This category helps you add KVO using blocks to make it easier to use and make your code more readable.

Associate a Objective-C block to a keyPath, and your block will be executed each time the value of the KeyPath is changed to let you do whatever you want. No need to centralize everything in -(void)observeValueForKeyPath:ofObject:change:context:, and no need for removeObserver:forKeyPath: anymore either as the observer blocks are automatically removed when the object is deallocated!

Example 1: observing a CGRect property

[self onKeyPathValueChange:@"frame"
 execute:^(id obj, NSString* kp, id old, id new)
 {
    if (![old isEqual:new])
    {
      NSLog(@"The frame changed from %@ to %@", old, new);
      CGRect newFrame = [new CGRectValue];
      ...
    } else {
      NSLog(@"The frame property was reaffected to the same CGRect value %@", new);
    }
 }];

Example 2: Observing an UIColor object and using a composite keyPath

[self onKeyPathValueChange:@"view.backgroundColor"
 execute:^(id obj, NSString* kp, id old, id new)
 {
    if (old != new)
    {
      NSLog(@"The view background color has been changed");
    }
 }];

Be careful to avoid retain cycles when using this helper method (as with any other usage of blocks retained by self).

Especially, the blocks you provide to onKeyPathValueChange:execute: are retained by self, so you should not use self directly in the block to avoid self to be retained by the block and creating a retain cycle.

Instead, you should declare a weak non-retaining reference to self like this before your block:

__block __weak typeof(self) weakSelf = self;

And use weakSelf instead of self in the block body, so that self won't be retained by the block.