I have a VCL TMemo
control and need to be notified every time the text is scrolled. There is no OnScroll
event and the scroll messages doesn't seem to propagate up to the parent form.
Any idea of how to get the notification? As a last resort I can place an external TScrollBar
and update the TMemo
in the OnScroll
event, but then I have to keep them in sync when I move the cursor or scroll the mouse wheel in TMemo
...
You can use a interposer class to handle the WM_VSCROLL
and WM_HSCROLL
messages and the EN_VSCROLL
and EN_HSCROLL
notification codes (exposed through the WM_COMMAND message).
Try this sample
type
TMemo = class(Vcl.StdCtrls.TMemo)
private
procedure CNCommand(var Message: TWMCommand); message CN_COMMAND;
procedure WMVScroll(var Msg: TWMHScroll); message WM_VSCROLL;
procedure WMHScroll(var Msg: TWMHScroll); message WM_HSCROLL;
end;
TForm16 = class(TForm)
Memo1: TMemo;
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form16: TForm16;
implementation
{$R *.dfm}
{ TMemo }
procedure TMemo.CNCommand(var Message: TWMCommand);
begin
case Message.NotifyCode of
EN_VSCROLL : OutputDebugString('EN_VSCROLL');
EN_HSCROLL : OutputDebugString('EN_HSCROLL');
end;
inherited ;
end;
procedure TMemo.WMHScroll(var Msg: TWMHScroll);
begin
OutputDebugString('WM_HSCROLL') ;
inherited;
end;
procedure TMemo.WMVScroll(var Msg: TWMHScroll);
begin
OutputDebugString('WM_HSCROLL') ;
inherited;
end;
You can subclass the Memo's WindowProc
property at runtime to catch all of messages sent to the Memo, eg:
private:
TWndMethod PrevMemoWndProc;
void __fastcall MemoWndProc(TMessage &Message);
__fastcall TMyForm::TMyForm(TComponent *Owner)
: TForm(Owner)
{
PrevMemoWndProc = Memo1->WindowProc;
Memo1->WindowProc = MemoWndProc;
}
void __fastcall TMyForm::MemoWndProc(TMessage &Message)
{
switch (Message.Msg)
{
case CN_COMMAND:
{
switch (reinterpret_cast<TWMCommand&>(Message).NotifyCode)
{
case EN_VSCROLL:
{
//...
break;
}
case EN_HSCROLL:
{
//...
break;
}
}
break;
}
case WM_HSCROLL:
{
//...
break;
}
case WM_VSCROLL:
{
//...
break;
}
}
PrevMemoWndProc(Message);
}
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