Disable and Enable an Event Handler

Sometimes you want to disable an event handler after the FIRST time that the event is "fired". Example: you want to disable all clicks on Label1 after it has been clicked:

procedure TForm1.Label1OnClick(Sender: TObject);
begin
  // Code to execute, e.g.:
  Label1.Caption := 'Label1 has been clicked';
  //...
  // Disable future clicks on Label1:
  Label1.OnClick := nil;
end;   

Maybe at some later time, you want to re-enable the label's OnClick event handler. Code it like this:

procedure TForm1.Button1OnClick(Sender: TObject);
begin
  // Code to execute, e.g.:
  Label1.Caption := 'Click on this text';
  //...
  // Re-enable click events for Label1:
  Label1.OnClick := Label1OnClick;
end;

For a Button, Checkbox, RadioGroup... you can obtain the same effect by disabling the component itself, for example: Button1.Enabled := FALSE;  -- at the same time, you give visual feedback to the user. The trick above makes only sense for other components, such as Panel, Label, Form,...