https://wedelphi.com/t/269743/

private
procedure CMGOTFOCUS(var Message: TMessage); message CM_GOTFOCUS;
procedure CMLOSTFOCUS(var Message: TMessage); message CM_LOSTFOCUS;
procedure CMACTIVATE(var Message: TMessage); message CM_ACTIVATE;
procedure CMDEACTIVATE(var Message: TMessage); message CM_DEACTIVATE;

 

查了以前的贴子，也亲自实验过，下面的代码确实可行！
unit Unit1;

interface

uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs;
type
TForm1 = class(TForm)
private
{ Private declarations }
procedure WMACTIVATEAPP( var message:TWMACTIVATEAPP); message WM_ACTIVATEAPP;
public
{ Public declarations }
end;
var
Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WMACTIVATEAPP(var message: TWMACTIVATEAPP);
begin
if Message.Active then
caption:='Form is Active'
else
caption:='Form is Not Active';
end;
end.