[DUG] MouseWheel and DBGrid in D6

[Wayne Roser]

Hi Stefan

Thanks for your explanation. 

I tried:

    if (HiWord(Msg.wParam) and $8000)=0
    then Msg.wParam := VK_UP else Msg.wParam := VK_DOWN;

but got the same error message as previously.

Also tried Msg.wParamHi instead of HiWord() but that appears to be available for a TMessage and
I've got Msg: tagMSG. Should I be using an event that gives me a TMessage instead of the
TApplicationEvents.OnMessage that I am using?

Wayne

Appendix 1: tagMSG in Windows.pas

{ Message structure }
  PMsg = ^TMsg;
  tagMSG = packed record
    hwnd: HWND;
    message: UINT;
    wParam: WPARAM;
    lParam: LPARAM;
    time: DWORD;
    pt: TPoint;
  end;

Appendix 2: TMessage in Messages.pas

{ Generic window message record }
  PMessage = ^TMessage;
  TMessage = packed record
    Msg: Cardinal;
    case Integer of
      0: (
        WParam: Longint;
        LParam: Longint;
        Result: Longint);
      1: (
        WParamLo: Word;
        WParamHi: Word;
        LParamLo: Word;
        LParamHi: Word;
        ResultLo: Word;
        ResultHi: Word);
  end;


NZ Borland Developers Group - Delphi List <delphi at ns3.123.co.nz> writes:
>I would try:
>
>I := smallint(msg.paramhi);
>
>or .. my preferred way:
>
>If (Msg.wParamHi and $8000)=0
>  then vk_down
>  else vk_up;
>
>the problem is with mixing signed and unsigned datatypes. "Word" datatype is
>unsigned (only has positive range) .. while smallint is "signed" (has +/-
>range). 
>
>Range of datatype:
>  Word = 0.. 65534;
>  smallint = -32768..+32767
>
>a value in a signed datatype is negative if the highest bit is set:
>
>  +2 = $0002
>  +1 = $0001
>   0 = $0000 
>  -1 = $ffff
>  -2 = $fffe
>
>As you can see ... the value of -1 is stored as $ffff in your wparamhi word.
>Assigning $ffff to a smallint will raise a range error as the compiler
>thinks you try to assign $ffff=65535 to it. 
>
>Kind Regards,
>Stefan Mueller 
>
>
>
>-----Original Message-----
>From: delphi-bounces at ns3.123.co.nz [mailto:delphi-bounces at ns3.123.co.nz] On
>Behalf Of Wayne Roser
>Sent: Tuesday, 30 May 2006 2:47 p.m.
>To: NZ Borland Developers Group - Delphi List
>Subject: [DUG] MouseWheel and DBGrid in D6
>
>I found an article in Delphi.about.com and got this code to fix MouseWheel
>behaviour in a DBGrid:
>
>procedure TForm1.ApplicationEvents1Message (var Msg: TMsg; var Handled:
>Boolean) ;
>var
>   i: SmallInt;
>begin
>   if Msg.message = WM_MOUSEWHEEL then  begin
>     Msg.message := WM_KEYDOWN;
>     Msg.lParam := 0;
>     i := HiWord(Msg.wParam) ; //  this line gives me a problem
>     if i > 0 then 
>       Msg.wParam := VK_UP
>     else
>       Msg.wParam := VK_DOWN;
>     Handled := False;
>   end;
>end;
>
>So I dropped a TApplicationEvents component on my form and used the code.
>
>I get a problem at the line 
>	 i := HiWord(Msg.wParam) ;
>Msg.wParam is -7864320 and an ERangeError pops up with message 'Range check
>error'
>
>My working version has 
>	    i := Msg.wParam div abs(Msg.wParam);
>in place of the offending line but I'd like to know wassup with the original
>code. 
>
>Wayne Roser
>
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