How to implement identical methods with 2 and more Classes? - delphi

Question

I want to write a TCheckBox and TRadioButton descendants having 3 identical methods.
TMyCheckBox = class(TCheckBox)
procedure DoSomething1;
procedure DoSomething2;
procedure WMSize(var Message: TWMSize); message WM_SIZE;
end;
TMyRadioButton = class(TRadioButton)
procedure DoSomething1;
procedure DoSomething2;
procedure WMSize(var Message: TWMSize); message WM_SIZE;
end;
// the following procedures are common for both classes, so in fact
// TMyCheckBox.DoSomething1 do the same as TMyRadioButton.DoSomething1
procedure DoSomething1;
begin
// here is the same code for TMyCheckBox as well as for TMyRadioButton
// but I don't want to write the same code many times but implement it
// for both classes at once in some common way
end;
procedure DoSomething2;
begin
// here is the same code for TMyCheckBox as well as for TMyRadioButton
// but I don't want to write the same code many times but implement it
// for both classes at once in some common way
end;
procedure WMSize(var Message: TWMSize); message WM_SIZE;
begin
// here is the same code for TMyCheckBox as well as for TMyRadioButton
// but I don't want to write the same code many times but implement it
// for both classes at once in some common way
end;
How can I do this?

Answer 1

Define an interface say IDoSomething with the the three method signatures.
Then change your class declaration to
TMyCheckBox = class(TCheckBox, IDoSomething)
and then implement.
If the implementations are common or very close.
Then define a helper class TDoSomething and then delegate the work.
e.g.
Procedure TMyCheckBox.DoSomething1; // implements IDoSomething1
Begin
TDoSomething.DoSomething1(Self); // given class method will suffice.
End;
Class Methods in delphi, equivalent to static methods in other languages.
Type
TDoSomethingHelper = Class(TObject)
Public
Class Procedure DoSomething1(aComponent : TComponent);
End;
...
implementation
Class Procedure TDoSomethingHelper.DoSomething1(aComponent : TComponent);
Begin
aComponent.Tag = 27;
End;

Answer 2

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.