getter and setter method

In object-oriented programming, it's generally accepted that you shouldn't expose any properties directly, but rather expose methods to get and set those properties. Swift actually does this for you automatically. For example, when you do something like this:

struct Person{
    var name = "Nydia"
}

var programmer = Person()
programmer.name // "Nydia"
programmer.name = "Michael"

you're actually invoking a method that figures out what programmer's name is, and then gives it to you, and another method that changes programmer's name to what you want it to be. Swift does this for every property you create on every class, struct, enum, and protocol. If you wanted to, though, you could override the default methods that Swift generates and write your own methods, instead. A common scenario is for something like this:

struct Temperature{
    var celsius: Double = 0
    var fahrenheit: Double{
        set{
            celsius = (newValue - 32) * (5/9)
        }
        get{
            return (celsius * 9/5) + 32
        }
    }
}

Although overridden getters and setters aren't used exclusively for this case, fahrenheit is called a "computed property" here because its value is entirely based off the value of celsius. This is accomplished by overriding its getter to return a value computed from the celsius variable, and also overriding its setter to set the celsius variable based on the desired value of fahrenheit.

In Swift, you can override a getter without overriding a setter, but you have to override the getter of a property if you override the setter. However, if you override just the getter of a property, Swift doesn't automatically create a setter, and the property becomes strictly read-only