unit Unit1;
{©Drkb v.3(2007): www.drkb.ru}
interface
uses
 Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms,
 Dialogs, StdCtrls;
type
 TForm1 = class(TForm)
  Memo1: TMemo;
  procedure FormCreate(Sender: TObject);
  procedure FormDestroy(Sender: TObject);
 private
  hwndViewer: THandle;
  procedure WMChangeCbChain(var Message: TWMChangeCBChain); message WM_CHANGECBCHAIN;
  procedure WMDrawClipboard(var Message: TMessage); message WM_DRAWCLIPBOARD;
 end;
var
 Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.FormCreate(Sender: TObject);
begin
 // Add the window to the clipboard viewer chain.
 hwndViewer := SetClipboardViewer(Handle);
 Memo1.Lines.Clear
end;
procedure TForm1.WMChangeCbChain(var Message: TWMChangeCBChain);
begin
 with Message do
 begin
  // If the next window is closing, repair the chain.
  if Remove = hwndViewer then
  hwndViewer :=
  // Otherwise, pass the message to the next link.
  else
  if hwndViewer <> 0 then
  SendMessage(hwndViewer, Msg, Remove, );
 end;
end;
// clipboard contents changed.
procedure TForm1.WMDrawClipboard(var Message: TMessage);
begin
 // Pass the message to the next window in clipboard
 // viewer chain.
 Memo1.Lines.Add('Сhanged');
 with Message do
  SendMessage(hwndViewer, Msg, WParam, LParam);
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
 ChangeClipboardChain(Handle, hwndViewer);
end;
end.
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<code>
{
 An application can be notified of changes in the data stored in the
 Windows clipboard by registering itself as a Clipboard Viewer.
 Clipboard viewers use two API calls and several messages to communicate
 with the Clipboard viewer chain. SetClipboardViewer adds a window to the
 beginning of the chain and returns a handle to the next viewer in the chain.
 ChangeClipboardChain removes a window from the chain. When a clipboard change occurs,
 the first window in the clipboard viewer chain is notified via the WM_DrawClipboard
 message and must pass the message on to the next window. To do this, our application
 must store the next window along in the chain to forward messages to and also respond
 to the WM_ChangeCBChain message which is sent whenever any other clipboard viewer on
 the system is added or removed to ensure the next window along is valid.
}

 unit Unit1;
 interface
 uses
  Windows, Messages, Forms, Classes, Controls, StdCtrls;
 type
  TForm1 = class(TForm)
  Button1: TButton;
  Button2: TButton;
  procedure Button1Click(Sender : TObject);
  procedure Button2Click(Sender : TObject);
  procedure FormCreate(Sender : TObject);
  procedure FormDestroy(Sender : TObject);
  private
  FClipboardViewer: HWND;
  procedure WMChangeCBChain(var Msg : TWMChangeCBChain); message WM_CHANGECBCHAIN;
  procedure WMDrawClipboard(var Msg : TWMDrawClipboard); message WM_DRAWCLIPBOARD;
  end;
 var
  Form1 : TForm1;
 implementation
 {$R *.DFM}
 procedure TForm1.FormCreate(Sender : TObject);
 begin
  { Initialize variable }
  FClipboardViewer := 0;
 end;

 procedure TForm1.Button1Click(Sender : TObject);
 begin
  if FClipboardViewer <> 0 then
  MessageBox(0, 'This window is already registered!', nil, 0)
  else
  { Add to clipboard chain }
  FClipboardViewer := SetClipboardViewer(Handle);
 end;

 procedure TForm1.Button2Click(Sender : TObject);
 begin
  { Remove from clipboard chain }
  ChangeClipboardChain(Handle, FClipboardViewer);
  FClipboardViewer := 0;
 end;

 procedure TForm1.WMChangeCBChain(var Msg : TWMChangeCBChain);
 begin
  inherited;
  { mark message as done }
  Msg.Result := 0;
  { the chain has changed }
  if Msg.Remove = FClipboardViewer then
  { The next window in the clipboard viewer chain had been removed. We recreate it. }
  FClipboardViewer := Msg.
  else
  { Inform the next window in the clipboard viewer chain }
  SendMessage(FClipboardViewer, WM_CHANGECBCHAIN, Msg.Remove, Msg.);
 end;

 procedure TForm1.WMDrawClipboard(var Msg : TWMDrawClipboard);
 begin
  inherited;
  { Clipboard content has changed }
  try
  MessageBox(0, 'Clipboard content has changed!', 'Clipboard Viewer', MB_ICONINFORMATION);
  finally
  { Inform the next window in the clipboard viewer chain }
  SendMessage(FClipboardViewer, WM_DRAWCLIPBOARD, 0, 0);
  end;
 end;

 procedure TForm1.FormDestroy(Sender : TObject);
 begin
  if FClipboardViewer <> 0 then
  begin
  { Remove from clipboard chain }
  ChangeClipboardChain(Handle, FClipboardViewer);
  FClipboardViewer := 0;
  end;
 end;
 end.