Using NSNotificationCenter in Swift

NSNotificationCenter is a convenient method to implement Observer pattern in iOS, something similar to what we say Signal/Slot in Qt framework.

To register one self as observer to some event, you can use addObserver method form NSNotificationCenter. In below code you are adding self as observer, selector is method which will be called when event is triggered, name is name of event or notification in which we are interested in, and object is object from which we want to receive event from, nil means we are ready to receive event from any object.

NSNotificationCenter.defaultCenter().addObserver(
    self, 
    selector: "gameViewClosed", 
    name: "gameViewClosed", 
    object: nil);

Now to trigger the notification, you can use postNotificationName method from NSNotificationCenter. Following code shows the same. Here we are sending notification for "gameViewClosed" event, object nil mean deliver event to all interested objects.

NSNotificationCenter.defaultCenter().postNotificationName(
    "gameViewClosed", 
    object: nil);

That's it, thanks for reading.

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
    forKeyPath:@"isPlaying"
       options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
       context:NULL];

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.

    @try{
        [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.