Inheriting forms and overriding properties

[djjd47130]

I have come across an awkward issue and resolved it, and wanted to document the trouble. What I have is 4 different forms which I embed into different tabs inside the main form. I intend to add more than just 4 forms/tabs, and was seeking ways of adding/removing pages. After the 4 separate forms were already designed, I created another new form "TBaseForm" which I in turn converted the 4 forms to inherit from "TBaseForm" instead of the standard "TForm". Here's the base form:

unit uBaseForm;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs;

type
  TFormBusyEvent = procedure(Sender: TObject; const Busy: Bool) of object;
  TFormProgress = procedure(Sender: TObject; const Position, Max: Integer) of object;

  TBaseForm = class(TForm)
  private
    FBusy: Bool;
    FOnBusy: TFormBusyEvent;
    FMax: Integer;
    FPosition: Integer;
    FOnProgress: TFormProgress;
    procedure SetBusy(const Value: Bool);
    procedure DoProgress(const Position, Max: Integer);
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
  published
    property Busy: Bool read FBusy write SetBusy;
    property Position: Integer read FPosition;
    property Max: Integer read FMax;
    property OnBusy: TFormBusyEvent read FOnBusy write FOnBusy;
    property OnProgress: TFormProgress read FOnProgress write FOnProgress;
  end;

var
  BaseForm: TBaseForm;

implementation

{$R *.dfm}

{ TBaseForm }

constructor TBaseForm.Create(AOwner: TComponent);
begin
  inherited;
  Busy:= True;
  try
    FPosition:= 0;
    FMax:= 0;
  finally
    Busy:= False;
  end;
end;

destructor TBaseForm.Destroy;
begin
  inherited;
end;

procedure TBaseForm.SetBusy(const Value: Bool);
begin
  FBusy := Value;
  if Value then begin
    Screen.Cursor:= crHourglass;
  end else begin
    Screen.Cursor:= crDefault;
  end;
end;

procedure DoProgress(const Position, Max: Integer);
begin
  FPosition:= Position;
  FMax:= Max;
  if assigned(FOnProgress) then
    FOnProgress(Self, FPosition, FMax);
end;

end.

The issue I was facing was when one of these inherited forms was created in runtime, I was getting "Property not found" errors.

The solution was that I was using the property name "Position", which is already used by the original "TForm". So I changed references from "Position" and "FPosition" to "Progress" and "FProgress". Everything works fine now.

So in the end, the problem was just using a property which was already reserved by its ancestor, and not properly overriding it.



JD Solutions