Question

I want to write a little component which shows me on which control mouse is currently over.
When it spot the choosen control it should fire the messaage (for example).

But I don’t know what should I do to form to get the position of the mouse all the time.
This is what I’ve got:

  TMouseOverControl = class(TComponent)
  private
    fActive: Boolean;
    fControl: TWinControl;
  public
    constructor Create(AOwner: TComponent); override;
    procedure Loaded; override;
    procedure SpotIt;
  published
    property Active: Boolean read fActive write fActive;
    property Control: TWinControl read fControl write fControl; // when mouse is over this control show me the message
  end;

constructor TMouseOverControl.Create(AOwner: TComponent);
begin
  // nothing interesting here
  // don't have control property here - so overrided the loaded method
  inherited;
end;

procedure TMouseOverControl.Loaded;
begin
  inherited;

  //  TForm(Owner).Mo.... := SpotIt.... 
  //  what should i do to make it work?
 end;

 procedure TMouseOverControl.SpotIt;
 begin
// IsMouseOverControl is easy to implement
// http://delphi.about.com/od/delphitips2010/qt/is-some-delphi-tcontrol-under-the-mouse.htm
       if IsMouseOverControl(Control) then 
         ShowMessage('Yep, U got it!');
     end;

Any ideas?

Answer 1

Well you only need to check/update when the mouse moves. So you could track WM_MOUSEMOVE messages by using TApplicationEvents.

// Edit: these variables are intended to be private fields of the component class
var
  FAppEvents: TApplicationEvents;
  FFoundControl: Boolean;
  FCurrentControl: TWinControl;

procedure TMyComponent.HandleAppMessage(var Msg: tagMSG; var Handled: Boolean);
var
  Control: TWinControl;
begin
  if (Msg.message = WM_MOUSEMOVE) and not FFoundControl then
  begin
    Control:= FindControl(Msg.hwnd);
    if Assigned(Control) then
    begin
      FCurrentControl:= Control;
      FFoundControl:= True;
    end;
  end else
  if (Msg.message = WM_MOUSELEAVE) then
    FFoundControl:= False;
end;

procedure TMyComponent.FormCreate(Sender: TObject);
begin
  FAppEvents:= TApplicationEvents.Create(nil);
  FAppEvents.OnMessage:= HandleAppMessage;
end;

This could certainly be optimized, e.g. by also checking for WM_MOUSELEAVE so you don’t have to FindControl on every mouse move. This solution works for TWinControls and descendants.

Edit: Made use of WM_MOUSELEAVE.