Util_Listeners

Listeners

Introduction

The Listeners package implements a simple observer/listener design pattern. A subscriber registers to a list. When a change is made on an object, the application can notify the subscribers which are then called with the object.

Creating the listener list

The listeners list contains a list of listener interfaces.

L : Util.Listeners.List;

The list is heterogeneous meaning that several kinds of listeners could be registered.

Creating the observers

First the Observers package must be instantiated with the type being observed. In the example below, we will observe a string:

package String_Observers is new Util.Listeners.Observers (String);

Implementing the observer

Now we must implement the string observer:

type String_Observer is new String_Observer.Observer with null record;
procedure Update (List : in String_Observer; Item : in String);

Registering the observer

An instance of the string observer must now be registered in the list.

O : aliased String_Observer;
L.Append (O'Access);

Publishing

Notifying the listeners is done by invoking the Notify operation provided by the String_Observers package:

String_Observer.Notify (L, "Hello");

---

Generated by Dynamo from util-listeners.ads