unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls;
const
  WM_TestMessage = WM_USER + 2000;
type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}
var
  HookHandle: HHOOK;

function TestHookProc(Code: Integer; WParam: Longint;Msg:Longint): Longint;stdcall;
begin
    if (Code = HC_ACTION) then
    if PMsg(Msg)^.Message = WM_TestMessage then
    begin
      showMessage('已經截獲該消息');
    end;
    Result := CallNextHookEx(HookHandle, Code, WParam, Longint(@Msg));
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  HookHandle:=SetWindowsHookEx(WH_GETMESSAGE,TestHookProc,0,GetCurrentThreadID);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  PostMessage(self.Handle,WM_TestMessage,0,0);
end;

end.