Object-oriented Software in Ada 95

244 Polymorphism
The method Update is overridden by an observed with appropriate code to display the state of the observable
object. This object is passed as the second parameter to the method.
procedure Update( The:in Observer; What:in Observable'Class ) is
begin
null;
--Should be overridden
end Update;
end Class_Observe_Observer;
Note:
The parameter What is of type Observable'Class so that re-dispatching can take place.
16.10 Using the observe-observer pattern
The following is an implementation of the game of noughts and crosses using the observe-observer pattern.
16.10.1 The observed board object
The class Board that implements the board for the game of noughts and crosses is now defined as:
with Class_Observe_Observer;
use Class_Observe_Observer;
package Class_Board is
type Board is new Observable with private;
type Game_State is ( Win, Playable, Draw );
procedure Add( The:in out Board; Pos:in Integer;
Piece:in Character );
function Valid( The:in Board; Pos:in Integer ) return Boolean;
function State( The:in Board ) return Game_State;
function Cell( The:in Board; Pos:in Integer ) return Character;
procedure Reset( The:in out Board );
private
subtype Board_Index is Integer range 1 .. 9;
type Board_Array is array( Board_Index ) of Character;
type Board is new Observable with record
Sqrs : Board_Array := ( others => ' '); --Initialize
Moves : Natural := 0;
end record;
end Class_Board;
Note: Apart from inheriting from the class Observable, this code is identical to that seen in Section 8.4.1.
The implementation of the class Board is defined in the body of the package Class_Board as follows:
package body Class_Board is
The procedure add adds a counter either the character 'X'or 'O' to the board.
procedure Add( The:in out Board; Pos:in Integer;
Piece:in Character ) is
begin
The.Sqrs( Pos ) := Piece;
end Add;
© M A Smith - May not be reproduced without permission