How do I override a getter for a KVO-compliant property?

Question

I want to create a subclass of NSOperation that needs to customize -isReady, the getter for a KVO-compliant property. My override would do a Boolean-AND of my custom test and super's version of the method. But the override still has to keep KVO-compliance. So, how?

Answer 1

Nothing too special, as synthesized properties are KVC and KVO compliant:

The property:

@property (nonatomic, readwrite, getter=isReady) BOOL ready;`

The implementation:

@synthesize ready;

- (BOOL) isReady {
   // your custom logic here.
}

For an NSOperation subclass, KVO notifications for those properties will be automatic. You do not need to do anything more (you do not need to call will/didSetValueForKey). If the behavior of the NSOperation set accessors such as isReady or isFinished is dependent on other properties or key paths, be sure to register them with KVO:

+ (NSSet *) keyPathsForValuesAffectingIsFinished {
    NSSet   *result = [NSSet setWithObject:@"finished"];
    return result;
}