Solved

Adding to a component's canvas

Posted on 1998-04-27
5
209 Views
Last Modified: 2010-04-06
I need a good abstarct way, hopefully off of TControl or TWinControl methods or props, that will allow me to make additions to the canvas of many components of unknown type at run time.  EX: Adding a small red circle to the upper left corner of a textbox that will remain when the form is minimized and restored.  If at all possible, I'd like to do this without redirecting WndProcs...   I have, in fact, gotten to draw on the canvas of a textbox using a Canvas i have created and set the Handle of to a GetDC call on the control, but i can't get the image i draw on the control to persist....  all ideas are worth a shot on this one!
0
Comment
Question by:SQLwill
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
5 Comments
 
LVL 1

Expert Comment

by:pjdb
ID: 1337849
Use the onpaint event of the components the circle will be redrawed each time (event after a minimize). To avoid to redraw every thing, you can use the form on paint event and use the form.canvas.cliprect (return the area that need to be redrawed) to test if the circle need to redraw or not.

JDB
0
 

Author Comment

by:SQLwill
ID: 1337850
This won't work in the manner in which I intend to use it.  I need a *abstract* method with which to do my assigned task whereupon i do not choose or mess with any components at design time other than to add them to a "WatchedComponents" StringList.
0
 
LVL 2

Expert Comment

by:mvz121697
ID: 1337851
I think you need something like :

function NewWindowProc (wnd:Hwnd; Msg:UINT; Wparam:wparam ;
lparam:lparam): longint; stdcall;
begin
  if Msg=WM_PAINT then begin
     // Do your painting
  end;
  Result := CallWindowProc (OldProc,Wnd,Msg,wParam,lParam);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  OldProc :=Pointer(SetWindowLong(      Edit1.Handle,gwl_wndproc,longint(@NewWindowProc)));
end;

Greetings,
MvZ
0
 

Author Comment

by:SQLwill
ID: 1337852
This looks like what i might have to do.  I'm trying, however, to do my best to avoid moving around WndProcs, since doing so gets extremely ugly when more than one WndProc stealing component is used.  =]
0
 
LVL 1

Accepted Solution

by:
dvaline earned 100 total points
ID: 1337853
Declare a TCanvas:
NewCanvas:= TCanvas;
Initialize the canvas:
NewCanvas := TCanvas.Create;

Next, each time an object is painted to the canvas you want to "keep" save the image to the TCanvas.  This way you have full control.

VisibleCanvas.Assign(NewCanvas) {this might be backwards, I didn't have a chance to check it out, but I know it works}

You can put into an event OnMinimize, OnMaximize, OnPaint, whatever to get the canvas back to the control by just reversing the operation.

NewCanvas.Assign(VisibleCanvas)

Make sure that you put some kind of switch if you use an onpaint event, assigning will cause a paint event, if you don't put a switch it will keep repainting, not a pretty picture, crashola if you know what I mean.




0

Featured Post

Announcing the Most Valuable Experts of 2016

MVEs are more concerned with the satisfaction of those they help than with the considerable points they can earn. They are the types of people you feel privileged to call colleagues. Join us in honoring this amazing group of Experts.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Introduction Raise your hands if you were as upset with FireMonkey as I was when I discovered that there was no TListview.  I use TListView in almost all of my applications I've written, and I was not going to compromise by resorting to TStringGrid…
In my programming career I have only very rarely run into situations where operator overloading would be of any use in my work.  Normally those situations involved math with either overly large numbers (hundreds of thousands of digits or accuracy re…
Nobody understands Phishing better than an anti-spam company. That’s why we are providing Phishing Awareness Training to our customers. According to a report by Verizon, only 3% of targeted users report malicious emails to management. With compan…
Finding and deleting duplicate (picture) files can be a time consuming task. My wife and I, our three kids and their families all share one dilemma: Managing our pictures. Between desktops, laptops, phones, tablets, and cameras; over the last decade…

737 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question