Should OnMouseMove events arrive endlessly?

Question

I'm trying to handle OnMouseMove in Delphi (XE3) and even when I don't move the mouse I get endless stream of OnMouseMove events, one after another.

Ex.: start new VCL Forms Application. Add OnMouseMove handler to the form with this code:

var s: string;
begin
  s := IntToStr(GetTickCount()) + ': MouseMove';
  OutputDebugString(PChar(s));
end;

Place the mouse over the form, observe endless OnMouseMoves in IDE's Messages log.

I would expect MouseMove messages to only arrive when I actually move the cursor (and maybe at some other exceptional situations). In fact, i've always though OnMouseMove worked this way.

Am I doing something wrong? Is this correct from a Delphi standpoint (i.e. by design)? Is this correct by WinAPI? What to do about it?

Answer 1

I can add a tip, OnMouseMove events arrive endlessly also when you show a Hint over the control.

I discovered the hard way: I was trying to show a hint when mouse moves over each element of a ListBox showing some 'extra' information about that element.

As soon as i call HintWindow.ActivateHint(...,HintWindow.Hint); the OnMouseMove is set to be triggered.

Since the first thing io do inside the OnMouseMove is to free and recreate the HintWindow, the Hint flickers a lot because OnMouseMove events arrive endlessly.

If i set OnMouseMove to Nil prior to .ActivateHint, the flicker ends and no more OnMouseMove events arrive.

Now i am trying to 'save' the event into a variable and restore after .ActivateHint, but i am not sure how to do it and what effect will have ... saving OnMouseLeave is easy, but OnMouseMove raises a compiler error Parameter List Differs.