Trap the OnEnter and OnLeave events

Problem/Question/Abstract:

This code shows how to get the OnEnter and OnLeave event from components without changing the component.

Answer:

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    Label1: TLabel;
    Button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
    FFocusControl: TControl;
    procedure ApplicationIdle(Sender: TObject; var Done: Boolean);
  public
    { Public declarations }
    procedure OnEnter(Sender: TObject);
    procedure OnExit(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.FormCreate(Sender: TObject);
begin
  FFocusControl := nil;
  Application.OnIdle := ApplicationIdle;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  Application.OnIdle := nil;
end;

procedure TForm1.ApplicationIdle(Sender: TObject; var Done: Boolean);
var
  CurControl: TControl;
  P: TPoint;
begin
  GetCursorPos(P);
  CurControl := FindDragTarget(P, True);
  if FFocusControl <> CurControl then
  begin
    if FFocusControl <> nil then
      OnExit(FFocusControl);
    FFocusControl := CurControl;
    if FFocusControl <> nil then
      OnEnter(FFocusControl);
  end;
end;

procedure TForm1.OnEnter(Sender: TObject);
begin
  //OnEnter code
  if sender = Button1 then
  begin
    Label1.caption := 'Hello';
    Button1.Caption := 'Exit';
  end;
end;

procedure TForm1.OnExit(Sender: TObject);
begin
  //OnExit code
  if sender = Button1 then
  begin
    Label1.caption := 'Godbye';
    Button1.Caption := 'Enter';
  end;
end;

end.