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objective-ckey-value-observingkvc

KVO notification not sent when using the super class' setter

I have a ClassA that defines a property:

@interface ClassA : NSObject
@property (nonatomic) CGPoint property;
@end

The implementation doesn't override the accessors.

ClassB overrides the setter to do some additional work:

- (void)setProperty:(CGPoint)property {
    [super setProperty:property];
    [self someAdditionalWork];
}

In an other method of ClassB I try to set this property via the super setter, to skip the additional work:

- (void)otherMethodInClassB {
    // ...
    super.property = newValue;
    // ...
}

When I do this, the KVO notifications for the property are not sent. If I do the same thing, but use self, the KVO notifications work as expected:

- (void)otherMethodInClassB {
    // ...
    self.property = newValue;
    // ...
}

What's going on here? Is this the expected behavior? I couldn't find anything that would say so.

Solution

  • I'm not sure if this is documented, but this is the expected behavior.

    Automatic KVO notifications work by silently changing your instance's class in runtime from original ClassB to an auto-generated subclass NSKVONotifying_ClassB which overrides all the required setter methods to do those willChange.../didChange... calls for you. By calling super, you effectively skip all that magic and invoke the original setter, which only does bare value assignment.

    P.S. This blog post digs into deeper details of this: https://www.mikeash.com/pyblog/friday-qa-2009-01-23.html