How to apply DRY to interface implementations involving Form components? - delphi

Question

I have an interface IComm declaring a routine SetMonitorLogLevel():
unit IFaceComm;
interface
type
TMonitorLogLevel = (mllOnlyImportant, mllAll);
IComm = Interface(IInterface)
procedure SetMonitorLogLevel(LogLevel: TMonitorLogLevel);
end;
end.
The interface is implemented by 2 Forms, which are similar to each other, frmBarComm and frmFooComm, that look like this:
TfrmBarComm = class(TForm, IFaceComm.IComm)
cboDebugLevel: TComboBox;
private
procedure SetMonitorLogLevel(LogLevel: IFaceComm.TMonitorLogLevel);
end;
Note that the 2 Forms have a lot of components in common, such as cboDebugLevel, but also can have components the other doesn't.
Both Forms implement IComm.SetMonitorLogLevel() in exactly the same way:
procedure TfrmBarComm.SetMonitorLogLevel(LogLevel: IFaceComm.TMonitorLogLevel);
begin
case LogLevel of
IFaceComm.TMonitorLogLevel.mllOnlyImportant:
Self.cboDebugLevel.ItemIndex := 0;
IFaceComm.TMonitorLogLevel.mllAll:
Self.cboDebugLevel.ItemIndex := 1;
end;
end;
How do I avoid violating the Don't Repeat Yourself (DRY) principle? I'm faced with this issue quite often, and it's particularly ugly when the copy-pasted routines are much bigger than the simple example I shown above.

Answer 1

The usual way to deal with this is to create another class which implements the interface. It might look like this:
type
TComboBoxCommImplementor = class(TInterfacedObject, IFaceComm.IComm)
private
FDebugLevel: TComboBox;
public
constructor Create(DebugLevel: TComboBox);
procedure SetMonitorLogLevel(LogLevel: TMonitorLogLevel);
end;
constructor TComboBoxCommImplementor.Create(DebugLevel: TComboBox);
begin
inherited Create;
FDebugLevel := DebugLevel;
end;
procedure TComboBoxCommImplementor.SetMonitorLogLevel(
LogLevel: IFaceComm.TMonitorLogLevel);
begin
case LogLevel of
IFaceComm.TMonitorLogLevel.mllOnlyImportant:
FDebugLevel.ItemIndex := 0;
IFaceComm.TMonitorLogLevel.mllAll:
FDebugLevel.ItemIndex := 1;
end;
end;
Then in your form implement the interface using delegation:
type
TfrmBarComm = class(TForm, IFaceComm.IComm)
cboDebugLevel: TComboBox;
private
FComm: IFaceComm.IComm;
property Comm: IFaceComm.IComm read FComm implements IFaceComm.IComm
public
constructor Create(AOwner: TComponent); override;
end;
constructor TfrmBarComm.Create(AOwner: TComponent);
begin
inherited;
FComm := TComboBoxCommImplementor.Create(cboDebugLevel);
end;

Answer 2

Create a frame that owns the components which are used on both forms.
The frame implements IComm. Both forms using the frame and both forms implement IComm.
The frame is accessible as property that delegates the interface implementation.
It looks like this:
type TfrmBarComm = class(TForm)
FFrameComm: TFrameComm;
public
property FrameComm: TFrameComm read FFrameComm implements IComm;
end;