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Indirection - Delphi OOP Part 8 - Chapter 17

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Example 8.2 Chaining through an intermediate object

To use chaining for this application, we create an intermediate class, TMuseumCount, that has the three TCounter objects as public data items. The client class (the user interface) then updates the counts through the TMuseumCount object.

Ex 8.2 step 1 Defining the intermediate object

The TCounter class remains as before. We’ll create the TMuseumCount class in a separate unit. Add another unit to the project from example 8.1, save it as MuseumCountU.pas, and edit it as follows:

unit MuseumCountU;

// using chaining

interface

uses CounterU;

type
   TMuseumCount = class(TObject)
   public
     VEntered, VInside, VLeft: TCounter;
     constructor Create;
   end;

implementation

{ TMuseumCount }

constructor TMuseumCount.Create;
begin
   // propagate construction
   inherited Create;
   VEntered := TCounter.Create;
   VInside := TCounter.Create;
   VLeft := TCounter.Create;
end;

end.

TMuseumCount is a very simple class. It is derived from TObject and contains three public data items of type TCounter. (That’s why we need the uses CounterU clause). TMuseumCount contains only one method, a constructor extending the inherited constructor to create the associated TCounter objects (lines 17–19). So whenever an object of type TMuseumCount is created it automatically creates the necessary TCounter objects.

Ex 8.2 step 2 Using chaining

We now modify the user interface class, TfrmAttendance, to use the intermediate class TMuseumCount instead of the individual TCounter objects:

unit UpAndDownU;

interface

uses
   Windows, Messages, SysUtils, Variants, Classes, Graphics,
   Controls, Forms, Dialogs, StdCtrls, Spin, MuseumCountU;

type
   TfrmAttendance = class(TForm)
...
   private
     MuseumCount: TMuseumCount;
   end;

var
   frmAttendance: TfrmAttendance;

implementation

{$R *.dfm}

procedure TfrmAttendance.btnEnterClick(Sender: TObject) ;
begin
   MuseumCount.VEntered.Add (sedEnter.Value) ;
   lblIn.Caption := IntToStr(MuseumCount.VEntered.Total) ;
   MuseumCount.VInside.Add (sedEnter.Value) ;
   lblVisitors.Caption := IntToStr(MuseumCount.VInside.Total) ;
   sedEnter.Value := 0;
end;

procedure TfrmAttendance.btnLeaveClick(Sender: TObject) ;
begin
   with MuseumCount do
   begin
     VLeft.Add (sedLeave.Value) ;
     lblOut.Caption := IntToStr(VLeft.Total) ;
     VInside.Subtract (sedLeave.Value) ;
     lblVisitors.Caption := IntToStr(VInside.Total) ;
     sedLeave.Value := 0;
   end;
end;

procedure TfrmAttendance.FormCreate(Sender: TObject) ;
begin
   MuseumCount := TMuseumCount.Create;
end;

We replace TfrmAttendance’s three private TCounter objects by a single TMuseumCount object and so we substitute MuseumCountU for CounterU in the uses clause. We also change the form’s OnCreate event handler to create a single TMuseumCount object in the place of the three TCounter objects.

TMuseumCount does not have its own data fields or methods, so how do we use it to calculate and store the necessary values? We use the dot operator to give us access to the TCounter objects through chaining or explicit delegation. (This is familiar from the RAD objects, and we look at them later in this chapter.) We see this chaining clearly in btnEnter’s OnClick event handler. For example, we have the statement:

MuseumCount.VEntered.Add (sedEnter.Value) ;

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