Application.HookMainWindow完全替代了原来的窗口过程（但是好像也会继续传递）

unit HookMain;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  StdCtrls, ExtCtrls;

type
  THookForm = class(TForm)
    SendBtn: TButton;
    GroupBox1: TGroupBox;
    LogList: TListBox;
    DoLog: TCheckBox;
    ExitBtn: TButton;
    Button1: TButton;
    procedure SendBtnClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure ExitBtnClick(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
    function AppWindowHook(var Message: TMessage): Boolean;
  end;

var
  HookForm: THookForm;

implementation

{$R *.DFM}

procedure THookForm.FormCreate(Sender: TObject);
begin
  Application.HookMainWindow(AppWindowHook);
end;

procedure THookForm.FormDestroy(Sender: TObject);
begin
  Application.UnhookMainWindow(AppWindowHook);
end;

function THookForm.AppWindowHook(var Message: TMessage): Boolean;
const
  LogStr = 'Message ID: $%x, WParam: $%x, LParam: $%x';
begin
  Result := True;
  if DoLog.Checked then
    with Message do
      LogList.Items.Add(Format(LogStr, [Msg, WParam, LParam]));
end;

procedure THookForm.SendBtnClick(Sender: TObject);
begin
  SendMessage(Application.Handle, WM_NULL, , );
end;

procedure THookForm.ExitBtnClick(Sender: TObject);
begin
  Close;
end;

procedure THookForm.Button1Click(Sender: TObject);
begin
    ShowMessage('ddd');
end;

end.

理论解释：

Enables a native Windows dialog box to receive messages sent to the application's main window.

Use HookMainWindow to ensure that a native Windows dialog box behaves correctly as a child of the application, not as a stand-alone window. For example, switching among applications with Alt+Tab treats the application as a single task after calling HookMainWindow, rather than treating the native Windows dialog box as a separate task.

When the window identified by the Handle property receives relevant dialog messages, it passes them to the dialog procedure passed as the Hook parameter.

There is no problem with leaving a dialog box hooked into the main window, even for extended periods. However, should the dialog box close, call the UnhookMainWindow method to release the hook.