Cocoa Programming for OS X: The Big Nerd Ranch Guide

Computed Properties

It is frequently useful to find a vector’s length or magnitude. You could do this by adding a function returning a Double:

s​t​r​u​c​t​ ​V​e​c​t​o​r​ ​{​
 ​ ​ ​ ​.​.​.​

 ​ ​ ​ ​f​u​n​c​ ​l​e​n​g​t​h​(​)​ ​-​>​ ​D​o​u​b​l​e​ ​{​
 ​ ​ ​ ​ ​ ​ ​ ​r​e​t​u​r​n​ ​s​q​r​t​(​x​*​x​ ​+​ ​y​*​y​)​
 ​ ​ ​ ​}​
}​

However, it is much more natural to think of this as a read-only property. In Swift the general term for this is computed property, which is in contrast to the stored properties you have been using so far. A read-only computed property version of length would look like this:

s​t​r​u​c​t​ ​V​e​c​t​o​r​ ​{​ ​ ​ ​ ​.​.​.​ ​ ​ ​ ​v​a​r​ ​l​e​n​g​t​h​:​ ​D​o​u​b​l​e​ ​{​ ​ ​ ​ ​ ​ ​ ​ ​g​e​t​ ​{​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ...