Object-oriented Software in Ada 95

242 Polymorphism
The Ada specification of this responsibility is:
type Observable is tagged private;
type P_Observer is access all Observer'Class;
procedure Add_Observer ( The:in out Observable;
O:in P_Observer );
procedure Delete_Observer( The:in out Observable;
O:in P_Observer );
procedure Notify_Observers( The:in out Observable'Class );
procedure Set_Changed( The:in out Observable );
Note:
Observers are manipulated by using their access values, rather than the instance of the observer object
directly.
16.9.3 Putting it all together
The complete class specifications for the observer and observable classes are combined into a single package as
follows:
package Class_Observe_Observer is
type Observable is tagged private;
type Observer is tagged private;
type P_Observer is access all Observer'Class;
procedure Add_Observer ( The:in out Observable;
O:in P_Observer );
procedure Delete_Observer( The:in out Observable;
O:in P_Observer );
procedure Notify_Observers( The:in out Observable'Class );
procedure Set_Changed( The:in out Observable );
procedure Update( The:in Observer; What:in Observable'Class );
private
Max_Observers : constant := 10;
subtype Viewers_Range is Integer range 0 .. Max_Observers;
subtype Viewers_Index is Viewers_Range range 1 .. Max_Observers;
type Viewers_Array is array( Viewers_Index ) of P_Observer;
type Observable is tagged record
Viewers : Viewers_Array := ( Others => null );
Last : Viewers_Range := 0;
State_Changed : Boolean := True;
end record;
type Observer is tagged null record;
end Class_Observe_Observer;
Note:
Unfortunately as the methods of the classes Observer and Observable are mutually
interdependent it is not possible to easily separate this package into two distinct packages.
The implementation of the package Class_Observe_Observer is as follows:
package body Class_Observe_Observer is
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