Swift Programming: The Big Nerd Ranch Guide

Protocols

A protocol allows you to define the interface you want a type to satisfy. A type that satisfies a protocol is said to conform to the protocol.

Define a protocol that specifies the interface you need for the printTable(_:withColumnLabels:) function. The function needs to know how many rows and columns there are, what the label for each column is, and what data should be displayed in each cell. It does not matter to the Swift compiler where in your playground file you put this protocol. But it probably makes the most sense to put it just before printTable(_:withColumnLabels:), because you are going to use the protocol in the function.

Listing 19.5 Defining a protocol

...
protocol TabularDataSource {
    var numberOfRows: ...