Event Handling - Pro Unity Game Development with C# - page 95

The Dictionary has two “dimensions,” based on key-value pairs: Dictionary<TKey, TValue> . For
the NotificationsManager , I've set the first dimension to be a list of strings (for Event Type names),
and the second as List< Component > (for Listener objects). MonoBehaviour derived components
ultimately descend from type Component . I could, of course, have chosen different data types if
I'd wanted, since the Dictionary class uses Generics. However, these two types, strings and
List<Component> , will suit our needs, as later sections will show. By using these two types, we can
access all listeners as a list for any event of name N , with the following code:
//N is an event name: such as "OnHealthChange" or "OnLevelComplete"
List MyListenersList = Listeners[ N ];
Registering As a Listener
So the NotificationsManager now has a private Dictionary member, which makes use of Generic
types , and this allows us to maintain a searchable list of potential listeners for events, and the
size of the list can grow and shrink over time. Each listener in the list is specified as being of type
Component , but thanks to class inheritance and polymorphism, it can really be of any type descended
from Component, including MonoBehaviour. For more information on polymorphism, see Chapter 10.
In essence, this means our list of listeners can be a wide mix of different types. In this section, we'll
add functionality to the NotificationsManager that lets us add a new Listener to the list. If an object
expects to be notified about any event, then it must previously have registered itself as a Listener
with the NotificationsManager. When an object registers itself as a Listener, it's effectively saying,
“Hey, NotificationsManager. I want you to tell me about every occurrence of event X, as and when
it happens!” To achieve this functionality, a new public method AddListener can be added to the
NotificationsManager (see Listing 3-8).
Listing 3-8. Adding Listeners to the NotificationsManager
01 //Function to add a listener for an notification to the listeners list
02 public void AddListener (Component Listener string NotificationName)
03 {
04 //Add listener to dictionary
05 if(!Listeners. ContainsKey (NotificationName))
06 Listeners.Add (NotificationName, new List<Component>());
07
08 //Add object to listener list for this notification
09 Listeners[NotificationName]. Add (Listener);
10 }
The AddListener method accepts two arguments: namely, Sender and NotificationName . The Sender
is a Component reference to the object that should become the registered listener. This is the object
that must be notified by the NotificationsManager, if and when the event occurs. NotificationName
is a string indicating the custom event type for which the Sender is listening. This is a user-defined
string naming the events for our game.