Intercepting maximization in a form

Posted on 1998-12-19
Medium Priority
Last Modified: 2008-02-20
I need to perform a procedure when a form is maximized or restored.
I used to write in OnResize, but it only works when you resize the window by its borders.
Any idea?

Question by:forns
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LVL 12

Accepted Solution

rwilson032697 earned 60 total points
ID: 1351965
Override the WM_MAXIMIZE method like this:

procedure WMMaximize(var Message : TMessage); message WM_MAXIMIZE;



Expert Comment

ID: 1351966
On Resize also captures the resizing of the Form by the border icons !!
If that doesn't work in your app, then there might be an error somewhere else.

Try yourself:

Make a new app, with one TForm only.

then write to the OnResize event:

procedure Form1.OnResize ( Sender: TObject );
    ShowMessage ( 'Resized!' );

It doesn' work, if the Form was minimized into the Windows task bar and re-opened. Is that, what you mean? In that case, use OnPaint. ( Because the Form will get repainted ).

Greetings, Oli

Expert Comment

ID: 1351967
The message to intercept is WM_SIZE. The wparam part of the message tells whether the window (or form, in this case) is maximized or restored (but not minimized - this is a different story).

Remember to call inherited in your message handler though.

To handle the restore situation completely, you need a little extra work. Due to the TApplication architecture in Delphi, you need to assign an event handler for Application.OnRestore. This event is fired when the main form is minimized then restored (as opposed to maximized then restored).


TForm1 = class(TForm)
procedure WMSize (var msg : TWMSize); message WM_SIZE;
procedure AppRestore ( Sender : TObject );


procedure TForm1.FormCreate ( Sender : TObject );
   Application.OnRestore := AppRestore;

procedure TForm1.AppRestore ( Sender : TObject );
   ShowMessage ('appRestore');

procedure TForm1.WMSize ( var msg: TWMSize );
   case msg.SizeType of
      SIZE_MAXIMIZED: ShowMessage ('maximized');
      SIZE_RESTORED: ShowMessage ('restored');


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