Object-oriented Software in Ada 95

Polymorphism 241
Screen
Observed
Observer
oxo
oxo_observer
Sqrs : Board_Array
Figure 16.4 Oxo observer and observed oxo board.
The observed object oxo is unaware of how its representation will be displayed, whilst the observer object
oxo_observer is unaware of how the observed object oxo represents and manipulates the board.
16.9.1 The Observer’s responsibilities
An observer class for an object is required to inherit from the type Observer and override the method Update
with code that will display the state of the observed object.
Method
Update
Responsibility
Display the state of the observed object.
An implementor of an Observer class overrides the method Update with a method that displays an
appropriate representation of the observed object passed as a formal parameter to the method. The method
Update is called when the state of the object being observed changes and an update of the representation of the
objects state is required.
The Ada specification of this responsibility is:
type Observer
is tagged private;
procedure Update( The:in Observer; What:in Observable'Class );
Note:
The object being observed (What) is passed as it base representation. This object will need to be
converted back into its true type before it can be interrogated.
16.9.2 The responsibilities of the observable object
An observable class for an object is required to inherit from the type Observable so that the following methods
may be made available to it.
Method
Add_Observer
Delete_Observer
Notify_Observers
Set_Changed
Responsibility
Add an observer to the observable object.
Removes an observer.
If the object has changed, tells all observers to update
their view of the object.
Sets a flag to indicate that the object has changed.
© M A Smith - May not be reproduced without permission