Question

What is the best way to respond to data changes when property setters are called. For example, if I have a property called data, how can I react when [object setData:newData] is called and still use the synthesised setter. Instinctively, I would override the synthesised setter like so:

- (void)setData:(DataObject *)newData {
    // defer to synthesised setter
    [super setData:newData];

    // react to new data
    ...
}

…but of course this doesn’t make sense – I can’t use super like this. So what is the best way to handle this situation? Should I be using KVO? Or something else?

Answer 1

There are a few different ways to do this, depending on how much control you want. One way to do it is to observe your own property:

[self addObserver:self forKeyPath:@"data" options:0 context:nil];

- (void)observeValueForKeyPath:(NSString *)path ofObject:(id)object change:(NSDictionary *)change context:(void *)context {
    if(object == self && [path isEqualToString:@"data"]) {
        //handle change here
    } else [super observeValueForKeyPath:path ofObject:object change:change context:context];
}

Make sure you remove yourself as an observer in your dealloc or finalize method, if not before.

Another way would be to override -didChangeValueForKey:. However, this method may not be called if there are no observers on the object.

- (void)didChangeValueForKey:(NSString *)key {
    [super didChangeValueForKey:key];
    if([key isEqualToString:@"data"]) {
        //handle change here
    }
}