Delphi control MouseDown/Move/Up passed to underlying panel

Question

I'm using Delphi 10.2.3. I have a descendent of a TCustomPanel for which I've overridden the MouseDown/MouseMove/MouseUp events to allow selection, dragging, custom painting, etc. Descendents of this panel will have various controls - TImage, TStaticText, TStringGrid, etc. - added dynamically at runtime, and some will have several such controls. The problem is that clicking anywhere in the panel that has some other child control in that location, that control gets the mouse events rather than the underlying panel.

Is there some way of having a control pass its mouse events to its parent, without having to dynamically assign OnMouseXxx handlers for every control added to the panel?

Answer 1

If a child is a TGraphicControl descendant, it does not have its own HWND in the OS, so for all practical purposes it doesn't exist to the OS. Thus, a click on the child will be sent to that child's immediate parent for processing. The VCL will then look for a child at the click coordinates, and if found then send the click to that child, otherwise handle it as a click on the parent. The methods involved in this lookup and delegation are not virtual or dynamic, so you can't override them, and they are invoked before the virtual Mouse(Down|Move|Up) methods are called on the determined target control.

If a child is a TWinControl descendant, it has its own HWND in the OS, thus a click on the child will be sent directly to that child for processing. The parent will not know the child has been clicked on, unless the parent has assigned appropriate event handlers to the child, or the child otherwise notifies the parent.

So, your TCustomPanel (which is a TWinControl descendant) can handle clicks transparently but only for non-windowed children, if it handles the WM_(L|M|R)BUTTON(DOWN|UP)/WM_MOUSEMOVE messages directly before passing them along to any inherited handler. That is not the case for windowed children. To intercept those messages, you would need to use a thread-specific mouse/message hook via the Win32 SetWindowsHookEx() function, looking for HWNDs that have your Panel's HWND as their (grand)parent.