How can a control receive mouse events after the mouse is dragged beyond its borders?

Question

I'm creating a custom control which recognizes when the mouse is dragging, specifically using messages WM_LBUTTONDOWN, WM_LBUTTONUP, and WM_MOUSEMOVE. When the mouse goes down, I capture the position on the control, and then when the mouse moves, if the left mouse button is down, I do more handling (calculating between starting and ending points).

The problem is, I'm expecting the mouse to go out of the control, and even out of the form, but when the mouse leaves the control, it no longer captures mouse events. Is there a way I can handle specifically the WM_MOUSEMOVE and WM_LBUTTONUP messages without the mouse being over the control?

Answer 1

Releasecapture will work for Wincontrols, an other way could be a Mousehook. That's just a demo ....

unit MouseHook;
// 2012 by Thomas Wassermann
interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs;

type

  TForm3 = class(TForm)
    procedure FormDestroy(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private-Deklarationen }
  public
    { Public-Deklarationen }
  end;

var
  Form3: TForm3;

implementation

var
  HookHandle: Cardinal;

Type
  tagMSLLHOOKSTRUCT = record
    POINT: TPoint;
    mouseData: DWORD;
    flags: DWORD;
    time: DWORD;
    dwExtraInfo: DWORD;
  end;
  TMSLLHOOKSTRUCT = tagMSLLHOOKSTRUCT;
  PMSLLHOOKSTRUCT = ^TMSLLHOOKSTRUCT;

{$R *.dfm}

function LowLevelMouseProc(nCode: Integer; wParam: wParam; lParam: lParam): LRESULT; stdcall;
var
 Delta:Smallint;
begin
  if (nCode >= 0) then
  begin
    Form3.Caption := Format('X: %d  Y: %d ', [PMSLLHOOKSTRUCT(lParam)^.Point.X,  PMSLLHOOKSTRUCT(lParam)^.Point.Y]);
    if wParam = WM_LButtonDOWN then Form3.Caption := Form3.Caption + ' LD';
    if wParam = WM_LButtonUP then Form3.Caption := Form3.Caption + ' LU';
    if wParam = WM_RButtonDOWN then Form3.Caption := Form3.Caption + ' RD';
    if wParam = WM_RButtonUP then Form3.Caption := Form3.Caption + ' RU';
    if wParam =  WM_MOUSEMOVE then Form3.Caption := Form3.Caption + ' Move';
    Delta := PMSLLHOOKSTRUCT(lParam)^.mouseData shr 16;
    if wParam =  WM_MOUSEWHEEL then
          begin

            Form3.Caption := Form3.Caption + ' Wheel ' ;
            if Delta=120 then Form3.Caption := Form3.Caption + ' KLICK'
            else if Delta > 0  then Form3.Caption := Form3.Caption +' UP'
            else if Delta < 0  then Form3.Caption := Form3.Caption +' DOWN'
          end;
    if wParam =  WM_MOUSEHWHEEL then
          begin
            Form3.Caption := Form3.Caption + ' HWheel';
            if Delta=120 then Form3.Caption := Form3.Caption + ' KLICK'
            else if Delta > 0  then Form3.Caption := Form3.Caption +' UP'
            else if Delta < 0  then Form3.Caption := Form3.Caption +' DOWN'

          end;
     Form3.Caption := Form3.Caption +' >> '+ IntToStr(Delta)

  end;
  Result := CallNextHookEx(HookHandle, nCode, wParam, lParam);
end;

function InstallMouseHook: Boolean;
begin
  Result := False;
  if HookHandle = 0 then
  begin
    HookHandle := SetWindowsHookEx(WH_MOUSE_LL, @LowLevelMouseProc, hInstance, 0);
    Result := HookHandle <> 0;
  end;
end;

procedure TForm3.FormCreate(Sender: TObject);
begin
  InstallMouseHook;
end;

procedure TForm3.FormDestroy(Sender: TObject);
begin
  if HookHandle <> 0 then
    UnhookWindowsHookEx(HookHandle);
end;

end.