Delphi how to implement baseclass event? - delphi

Question

I have an abstract Delphi XE form that acts as base class for a family of forms used in my application. I am trying to figure out the best way to make a help function (on F1 keypress) that opens a wiki-page for the active form.
I'd very much like this function to be implemented at base-class level and to call when the user presses F1 but I need some advice on how to do this in a smart way. Currently I just put a KeyDown-event on the base form but this gets overwritten if the subform receives its own KeyDown, at which point I have to manually call the baseKeyDown. Obviously, this is not optimal.. Is there a way to ensure I catch the F1 keypress at baseclass level, random overloads nonwithstanding?
I am running Delphi XE on Windows 7

Answer 1

The straight answer to your question is that a library writer should not assign to event handlers. That's because doing so makes it hard for library consumers to also consume those events. And when you are writing a base class, you are taking the role of the library author.
So, instead of implementing the handler in the OnKeyDown event, override the KeyDown method.
type
TBaseForm = class(TForm)
protected
procedure KeyDown(var Key: Word; Shift: TShiftState); override;
end;
....
procedure TBaseForm.KeyDown(var Key: Word; Shift: TShiftState);
begin
inherited; // this will fire the OnKeyDown event
// your processing for F1 goes here
end;
However, I wonder if you would not be better off using the built in help system. Personally I'd add an OnHelp event for the Application object and place the centralised logic there.

Answer 2

You should override KeyDown method in your base form class.
type
TBaseForm = class(TForm)
protected
procedure KeyDown(var Key: Word; Shift: TShiftState); override;
end;
procedure TBaseForm.KeyDown(var Key: Word; Shift: TShiftState);
begin
// do your processing here
// ...
inherited; // call inherited method that will call OnKeyDown if assigned
end;
This is default implementation of KewDown method that calls OnKeyDown events
procedure TWinControl.KeyDown(var Key: Word; Shift: TShiftState);
begin
if Assigned(FOnKeyDown) then FOnKeyDown(Self, Key, Shift);
end;