How to implement identical methods with 2 and more Classes?

Question

I want to write a TCheckBox and TRadioButton descendants having 3 identical methods.

TMyCheckBox = class(TCheckBox)
  procedure DoS         


        

Answer 1

You are looking for implementation inheritance rather than interface inheritance. This is only achievable in Delphi if you can derive classes from a single common ancestor. This limitation is inherent because the language only supports single-inheritance.

The best you can do is something like this:

type
  TMyWinControlExtender = class
  private
    FTarget: TWinControl;
  public
    constructor Create(Target: TWinControl);
    procedure WMSize(var Message: TWMSize; out CallInherited: Boolean);
    procedure DoSomething;
  end;

  TMyCheckBox = class(TCheckBox)
  private
    FExtender: TMyWinControlExtender;
  protected
    procedure WMSize(var Message: TWMSize); message WM_SIZE;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
    procedure DoSomething;
  end;

  TMyRadioButton = class(TRadioButton)
  private
    FExtender: TMyWinControlExtender;
  protected
    procedure WMSize(var Message: TWMSize); message WM_SIZE;
  public
    constructor Create(AOwner: TComponent); override;
    destructor Destroy; override;
    procedure DoSomething;
  end;

{ TMyWinControlExtender }

constructor TMyWinControlExtender.Create(Target: TWinControl);
begin
  inherited Create;
  FTarget := Target;
end;

procedure TMyWinControlExtender.WMSize(var Message: TWMSize; out CallInherited: Boolean);
begin
  if FTarget.... then
    ....
  CallInherited := ...;
  //etc.
end;

procedure TMyWinControlExtender.DoSomething;
begin
  if FTarget.... then
    ....
  //etc.
end;

{ TMyCheckBox }

constructor TMyCheckBox.Create(AOwner: TComponent);
begin
  inherited;
  FExtender := TMyWinControlExtender.Create(Self);
end;

destructor TMyCheckBox.Destroy;
begin
  FExtender.Free;
  inherited;
end;

procedure TMyCheckBox.DoSomething;
begin
  FExtender.DoSomething;
end;

procedure TMyCheckBox.WMSize(var Message: TWMSize);
var
  CallInherited: Boolean;
begin
  FExtender.WMSize(Message, CallInherited);
  if CallInherited then
    inherited;
end;

And likewise for TMyRadioButton etc.

Now, you could use interfaces and delegation to reduce some of the boilerplate, but there's no way for that to help with a message handler like WMSize.