What is a computed property?

Written by Paul Hudson    @twostraws

Swift offers us two kinds of property: a stored property is one that saves a value for use later, and a computed property is one that runs some code in order to calculate the value.

As comparison, here’s a Person struct with four stored properties and one computed one. The computed property returns a string based on joining the four stored properties into a sentence.

struct Person {
    var name = "Taylor"
    var favoriteColor = "red"
    var favoriteCity = "Tokyo"
    var favoriteFood = "tea"

    var greeting: String {
        return "Hello, my name is \(name), and I like \(favoriteFood), \(favoriteCity), and the color \(favoriteColor)."
    }
}

Note: Unlike stored properties, Swift requires you to use an explicit type with your computed properties.

Available from iOS 8.0

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This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.