When to use a colon with a @selector

Question

Just getting going with iPhone development and Objective-C.

Yesterday I was trying to addObserver for a notification in a view of mine, and I kept getti

Answer 1

A selector represents a method name, and the number of colons in a selector matches the number of arguments in the corresponding method:

1. mySelector — no colon, no arguments, e.g. - (void)mySelector;, [self mySelector];
2. mySelectorWithFoo: — one colon, a single argument, e.g. - (void)mySelectorWithFoo:(Foo *)foo;, [self mySelectorWithFoo:someFoo];
3. mySelectorWithFoo:withBar: — two colons, two arguments, e.g. - (void)mySelectorWithFoo:(Foo *)foo bar:(Bar *)bar;, [self mySelectorWithFoo:someFoo bar:someBar];

and so forth.

It is also possible to have a selector without ‘naming’ the parameters. It’s not recommended since it’s not immediately clear what the parameters are:

1. mySelector:: — two colons, two arguments, e.g. - (void)mySelector:(Foo *)foo :(Bar *)bar;, [self mySelector:someFoo :someBar];
2. mySelector::: — three colons, three arguments, e.g. - (void)mySelector:(int)x :(int)y :(int)z;, [self mySelector:2 :3 :5];