ListBox: Part IX

Whenever an event occurs, listbox can notify another object about it. That other object is called event handler or listbox observer, following GoF Observer pattern. ListBox observer can be any object that implements void HandleListBoxEventL(CEikListBox* listBox, TListBoxEvent eventType) method, so that listbox can invoke that method passing a pointer to itself and the information about the type of the event that happened.

HandleListBoxEventL method didn't have to be the only required method; listbox might have required from a listbox observer to implement other methods as well, even though it's not the case.

The way to specify what are the required observer methods is to create a class that lists them as pure virtual methods. Such class is called the observer interface, or the 'Mixin' in Symbian terminology.

The observer interface serves as a common point for objects that generate events and objects that receive events. Objects that generate events only know about the interface; they call virtual methods on it, without knowing or caring what is the exact object that implemented them. On the other side, objects that receive events inherit from the interface and implement methods specified in it in order to qualify as observers.

The observer interface is the essence of communication between the objects that otherwise might have nothing in common; any object can act as an observer of any other object, as long as it implements the observer interface defined by that other object.

Getting back to listbox -- MEikListBoxObserver is the observer interface that every object must implement in order to qualify as a listbox observer. Implementing MEikListBoxObserver interface means inheriting from MEikListBoxObserver class and implementing HandleListBoxEventL() method.

In our sample, ListBoxContainer (itself CCoeControl) implements MEikListBoxObserver interface in order to receive notifications from a listbox which happens to be its only child component.

ListBoxContainer registers itself as a listbox observer for a concrete listbox instance by calling listBox->SetListBoxObserver(this); Through observer's HandleListBoxEventL() method listbox sends the information about several different types of events, all defined in MEikListBoxObserver::TListBoxEvent enum: