CODEWITHSUNDEEP
delphiaccessibilitysubclassingdelphi-5
Ian Boyd
Ian Boyd

Reputation: 256731

How to receive TAB key press in edit box?

i want to receive OnKeyPress events when the user presses the Tab key.

procedure TForm1.Edit1(Sender: TObject; var Key: Char);
begin
   case Key of
   #09:
      begin
         //Snip - Stuff i want to do
      end;
   end;
end;

i try subclassing the Edit box, and handle the WM_GETDLGCODE message:

procedure TfrmEnableVIPMode.AccountNumberWindowProc(var Message: TMessage);
begin
   case Message.Msg of
   WM_GETDLGCODE: Message.Result := DLGC_WANTTAB;
   else
      FOldAccountNumberWindowProc(Message);
   end;
end;

And i now receive Tab KeyPress events (as i hoped), but now pressing the Left or Right cursor keys causes focus to shift to the previous, or next, control in the tab order.

What is the correct way to recieve Tab Key Press events?

Bonus Reading

i tried doing what the MSDN documentation says:

wParam
The virtual key, pressed by the user, that prompted Windows to issue this notification. The handler must selectively handle these keys. For instance, the handler might accept and process VK_RETURN but delegate VK_TAB to the owner window. For a list of values, see Virtual-Key Codes.

lParam A pointer to an MSG structure (or NULL if the system is performing a query).

but wParam and wParam are both zero.

Update Two

i realized i have the same bug as this answer:

if Message.Msg = WM_GETDLGCODE then
   Message.Result:= Message.Result or DLGC_WANTTAB
else
   if Assigned(FOldWndProc) then FOldWndProc(Message);

when i should actually use concepts from the correct code listed elsewhere in the same answer:

if Assigned(FOldWndProc) then FOldWndProc(Message);
if Message.Msg = WM_GETDLGCODE then
   Message.Result:= Message.Result or DLGC_WANTTAB;

That helps to explain why my original code is wrong. Setting Message.Result to DLGC_WANTTAB is wrong:

procedure TfrmEnableVIPMode.AccountNumberWindowProc(var Message: TMessage);
begin
   case Message.Msg of
   WM_GETDLGCODE: Message.Result := DLGC_WANTTAB;
   else
      FOldAccountNumberWindowProc(Message);
   end;
end;

it is also wrong to try to bitwise or the flag DLGC_WANTTAB into Message.Result, because Message.Result doesn't have a value yet:

procedure TfrmEnableVIPMode.AccountNumberWindowProc(var Message: TMessage);
begin
   case Message.Msg of
   WM_GETDLGCODE: Message.Result := Message.Result or DLGC_WANTTAB;
   else
      FOldAccountNumberWindowProc(Message);
   end;
end;

i must first call the original window procedure, to get Windows' EDIT control set correct values of Message.Result. Then i can bitwise combine DLGC_WANTTAB:

procedure TfrmEnableVIPMode.AccountNumberWindowProc(var Message: TMessage);
begin
    FOldAccountNumberWindowProc(Message);

    case Message.Msg of
    WM_GETDLGCODE: Message.Result := Message.Result or DLGC_WANTTAB;
    end;
end;

To paraphrase the Raymond Chen blog entry, and adding emphasis as required:

After asking the original control what behavior it thinks it wants, we turn on the DLGC_WANTTAB flag

So this is better. Cursor keys continue to navigate text in the Edit control (rather than shifting focus), and i receive OnKeyPress (and OnKeyDown and OnKeyUp) events for the Tab key.

The remaining problem is that the user pressing Tab no longer shifts focus.

i tried to start to start manually hacking in focus changing myself:

procedure TfrmEnableVIPMode.edAccountNumberKeyPress(Sender: TObject; var Key: Char);
begin
   case Key of
   #09:
      begin
         //Snip - Stuff i want to do

         { 
            The DLGC_WANTTAB technique broke Windows focus change. 
            Keep throwing in hacks until it's no longer obviously broken
         }
         //Perform(CM_DialogKey, VK_TAB, 0); //doesn't work
         Self.ActiveControl := Self.FindNextControl(edAccountNumber, True, True, False);
      end;
   end;
end;

The above code works - if the user pressed the Tab key. But the code is broken, as Raymond Chen notes six year ago:

There are many things wrong with this approach. You can spend quite a lot of time nitpicking the little details, how this code fails to set focus in a dialog box properly, how it fails to take nested dialogs into account, how it fails to handle the Shift+Tab navigation key

In my case, i broke Shift+Tab. And who knows what else.

So, my question:

How to receive TAB key press in edit box?

i don't want to eat them, i just want to know that the user pressed the Tab key.

Bonus Chatter

Upvotes: 7

Views: 7652

Answers (2)

Remy Lebeau
Remy Lebeau

Reputation: 596352

You need to call the previous WndProc first so the Message.Result gets a default value for the key codes that TEdit natively wants, then append your DLGC_WANTTAB flag to that result, eg:

procedure TfrmEnableVIPMode.AccountNumberWindowProc(var Message: TMessage);
begin
  FOldAccountNumberWindowProc(Message);
  if Message.Msg = WM_GETDLGCODE then
    Message.Result := Message.Result or DLGC_WANTTAB;
end;

Upvotes: 5

Sertac Akyuz
Sertac Akyuz

Reputation: 54812

You can handle the CN_KEYDOWN message:

procedure TfrmEnableVIPMode.AccountNumberWindowProc(var Message: TMessage);
begin
   case Message.Msg of
   CN_KEYDOWN:
      if TWMKey(Message).CharCode = VK_TAB then
         ....
   end;
   FOldAccountNumberWindowProc(Message);
end;


It is also possible to detect the key down message at the form level, without subclassing the edit:

procedure TfrmEnableVIPMode.CMDialogKey(var Message: TCMDialogKey);
begin
  if (Message.CharCode = VK_TAB) and (ActiveControl = edAccountNumber) then
    ...

  inherited;
end;

Upvotes: 7

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