Understanding the Sender parameter in Delphi Event Handlers

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Event handlers and the Sender

 procedure TForm1.Button1Click(Sender: TObject) ;
begin
  ...
end; 
Button1Click
OnClick event

The parameter "Sender" references the control that was used to call the method. If you click on the Button1 control, causing the Button1Click method to be called, a reference or pointer to the Button1 object is passed to Button1Click in the parameter called Sender.

Let's Share Some Code

For example, suppose we want to have a button and a menu item do the same thing. It would be silly to have to write the same event handler twice.

To share an event handler in Delphi, do the following:

  1. Write the event handler for the first object (e.g. button on the SpeedBar)
  2. Select the new object or objects - yes, more than two can share (e.g. MenuItem1)
  3. Go to the Event page on the Object Inspector.
  4. Click the down arrow next to the event to open a list of previously written event handlers. (Delphi will give you a list of all the compatible event handlers that exist on the form)
  5. Select the event from the drop-down list. (e.g. Button1Click)
 procedure TForm1.Button1Click(Sender: TObject) ;
begin
  {code for both a button and a menu item}
  ...
  {some specific code:}
  if Sender = Button1 then
   ShowMessage('Button1 clicked!')
  else if Sender = MenuItem1 then
   ShowMessage('MenuItem1 clicked!')
  else
   ShowMessage('??? clicked!') ;
end; 

Note: the second else in the if-then-else statement handles the situation when neither the Button1 nor the MenuItem1 have caused the event. But, who else might call the handler, you could ask. Try this (you'll need a second button: Button2) :

 procedure TForm1.Button2Click(Sender: TObject) ;
begin
   Button1Click(Button2) ;
   {this will result in: '??? clicked!'}
end; 

IS and AS

 if Sender is TButton then
   DoSomething
else
   DoSomethingElse; 
 procedure TForm1.Edit1Exit(Sender: TObject) ;
begin
  Button1Click(Edit1) ;
end; 
 {... else}
begin
  if Sender is TButton then
    ShowMessage('Some other button triggered this event!')
  else if Sender is TEdit then
    with Sender as TEdit do
     begin
      Text := 'Edit1Exit has happened';
      Width := Width * 2;
      Height := Height * 2;
     end {begin with}
end; 

Conclusion

As we can see, the Sender parameter can be very useful when used properly. Suppose we have a bunch of Edit boxes and Labels that share the same event handler. If we want to find out who triggered the event and act, we'll have to deal with Object variables. But, let's leave this for some other occasion.