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Thomas Fuchs
Guest
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 Posted: Wed Oct 22, 2003 6:52 pm Post subject: Cannot find the mistake in this code |
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Hi,
I'm desperate! I'm new to writing components and just wanted to start very
simple with a panel which becomes red if the mouse moves over it (I know I'd
better catch the WM_MOUSEENTER message for this but this doesn't matter
here) AND if the panel's "Bla"-property = True.
But nothing happens. If I set Bla to True during design time, it's false
during runtime.
What's wrong here? Perhaps it's just a small mistake but I can't find it and
I searched for hours!
==================
unit Test;
interface
uses
SysUtils, Classes, Controls, ExtCtrls, Forms, Graphics, Windows, Messages;
type
TTEST = class(TPanel)
private
Fbla: Boolean;
public
procedure MouseMove(Shift: TShiftState; X, Y: Integer); override;
published
property Bla: Boolean read Fbla write Fbla default True;
end;
procedure Register;
implementation
procedure Register;
begin
RegisterComponents('Desperate research', [TTEST]);
end;
procedure TTEST.MouseMove(Shift: TShiftState; X, Y: Integer);
begin
inherited MouseMove(Shift, X, Y);
if Fbla then
color := clred;
end;
end.
==========
Thank you!
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Paul van Kan
Guest
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| Posted: Wed Oct 22, 2003 7:50 pm Post subject: Re: Cannot find the mistake in this code |
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From the D7 Helpfile (topic Default property values):
<quote>
Note
Declaring a default value does not set the property to that value. The
component's constructor method should initialize property values when
appropriate. However, since objects always initialize their fields to 0, it
is not strictly necessary for the constructor to set integer properties to
0, string properties to null, or Boolean properties to False.
</quote>
constructor TTEST.Create(AOwner: TComponent)
begin
inherited Create(AOwner);
FBla := True;
end;
Good luck,
Paul
"Thomas Fuchs" <tommifuchs (AT) gmx (DOT) de> schreef in bericht
news:bn6jjb$l8j$01$1 (AT) news (DOT) t-online.com...
| Quote: | Hi,
I'm desperate! I'm new to writing components and just wanted to start very
simple with a panel which becomes red if the mouse moves over it (I know
I'd
better catch the WM_MOUSEENTER message for this but this doesn't matter
here) AND if the panel's "Bla"-property = True.
But nothing happens. If I set Bla to True during design time, it's false
during runtime.
What's wrong here? Perhaps it's just a small mistake but I can't find it
and
I searched for hours!
==================
unit Test;
interface
uses
SysUtils, Classes, Controls, ExtCtrls, Forms, Graphics, Windows,
Messages;
type
TTEST = class(TPanel)
private
Fbla: Boolean;
public
procedure MouseMove(Shift: TShiftState; X, Y: Integer); override;
published
property Bla: Boolean read Fbla write Fbla default True;
end;
procedure Register;
implementation
procedure Register;
begin
RegisterComponents('Desperate research', [TTEST]);
end;
procedure TTEST.MouseMove(Shift: TShiftState; X, Y: Integer);
begin
inherited MouseMove(Shift, X, Y);
if Fbla then
color := clred;
end;
end.
==========
Thank you!
|
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Rob Kennedy
Guest
|
| Posted: Wed Oct 22, 2003 7:53 pm Post subject: Re: Cannot find the mistake in this code |
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|
Thomas Fuchs wrote:
| Quote: | If I set Bla to True during design time, it's false
during runtime.
[...]
property Bla: Boolean read Fbla write Fbla default True;
|
The "default" directive tells the streaming system that if the property
is set to True as design time, that property's value should not be
stored in the DFM file.
When a new instance of TTest is created, all fields are set to 0, which
means that the initial value of the FBla field will be False. Since
there is no entry in the DFM file for the Bla property, that property
won't be changed from its initial value. That's why the Bla property is
False at run time.
If you include a "default" value for a property, you need to set that
value in the control's constructor:
constructor TTest.Create(AOwner: TComponent);
begin
inherited;
FBla := True;
end;
--
Rob
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Brian Cook
Guest
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| Posted: Wed Oct 22, 2003 8:42 pm Post subject: Re: Cannot find the mistake in this code |
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Greetings.
Aren't we all.
| Quote: | What's wrong here? Perhaps it's just a small mistake but I can't find it and
I searched for hours!
|
Fbla needs to be initialized...
| Quote: | type
TTEST = class(TPanel)
private
Fbla: Boolean;
public
|
constructor Create( AOwner: TComponent ); override;
| Quote: | procedure MouseMove(Shift: TShiftState; X, Y: Integer); override;
published
property Bla: Boolean read Fbla write Fbla default True;
end;
implementation
|
constructor TTEST.Create( AOwner: TComponent );
begin
inherited Create( AOwner );
Fbla := True;
end;
Good luck, Brian
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