Search
Support DFF
If you shop at Amazon anyway,
consider using this link. We receive a few cents from each purchase.
Thanks.
Support DFF
If you benefit from the website, in terms of
knowledge, entertainment value, or something otherwise useful,
consider making a donation via PayPal to help defray the
costs. (No PayPal account necessary to donate via credit
card.) Transaction is secure.
Contact
Feedback:
Send an e-mail with your
comments about this program (or anything else).
|
| |
Problem Description
Here is a program that demonstrates how to
add an "OnPaint" event to a TGraphic control which does not have
that event defined. Specifically the problem is to to add
an identifying name or number to TShape controls.
Background & Techniques
I recently became involved in an interesting project to design software
to drive nozzles for a children's water fountain. We do not have the
Digital IO card or the solenoids yet, but I decided to write a simulator to
help my buddy define locations and trial "scripts" to drive the fountain.
I am representing the nozzles with TShape controls but needed to identify
each with a number. An OnPaint event exit was the logical way to do
this, but TShape does not support that event.
Here is a summary of the steps necessary to add the feature:
 | Define a new class derived from the control. in this case:
 | TNozzle = class(TShape) |
|
| Expose FOnPaint and Paint methods for the new class
| private
FOnpaint:TNotifyEvent;
protected
procedure Paint; override; |
|
| Publish the OnPaint event property as a new event to make it
accessible to the using program:
| published
property OnPaint: TNotifyEvent read FOnPaint write FOnPaint; |
|
| Add a line to the Paint method to check for assignment and call
FOnPaint inherited paint;
| procedure TNozzle.Paint;
begin
inherited paint; {Let TShape paint do its thing}
if assigned(FOnPaint) then FonPaint(self); {call our paint
event}
end; |
|
| The new class could be installed as a new component based on the
above four steps. I prefer to reintroduce the Create constructor to pass
additional initial property values and to use the existing graphic
control as a "prototype" to source the new pertinent properties such as
owner, parent, left, top, height, width, shape, color, etc. for the new
control. This allows us to use Delphi's IDE to visually define the the
characteristics we want. The prototype control may be freed
after transferring its properties to the new control;
| constructor create(NewNozzleId:integer;
s:TShape); reintroduce; |
|
| 6. Define your own OnPaint TNotifyEvent and assign it to
OnPaint as new control is initialized.
| procedure TForm1.NozzlePaint(Sender:
TObject);
begin
With TNozzle(sender), canvas do
begin
{Draw NozzleId in center of the nozzle}
textout(width div 2 -4, height div 2 -4,nozzleId);
end
end;
|
|
A similar technique could be used to customize the appearance of
TLabel or TBevel controls, both TGraphic descendents without an OnPaint
event..
Running/Exploring the Program
|
Original Date: August 21,2006 |
Modified:
May 18, 2009 |
|
|
|
|
