When we write a for-in loop like this:

for each in someCollection {
    //...
}

Swift calls the makeIterator() method on the collection and then make repeated calls next() on the result until it returns nil.

The object returned by the makeIterator() is of a type that conforms to the IteratorProtocol protocol.

You can make use of IteratorProtocol directly. For example, if you might a list of values and you want to be able to pick the next value from the list and perform an action with it. You could implement this using list-like collection, often a FIFO queue. Alternatively, with iterators:

var i = [1, 2, 3].makeIterator()
i.next() //1
i.next() //2
i.next() //3
i.next() //nil, we are done

Have fun!

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