This post is going to explain what a computed property is and how to use it.
There is a Swift Playground with examples in this post. It can be found at this link. Feel free to copy the files or clone the entire repo, but the repo has more than just this post's sample code.
Topics covered in this post
 What is a Computed Property
 Examples using getters and Setters
 Why use a Computed Property
What is a Computed Property?
A computed property is similar to a stored property, except a computed property does not hold a value. A computed property's value is calculated based on the provided getter defined for the property when the computed property is called.
A computed property can also affect other properties in the class, struct, or enumeration by using a setter.
Important Things about Computed Properties
 Always declare computed properties as a variable,
var
. A computed property is never declared as a constant,let
, since the values are never fixed.  The code inside the getter runs every time the property is called.
 Setters can affect other properties
 When declaring a computed property, a type has to be specified
Example of a computed property with only a getter
struct Example {
// 1.
var originalNum: Int = 1
// 2.
var addTwo: Int {
// 3.
get {
return originalNum + 2
}
}
}
Here's an explanation of what happens in the example above.

orginalNum
is a stored property that will affectaddTwo

addTwo
is the computed property. Every timeaddTwo
is called it will run the code inside the getter. Notice thataddTwo
has a type ofInt
. 
get
is optional if the computed property is readonly.addTwo
could have been written as:
var addTwo: Int {
retun orginalNum + 2
}
Another example
struct Rectangle {
var width: Int
var height: Int
var area: Int{
get {
return width*height
}
}
}
var rect: Rectangle = Rectangle(width: 12, height: 5)
rect.area // 60
In the example above, area
is a computed property. If rect.width
, changes to 15, then area
will change the next time it's called, which is seen in the code below.
rect.width = 15
rect.area // 75
Example of the computed property with a getter and setter
struct Square {
var width: Int
// 1.
var area: Int {
get {
return width * width
}
// 2.
set(newArea) {
// 3.
width = Int(sqrt(Double(newArea)))
}
}
}
var sqr: Square = Square(width: 12)
sqr.area // 144
sqr.area = 25
sqr.width // 5
Here's an explanation of what happens in the example above.
 Declares the computed property as a variable as an Int
 Declares the setter that will take an Int called
newArea
 Inside the setter, the width is assigned the square root of the
newArea
Why use a Computed Property
A computed property is useful when wanting to recall a value that can be calculated from other properties in the class, structure, or enumeration.
In the Square
structure, area
is a computed property because the area is always dependent on the width of the square. area
could have also been written as two functions, one to set the width based on the newArea
and another to get the area. Since, both would effect the width and gets/sets a value, a computed property makes a bit more sense and makes the code look a bit cleaner.
When choosing between a computed property and a function, keep in mind the cost of the operation. Every time a computed property is called, it is recalculated. Also, a computed property can't take any parameters when getting a value. The example below shows a computed property and a function.
struct Circle {
var radius: Double
// 1.
var area: Double {
get {
return Double.pi * radius
}
set(newArea) {
radius = newArea / Double.pi
}
}
// 2.
func getBiggerCirclesArea(times: Int) > Double{
return radius * Double(times) * Double.pi
}
}
Here's an explanation for the code above.

area
is a computed property. It looks similar toSquare
's area, but it has a circle formula. A computed property works forarea
since the radius of the circle might change, 
getBiggerCirclesArea
is a function. Having it be a function will allow the bigger circle be specified without adjusting the originalCircle
's radius.
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