, higher portability and ultimately converts your code into othogonal “modules” of awesomeness.

However, as soon as you use manual observer notification things get a little messy – you need to bracket every change in a pair of willChangeValueForKey: and didChangeValueForKey:.

KVOChangeScope provides a convenient way to mark manually managed properties as edited and consequently notify observers:

- (void)doingSomething {
   kvo_scoped_change(foo);
   // code which manipulate foo, which might return at any moment
}

This works by placing a C++ KVOChangeScope object on the stack, which will take care of sending |willChangeValueForKey| when created and automatically send |didChangeValueForKey| as soon as the method return. Note that you need to compile your source as Objective-C++ (file suffix “.mm” instead of “.m”).

There are also some convenience macros available, like |kvo_scoped_change| used in the example above. The same example, without using any macros, is equivalent to the following code:

- (void)doingSomething {
   KVOChangeScope change_scope(self, @"foo");
   // code which manipulate foo, which might return at any moment
}

Another useful macro is the limited scope |kvo_change| used for a more fine- grained control of the “will”-to-“did” scope. Here’s an illustrating example:

- (void)doSomethingComplex {
   // modify value of foo
   kvo_change(foo) {
     foo_ = @"Foo value 1";
     if (bar)
       foo_ = @"Foo value 2";
   } // <-- didChangeValueForKey:@"foo" called here
   // maybe perform slow, blocking I/O here
   // modify value of interwebs
   kvo_change(interwebs) interwebs_ = @"awesome";
   // didChangeValueForKey:@"interwebs" called here
   // maybe do some more slow stuff, not holding "edit locks" thus
   // avoiding other threads to wait on edit completion.
}

The whole thing is contained within a single small header which can be grabbed here:

https://gist.github.com/714763#file_kvo_change_scope.hh

You’re free to use this code for fun or profit with the only restriction of retaining the embedded copyright notice.