Author: Jonas Bilinkevicius
In my application, I want to be able to iconize the main form and still leave the
secondary forms displaying on the desktop. Likewise, I want to be able to select
secondary forms without the main form popping up. How can I do this?
Answer:
Recently a user asked me about this, and I had to do a bit of experimentation
before I finally figured it out. And the solution to this problem is actually so
simple, you'll scream (actually, I did all the screaming myself). But it's not
something that's necessarily easy to find out nor intuitive (maybe it is for some,
but it wasn't for me). But before I give you the solution, let's discuss the
concept that's behind it.
All windowed controls have a parent of some sort; that is, some control that
maintains visual control (ie. display) over it. Main forms of an application all
point to the Application as their parent. Likewise, by default, secondary forms
point to the main form of the application for parentage. But the neat thing about
creating windowed objects in Delphi (though you need to be careful with some
controls) is that you can change the parentage of a control to isolate its visual
control, essentially giving it independence from its default parent. Okay, so how
do you do it? You might think that you can reset parentage at FormCreate, but
that's not the right place to do it. The only way to do this is before the window
gets created in the first place, and that place is in the CreateParams procedure.
I've discussed CreateParams in previous articles, so I won't go into details about
it, though I will brush over what it does. CreateParams is an inherited procedure
that wraps the WinAPI functions CreateWindow and CreateWindowEx that are
responsible for a window's initial appearance. It's a convenient way to set display
parameters. With it, we can change the a variable parameter called Params that is a
TCreateParams structure (you should look this structure up in the online help) to
affect a number of different things about a form. One of the fields in the
TCreateParams structure is WndParent. This parameter specifies the handle of the
window that controls the display of the window being created. By changing this
parameter to point another window handle (hint, hint), we can change the default
parentage.
So now it's a matter of deciding what window is going to be the secondary form's
new parent. In this case, whenever we want to make a secondary form independent of
the main form, we're essentially turning it into its own mini-application without
creating a new EXE. So it's best to choose a parent that's at the highest order in
the system. That window is Windows' Desktop Window. Fortunately we have a way of
getting its handle by using the WinAPI call GetDesktopWindow, which returns the
handle of the Desktop.
Okay, we've covered all the bases. Now you're going to kill me for belaboring the
point. Here's the code:
1 unit Unit2;
2
3 interface
4
5 uses
6 Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs;
7
8 type
9 TForm2 = class(TForm)
10 private
11 { Private declarations }
12 //override the CreateParams procedure for any child forms you want to
13 //make independent of the main form
14 procedure CreateParams(var Params: TCreateParams); override;
15 public
16 { Public declarations }
17 end;
18
19 var
20 Form2: TForm2;
21
22 implementation
23
24 {$R *.DFM}
25
26 //Here's the implementation of CreateParams
27
28 procedure TForm2.CreateParams(var Params: TCreateParams);
29 begin
30 inherited CreateParams(Params); //Don't ever forget to do this!!!
31 Params.WndParent := GetDesktopWindow;
32 end;
33
34 end.
Insanely simple, huh? Sorry I took so long to lead up to it, but while the solution was simple, I just couldn't get away from explaining at least a bit of background information to help those who aren't familiar with the internal workings of the WinAPI. In any case, HAVE AT IT!!!
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