1. Properties (property observer and computed properties)
2. Static properties and methods
3. Access control
4. Polymorphism and typecasting (converting types with type casting initializers)
5. Closures

1. Property Observer

Swift lets you add code to be run when a property is about to be changed or has been changed. This is frequently a good way to have a user interface update when a value changes, for example.

There are two kinds of property observer: willSet and didSet, and they are called before or after a property is changed. In willSet Swift provides your code with a special value called newValue that contains what the new property value is going to be, and in didSet you are given oldValue to represent the previous value.

Let's attach two property observers to the clothes property of a Person struct:

struct Person {
    var clothes: String {
        willSet {
            updateUI(msg: "I'm changing from \(clothes) to \(newValue)")
        }

        didSet {
            updateUI(msg: "I just changed from \(oldValue) to \(clothes)")
        }
    }
}

func updateUI(msg: String) {
    print(msg)
}

var taylor = Person(clothes: "T-shirts")
taylor.clothes = "short skirts"

output

I'm changing from T-shirts to short skirts I just changes from T-shirts to short skirts

2. Computed properties

It's possible to make properties that are actually code behind the scenes. We already used the uppercased() method of strings, for example, but there’s also a property called capitalized that gets calculated as needed, rather than every string always storing a capitalized version of itself.

To make a computed property, place an open brace after your property then use either get or set to make an action happen at the appropriate time. For example, if we wanted to add a ageInDogYears property that automatically returned a person's age multiplied by seven, we'd do this:

struct Person {
    var age: Int

    var ageInDogYears: Int {
        get {
            return age * 7
        }
    }
}

var fan = Person(age: 25)
print(fan.ageInDogYears)

output