Computed property vs Method: Which one to use
Table of Contents
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
}
}
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