When to use computed properties over methods?

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I prefer to use computed properties when I need to compose new data from existing data sources or I want to reduce a complicated, nested property name to a more readable and easy to use one (but update it when the original property changes). If the property body contains complex logic or is not following O(1), then I move to methods. Properties are tempting to use. They are often seen as lightweight values which makes it a risk if it contains heavy operations. We should use these properties to display data relative to existing data.

Readability and smooth understanding of the code are key to make the decision.

KEY TAKEWAYS

You can use computed properties inside extensions. It performs its operations every time it’s called which can easily result in a performance decrease in costly operation. Computed properties won’t take any arguments. 
struct Person {
    var firstName: String
    var lastName: String
}

extension Person {
    // O(1) Calucaltion
    var fullName: String {
        return firstName + " " + lastName
    }

    // Prefer to use method as it needs
    // calculation more than O(1)
    func generateRandomEmail() -> String {
        let domainSuffixes = ["com", "net", "org", "io", "co.uk"]
        let letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        let randomText = String((0..<5).map{ _ in letters.randomElement()! })
        let randomDomainSuffixIndex = Int(arc4random_uniform(UInt32(domainSuffixes.count)))
        let domainSuffix = domainSuffixes[randomDomainSuffixIndex]
        let text = "\(firstName).\(randomText)" + "@" + "gmail" + "." + domainSuffix
        return text
    }
}

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