[solved] Dragging a control - TControl.BeginDrag oddity

[Fr0sT.Brutal 897]

SOLVED - SEE BELOW

 

I have a control that starts dragging on mouse down and finishes on up. I noticed weird bug - the drag won't start though MouseDown is executed and  TControl.BeginDrag is called. Stepping into that method I found this:

...
  if (DragControl = nil) or (DragControl = FlagControl) then
  begin
    DragControl := nil;
    if csLButtonDown in ControlState then
    begin
      GetCursorPos(P);
      Perform(WM_LBUTTONUP, 0, PointToLParam(ScreenToClient(P))); // ???
    end;
...

for some reason the method calls my MouseUp which, in turn, finishes dragging.

I tried adding a check to my MouseUp (ignore if in dragging mode, finish it in DragOver) but this doesn't work when the dragging starts from MouseDown handler. Of course I could add internal flag field like

fCallingBeginDrag := True;
BeginDrag;
fCallingBeginDrag := False

but this looks too awkward. Probably there's any smarter way?

Edited November 8, 2022 by Fr0sT.Brutal
solved

[aehimself 388]

You can check the tab dragging in AE.Comp.PageControl, I'm handling the dragging of tabs almost the same way. The only difference I see is creating a DragObject in DoStartDrag but that shouldn't affect the check above as it's called from DragInitControl, which is called later.

[Fr0sT.Brutal 897]

Thanks, I'll take a look.

I see you are using zero point as null value but it's a valid click point in fact. I'm using to (-1,-1) for this purpose

[aehimself 388]

That makes a lot of sense, thanks for pointing it out! As it caused no issues until now I didn’t even notice :)

Maybe I’ll update it if I have to make a change in the component.

[Fr0sT.Brutal 897]

I guess I figured things out. The catch is that when dragging, no "mouse up" event is fired when a user releases the button. I wasn't aware of that. So the solution is:

- in MouseUp ignore the event when in dragging mode (it's a MLB Up event fired by TControl.BeginDrag)

- in DragOver finish dragging mode when State is dsDragLeave

[Fr0sT.Brutal 897]

3 hours ago, Fr0sT.Brutal said:

- in DragOver finish dragging mode when State is dsDragLeave

Update: this is unreliable. The event is not called if there was just a simple click. I had to catch DoEndDrag as well.