Explanation of declaration - (IBAction)changeGreeting:(id)sender; iPhone

Question

everywhere they say what IBAction is but do not explain what the whole declaration means.

What is sender and id?

Regards, Namratha

Answer 1

1. According to my knowledge IBAction just implies void

2. id is a generic C type, which accepts objects of any kind/class. It is some what similar to void pointer in C. We can use id as a parameter of a method, in case, if objects of difference kind of classes will access that method.

3. sender, here,

   • is a human-named parameter name (you can name it anything)
   • it refers the object which calls the method changeGreeting:

An Example:

Lets assume the method changeGreeting: is defined in ClassA, and it reads like the following.

- (IBAction)changeGreeting:(id)sender {

    [sender setText:@"Hello %@", [sender class]];

    // Nothing can be returned from this method
}

And consider we have the following lines in Class B.

// LINE 1
[aLabel addTarget:objOfClassA action:@selector(changeGreeting:) forControlEvents:UIControlEventTouchUpInside];

// LINE 2
[aTextField addTarget:objOfClassA action:@selector(changeGreeting:) forControlEvents:UIControlEventTouchUpInside];

In LINE 1, aLabel is the sender and objOfClassA is the receiver. And in LINE 2: aTextField is the sender and objOfClassA is the receiver. HereaLabel & aTextField are called senders because they are calling the method changeGreeting:. And objOfClassA is called receiver because objOfClassA's changeGreeting: method is called here.

When the user touches inside the aLabel or aTextField, their text will be changed to Hello UILabel or Hello UITextField respectively.

The other Way: We can also call changeGreeting: method of objOfClassA from Class B like the following.

[objOfClassA changeGreeting:aLabel];
[objOfClassA changeGreeting:aTextField];

As the above code is self explanatory, objOfClassA is the receiver. But aLabel & aTextField are not the senders. Because they are not invoking the method. Here they are just the arguments. Here the actual sender is Class B, but aLabel and aTextField are passed in the argument (id)sender. Either way the result of the method execution is same.

Answer 2

Maybe you should check out a Objective-C tutorial (such as this), as this is pretty basic.

The IBAction is defined as "void", so it means that the method does not have a return value. It is useful only for Interface Builder, so it knows which of your methods are actually action you can link.

The (id)sender part is a (the only) parameter to your method, which is named sender. (id) says that the type of the parameter can be any object:

From Objective-C FAQ:

What is id?
It's a generic C type that Objective-C uses for an arbitrary object. [...]

Answer 3

IBAction is a keyword (#define'd to void) that Interface Builder looks for in your source code, and if it finds it as a return type, it will add it to the list of possibile actions you can wire up your IB elements to.

id is a generic object type in Objective-C that denotes any Objective-C object.

and sender is just the name of the id-typed parameter.

Answer 4

Functionally saying: IBAction is a keyword to identify methods you could link to through Interface Builder. (id)Sender is necessary to identify the object is triggering the method in case you use the same method for several controls.