Sent message or received message?

Question

1. As I known, [self setDate:...] mean self is receiver and setDate: is message. So which is the sender ?

2. I set a button with action (IBAction)buttonPressed:(id)sender. I understood that this button is the targer and buttonPressed is the action. So which is can be sender ?

Answer 1

Actually, In objective- c messaging system don't have any formal notion of "sender".

• (IBAction)buttonPressed:(id)sender.

  Here sender is just ordinary method argument ,not a part of objective c messaging system.Just a way argument is named in Appkit or UIKIt.

• [self setDate:...] here "self" is receiver and setDate: is message.

Please note : in ordinary speech "sending a message" implies three thing: sender,receiver and message.Where sender is function or block of code that contain message. But that analogy is not applicable for objective c.

Answer 2

1) As I known, [self setDate:...] mean self is receiver and setDate: is message. So which is the sender ?

The sender is the object—if any—that sent the message.

So which one is that?

1. You wrote the message in a statement.

2. You wrote that statement within a function, class method, or instance method.

   • If it was a function, then there is no object that was the sender.
   • If it was a class method, arguably the class is the sender (depending on how much you consider classes to be objects).

   • If it was an instance method, whichever object had that method called on it—i.e., received that message—is the sender of the setDate: message.

     For example, if you sent your setDate: message from an instance method named buttonPressed: in an NSWindowController subclass, then your window controller—the instance (object) that received the buttonPressed: message—is the object that sent the setDate: message.

Of course, if the receiver of the message is self, then you are sending the message to yourself, so you are both the receiver and the sender.

2) I set a button with action (IBAction)buttonPressed:(id)sender. I understood that this button is the targer and buttonPressed is the action.

Nope! The button is not the target; the button has a target, which is another object. The target is the object that receives the action message.

Think of an archery target. The target doesn't send the arrow! It receives the arrow.

So which is can be sender ?

The object that is referred to by the sender variable generally is the object that sent the message.

Usually, it is a control (such as a button) or menu item: when pressed, the control or menu item sends its action message to its target, passing itself (the control or menu item) as the “sender”.

So your button would send a message to its target, the message being whichever one is set as the button's action, and because it's sending the message, it would pass itself as the sender.

The only reason to look at the sender at all is to examine the sender's tag or representedObject. An action method is about the only kind of method that may need to care at all about the sender (which is why it has this argument), and even then, usually not.

Answer 3

1) Yes, self is the receiver of the message and setDate: is the message (the method call). There is no "sender". What do you mean by "sender" anyway?

2) In code, setting up the button to call the buttonPressed: method is done like this:

[someButton addTarget:someTarget action:@selector(buttonPressed:) forControlEvents: UIControlEventTouchUpInside];

someTarget is typically self. The "target" is the object that has the "action" method. The method looks like:

- (void)buttonPressed:(UIButton *)button {
}

The "someButton" is the "sender" since it is the button that had the event.

Answer 4

The sender is the object preforming the task....assuming your example (IBAction)buttonPressed:

If you are pressing a button, that button is doing the sending of whatever code follows. See my example

- (IBAction)pushButton:(id)sender {
UIButton *pushedButton = (UIButton *)sender;
[button setTitle:@"Press me to Send" forState:UIControlStateNormal];
}