How to reduce the number of Set methods in a component #108

Question

How to reduce the number of methods in a component I have the following class:

type TMyClass = class
  protected
    FProp1: string;
    bIsChanged: Boolean;
  public
    property Prop1: string read FProp1 write SetProp1;
end;

The Prop1 property is simply for storing a value, it does no work on the class itself. The reason I have SetProp1 is purely to update the bIsChanged variable. When the class is instantiated bIsChanged is defaulted to False. When Prop1 is set bIsChanged is updated to true. This works fine as it is but sometimes I have like 20 properties in the class and for each one I have to create a Set{Propery Name} method so that bIsChanged can be set to true. What I would like to know is is there a way to have the class "notified" when any of it's properties are modified. This way I can declare my properties as:

property Prop1: string read FProp1 write FProp1;

and eliminate all the Set{Property Name} methods. When the class is "notified" bIsChanged is set to true and viola - I am set. I know I can create a IsChanged property and expose it to the user of the class and then it is up to them to set it but I would like to not do this.

There is no way to avoid having a SetProp method, however the number of methods can be reduced by sharing a single SetProp method for all properties of a single type. The secret is to use the Indexed Property feature. The following is extracted from my TVersionInfo component:

{ ... }
  private
    procedure SetVerProp(index: integer; value: TControl);
    function GetVerProp(index: integer): TControl;
  published
    property CtlCompanyName: TControl index 1
      read GetVerProp write SetVerProp;
    property CtlFileDescription: TControl index 2
      read GetVerProp write SetVerProp;
    property CtlFileVersion: TControl index 3
      read GetVerProp write SetVerProp;
    property CtlInternalName: TControl index 4
      read GetVerProp write SetVerProp;
    property CtlLegalCopyRight: TControl index 5
      read GetVerProp write SetVerProp;
    property CtlOriginalFileName: TControl index 6
      read GetVerProp write SetVerProp;
    property CtlProductName: TControl index 7
      read GetVerProp write SetVerProp;
    property CtlProductVersion: TControl index 8
      read GetVerProp write SetVerProp;
  end;

In your case you do not need the GetXXX method though it could also be used. The SetXXX method uses a simple case statement to set the right member:

procedure TVersionInfo.SetVerProp(index: integer; value: TControl);
begin
  case index of
    1: FCtlCompanyName := Value;
    2: FCtlFileDescription := Value;
    3: FCtlFileVersion := Value;
    4: FCtlInternalName := Value;
    5: FCtlLegalCopyRight := Value;
    6: FCtlOriginalFileName := Value;
    7: FCtlProductName := Value;
    8: FCtlProductVersion := Value;
  end;
  Refresh;
end;

In your case, you can replace Refresh with setting the bIsChanged value.

Original resource:	The Delphi Pool
Author:	Wayne Niddery
Added:	2009/10/26
Last updated:	2009/10/26