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Respond to arrow keys in form

Posted on 1998-05-23
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Last Modified: 2010-04-06
Hello,

How do I make my form respond to the arrow keys (assigning my own procedures behind them) and ignore the default behavior (movement between components on the form) when pressing the arrow keys?

Regards,
Frank.
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Question by:franksymons
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12 Comments
 
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by:ssite
Comment Utility
Apparently, the code that does this is deep down in controls.pas and you can't effectively override it just in one place.

If you really want to do this, you need to inherit a new component from each one where you want the behaviour to change and then you need to watch for the CN_KeyDown message and then check if message.charcode is one of the VK_DOWN VK_LEFT VK_RIGHT or VK_UP. If so, dont call inherited and do your thing. otherwise, call inherited.

I'm pretty sure this would work.

Another solution for the more professional would be to hook your own process and then look out for WM_Keydown messages.

If you want the second one, write your e-mail and I'll tell you more.
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by:franksymons
Comment Utility
Could you be more specific?
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by:mvz121697
Comment Utility
Set the form's KeyPreview property to True (in object inspector).

Then add code to the 'OnKeyDown' event.

Greetings,
MvZ
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by:mvz121697
Comment Utility
In the OnkeyDown event code something like:

 if Key=VK_DOWN then begin
   MessageBeep(0);
   Key:=0;    
 end;

Set Key to 0 to prevent the default behaviour.
other key-values are VK_LEFT, VK_RIGHT.

Greetings,
MvZ
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Author Comment

by:franksymons
Comment Utility
Thanks for your answer, providing there is nothing on the form this works, but try adding a button and an edit box on the form. Use tab to make sure the button has the focus, now when pushing the down arrow key focus whill shift from the button to the edit box without executing the procedure. Pushing the down key with the editbox in focus will perform the procedure.
That's not what I'm looking for, I want the procedure to run every time, regardless of what may be on the form.
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by:rickpet
Comment Utility
Okay...the fix all for capturing keystrokes before they go to components...

use TPopUpMenu.  The menu items have a visible property which you can set so that items are not shown to your users.  If you need to know what component had focus look at Screen.ActiveControl.  The Shortcut function will take virtual keystrokes and convert them to TShortCuts.

procedure TForm1.FormCreate(Sender: TObject);
begin
//Note the forms PopUpMenu Property is set to the PopMenu
  Left1.ShortCut := ShortCut(vk_down,[]);
  Right1.ShortCut := ShortCut(vk_right,[]);
end;

procedure TForm1.left1Click(Sender: TObject);
begin
  Memo1.SetFocus;
end;


Rick
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Author Comment

by:franksymons
Comment Utility
Thanks for the answer, but I don't wan't to solve it that way. There has to be a way to catch the windows message itself and then send it to whereever I want it to.
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by:titz
Comment Utility
hallo,
i think you should kill the focus of the button or the edit field.
i did this with
button.enable:=false;
button.enable:=true;
then the button has no focus and you can handle the arrowkeys with the OnKeydownEvent (or OnKeypressedEvent)
i think its not elegant but it works.
regards
titz

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Expert Comment

by:rickpet
Comment Utility
Okay...tried this a couple different ways...tried to create a new button that overrode WndProc...but the Wm_ButtonDown never comes through this handler...it's dispatched by the Component ahead of it in Tab Order(watched it in WinSight to verify this).

So we went to the ultimate message handler...Application.OnMessage...that works...

procedure TForm1.FormCreate(Sender: TObject);
var
  myButton: TMybutton;
begin
  Application.OnMessage := MyMessage;
end;

procedure TForm1.MyMessage(var Msg: TMsg; var Handled: Boolean);
begin
if (Screen.ActiveControl is TButton) and (Msg.message = WM_KeyDown) then
  begin
    if (Msg.WParam = vk_Left) or (Msg.WParam = vk_Right) then
      Handled := true;
  end;
end;


Rick
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by:rickpet
Comment Utility
Note certain controls handle the Left Arrow and Right Arrow different than Buttons...i.e. TEdit.

Rick
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Author Comment

by:franksymons
Comment Utility
Thanks titz, but I'll go with the comments that rickpet gave me. It's exactly what I need. Rickpet if you want the points form it in a question and they're yours.
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rickpet earned 200 total points
Comment Utility
Thanks...

Rick
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