How do I get the coordinates of the mouse when a control is clicked?

Question

In a TImage's OnClick event, I would like to extract the x,y coordinates of the mouse. I would prefer them in relation to the image, but in relation to the form or window is just as good.

Answer 1

Mouse.CursorPos contains the TPoint, which in turn contains the X and Y position. This value is in global coordinates, so you can translate to your form by using the ScreenToClient routine which will translate screen coordinates to window coordinates.

According to the Delphi help file, Windows.GetCursorPos can fail, Mouse.CursorPos wraps this to raise an EOsException if it fails.

var
  pt : tPoint;
begin
  pt := Mouse.CursorPos; 
  // now have SCREEN position
  Label1.Caption := 'X = '+IntToStr(pt.x)+', Y = '+IntToStr(pt.y);
  pt := ScreenToClient(pt);
  // now have FORM position
  Label2.Caption := 'X = '+IntToStr(pt.x)+', Y = '+IntToStr(pt.y);
end;

Answer 2

The Mouse.CursorPos property will tell you the current position of the mouse. If the computer is running sluggishly, or if your program is slow to respond to messages, then it might not be the same as the position the mouse had when the OnClick event first occurred. To get the position of the mouse at the time the mouse button was clicked, use GetMessagePos. It reports screen coordinates; translate to client coordinates with TImage.ScreenToClient.

The alternative is to handle the OnMouseDown and OnMouseUp events yourself; their parameters include the coordinates. Remember that both events need to occur in order for a click to occur. You may also want to detect drag operations, since you probably wouldn't want to consider a drag to count as a click.