Saturday, January 24, 2015

Notification on property change in iOS SDK

I have used QML quite a lot in my projects and there is one quite nice feature in QML that makes implementation quite easy. The feature is Property change notification.

Apple iOS SDK also similar mechanism named Key-Value Observing mechanism. Here is Apple's documentation for the same.

Following is simple code which demonstrate how we can use Key-Value Observer API to get notification on property changes. First we need to register observer and in which property observer is interested in. Code shows, class "self" is interested in property change event from propertyOwner for "isPlaying" property.
[propertyOwner addObserver:self
       options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
Now, we have registered observer for property change, we need to implement method which will be called in case of property is changed. Following code shows the same. We need to check which object sent event and for which property, then we can take appropriate action.
- (void) observeValueForKeyPath:(NSString *)path ofObject:(id) object change:(NSDictionary *) change context:(void *)context
    // this method is used for all observations, so you need to make sure
    // you are responding to the right one.
    if (object == propertyOwner && [path isEqualToString:@"isPlaying"]) {
        // now we know which property is modified
When we are done and don't need notification any more we can remove observer. Below is code for the same. It's safe to use try and catch as we might get exception if we try to remove observer when none is registered.
        [propertyOwner removeObserver:self forKeyPath:@"isPlaying"];
    }@catch(id anException){
        //do nothing, obviously it wasn't attached because an exception was thrown
Now whenever "propertyOwner" changes's "isPlaying" property then "self" will get notification and "observeValueForKeyPath" method will get called.