Mega Code Archive
Adding new methods and properties without registering new components
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Title: Adding new methods and properties without registering new components
Question: Is there a way to add new methods and properties to a component without having its soruce code and having to install a descendant component?
Answer:
Adding new methods and properties
---------------------------------
Sometimes we need to add new methods and properties to an existing
component (or change the visibility of existing properties). One way
of doing this is modifiying the component, but this implies having to
recompile its package and we would have to redistribute our changes
if we wanted our application to be compiled by others, and that would
be a bother for the recipients. Sometimes we may not even have that
choice because we may not have the source code. In these situations,
better would be to subclass (derive) the component and add new
properties and methods. For example:
type
TEditX = class(TEdit)
public
function GetForeColor: TColor;
procedure SetForeColor(color: TColor);
property ForeColor: TColor read GetForeColor write SetForeColor;
end;
These methods could for example be implemented this way:
function TEditX.GetForeColor: TColor;
begin
Result := Font.Color;
end;
procedure TEditX.SetForeColor(color: TColor);
begin
Font.Color := Color;
end;
It's a silly example, of course, but it serves the purpose.
Casting to the new class
------------------------
We don't need to intall this new component and register it in the
components palette or replace existing controls in our applications
(which would be an unpayable penalty for such small changes and/or
additions). Instead, any time we want to access the new properties
and methods, we can just cast the object (for example Edit1) to our
new class. For example:
TEditX(Edit1).ForeColor := clRed;
or
TEditX(Edit1).SetForeColor(clRed);
Warning: This casting to a descendant class can only be done if the
new class adds new properties and static methods, but without adding
new fields and new virtual or dynamic methods, although in theory you
can override existing virtual methods. Also, the visibility of
existing properties can be changed, as in the InplaceEditor example
explained in the article "Accessing hidden properties".
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