What is the difference between an Interposer Class and a Helper Class in Delphi?

• Mediator Class

A Mediator Class ( Interposer Class ) is a class that, hierarchically speaking, positions itself between a classe ancestral ( from which it derives ) and an object of the type of this class . It has the main characteristic of having the same name of its ancestor class , which makes its application simplified in systems that already have declared objects of this class type.

In order for a CM ( Mediator Class ) to work, it is imperative that the CM declaration is between the declaration of the original class class > and the object declaration of the CM type. The interposer Class can even be in a unit , as long as this unit is positioned AFTER the unit that contains the ancestral class in the uses and provided that the uses clause in question is BEFORE the declaration of CM-type objects.

Implementing the Mediator Class

Now let's increase the mediator class so that it does what we want. The implementation does not differ from what would be done if we had made a component correctly, for example. The cool thing about CM is that it has nothing special about it, other than its specific position within the hierarchy. Let's first get our TEdit to accept numbers only. This is well manjado. Here is the code:

01  unit UFormPrincipal;
02
03  interface
04
05  uses
06    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, 
07    System.Classes, Vcl.Graphics, Vcl.Controls, Vcl.Forms, Vcl.Dialogs,
08    Vcl.StdCtrls;
09   
10  type
11     TEdit = class(Vcl.StdCtrls.TEdit)
12     protected
13       procedure KeyPress(var PKey: Char); override;
14       procedure Loaded; override;
15     public
16     end;
17
18  TFormPrincipal = class(TForm)
19    EDIT: TEdit;
20    private
21    { Private declarations }
22    public
23    { Public declarations }
24    end;
25
26  var
27  FormPrincipal: TFormPrincipal;
28
29  implementation
30 
31  {$R *.dfm}
32
33  { TEdit }
34
35  procedure TEdit.KeyPress(var PKey: Char);
36  begin
37  inherited;
38    if not (PKey in ['0'..'9',#8,'-']) then
39      PKey := #0;
40  end;
41
42  procedure TEdit.Loaded;
43  begin
44  inherited;
45    Text := '';
46  end;
47  
48  end.

Acehi in this Site :

• Class Helper

In Delphi, Class Helper is a feature that allows add methods and modify component behavior at runtime, without the need to inherit the class or use compositing features >, making it appear that the class contains the new behavior since its inception.

Another difference is that Class Helpers does not only apply to components. They also allow you to modify domain classes created by the developer, add methods to record structures, and primitive types (introduced from the XE3 version). In the latter case, however, you can not add properties.

In the code below is a unit that implements a Class Helper to add to the TEdit component the ability to validate your text, defining whether it represents a CPF or CNPJ.

01  unit UnitEditDocumento;
02     
03  interface
04  
05  uses
06     vcl.stdCtrls, SysUtils;
07  
08  type
09     TEditDocumento = class helper for TEdit
10     private
11        function TextoSemSinais: string;
12     public
13        function isPessoaFisica: boolean;
14        function isPessoaJuridica: boolean;
15  end;
16  
17  implementation
18  
19  uses
20     UnitUtils;
21  
22  { TEditDocumento }
23  
24  function TEditDocumento.TextoSemSinais: string;
25  var
26    //Retorna o texto sem caracteres especiais
27  end;
28  
29  function TEditDocumento.isPessoaFisica: boolean;
30  begin
31     result := length(TextoSemSinais) = 11;
32  end;
33  
34  function TEditDocumento.isPessoaJuridica: boolean;
35  begin
36     result := length(TextoSemSinais) = 14;
37  end;
38  end.

Notice that the entire structure of Class Helper is the same as a conventional class, changing only the form of declaration.

How to use Whenever it is necessary to use a component that has the new behavior, simply add the name of the unit where the Class Helper is declared in the uses section. From there, you can access the new methods added, as shown in Listing 2.

01  procedure TForm5.Edit1Exit(Sender: TObject);
02  begin
03     if Edit1.isPessoaFisica then
04        Label1.Caption:='Documento Pessoa Física'
05     else
06     begin
07        if Edit1.isPessoaJuridica then
08           Label1.Caption:='Documento Pessoa Jurídica'
09        else
10        begin
11           Label2.Caption:='Documento Inválido';
12           exit;
13        end;
14     end;
15  end;

In the code above using the methods added by Class Helper In this code, we are working with the OnExit event of the TEdit component, and on lines 3 and 7 we use the new methods added by Class Helper .

Modifying existing methods In addition to adding new methods, Class Helper allows us to modify existing methods in a component. In the code below we present an example of modifying the Clear method to a special situation in a form where the TEdit components that accept text receive the string "xxxx" when they are cleaned, and those that only accept numbers receive the text "0000 ". This behavior may be necessary on some system screens, from which we have, for example, to print the content on dot-matrix printers. In these cases, in continuous form sheets, fields can not be blank to avoid mischaracterising the print.

01  Unit UnitEditMatricial;
02  ...
03  type
04     TEditDocumento = class helper for TEdit
05     public
06        procedure Clear;
07     end;
08  
09  ...
10  
11  procedure TEditDocumento.Clear;
12  begin
13     if NumbersOnly then
14        Text:= StringOfChar('0',MaxLength)
15     else
16        Text:=StringOfChar('X',MaxLength);
17  end;

** Class Helper * to modify an existing method in TEdit *

Now, when we add the reference to this unit in the uses section and call the Clear method of a TEdit , this new method will be executed, instead of the default procedure that deletes all field text .

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