Perform selector when object is not nil ios

Question

I habe an object ImageURLObject, this object have property objectUrlImage, when I create this object property objectUrlImage is nil, becourse image url has not setted yet, how can I perform selector when property has actual value? It's not a method [self performSelector: withObject: afterDelay:]because I don't know after what delay property become not nil..

Answer 1

You can use Key-Value Observing (KVO) to detect a change to a property of an object.

In the setter of the ImageURLObject you need to perform the notification using willChangeValueForKey and didChangeValueForKey:

- (void)setObjectUrlImage:(NSURL *)url
{
    [self willChangeValueForKey:@"objectUrlImage"];
    _objectUrlImage = url;
    [self didChangeValueForKey:@"objectUrlImage"];
}

and establish an observation of that property from where ever it is you want:

[imageUrlObject addObserver:self
                 forKeyPath:@"objectUrlImage"
                    options:NSKeyValueObservingOptionNew
                    context:NULL];

and observations will be notified in the following method:

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if (object == imageUrlObject && [keyPath isEqualToString:@"objectUrlImage"])
    {
        // URL changed
    }
}

Don't forget to remove the observation of the property in the dealloc method of the observer:

- (void)dealloc
{
    [imageUrlObject removeObserver:self
                        forKeyPath:@"ObjectUrlImage"];
}

(note that this is one of the few reasons to subclass dealloc in an ARC environment).