Computed property vs Method: Which one to use

A read-only computed property can use interchangeability with a method with no arguments.

One of the common questions is when we should use one over another.

A read-only computed property, age.

struct User {
    var name: String
    let dateOfBirth: Date
    var age: Int {
        let comps = Calendar.current.dateComponents([.year], from: dateOfBirth, to: .now)
        return comps.year ?? 0
    }
}

A method, age().

struct User {
    var name: String
    let dateOfBirth: Date
    func age() -> Int {
        let comps = Calendar.current.dateComponents([.year], from: dateOfBirth, to: .now)
        return comps.year ?? 0
    }
}

There is no strict rule of which one to use, and everyone might have their own opinion about this.

In this article, I will give you my rule of thumb.

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When to use read-only Computed property over Method and vice-versa

My rule is easy. It is totally based on semantics.

• Property should represent a state or value associated with an object.
• Method is an action that you can perform on a particular type.

I use these semantics to decide which one to use. A similar syntax or calculation shouldn't be a factor of your choice.

Let's go back to our previous example. Can you guess which one I will use? A computed property or a method?

...

That's right. A computed property. Because for me, age is a property associated with a User struct.

struct User {
    var name: String
    let dateOfBirth: Date
    var age: Int {
        let comps = Calendar.current.dateComponents([.year], from: dateOfBirth, to: .now)
        return comps.year ?? 0
    }
}

What if the calculation is expensive

Even though I said I wouldn't care about the calculation's complexity, I still need to care in the real world.

If the age calculation is expensive and I know many parts of the program rely on this property, I might need to change it to a method.

But I won't just change it to an age() method. I will think of a new method that returns an age. This might sound tricky, but it is semantically different.

I might come up with a calculateAge() or getAge() method, which better reflects what the method is trying to do.

struct User {
    var name: String
    let dateOfBirth: Date
    
    func calculateAge() -> Int {
        let comps = Calendar.current.dateComponents([.year], from: dateOfBirth, to: .now)
        return comps.year ?? 0
    }
}