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05 Jan 2023

Computed properties, as the name suggests are properties whose value is derived from other stored properties at runtime and are a valuable tool to know in Swift programming. This eliminates the need to store these properties.

In this post, we’ll see how to use computed properties in Swift by creating a User struct that has two string properties: firstName and lastName and an integer that stores their age.

 struct User {
   let firstName: String
   let lastName: String
   let age: Int
}

Next we will add a computed property called fullName which will be computed from the firstName and lastName.

struct User {
  let firstName: String
  let lastName: String
  let age: Int
  var fullName: String { firstName + " " + lastName }
}

The advantage in doing this is that we do not need to store the fullName. It will always be readily available to us from the firstName and the lastName.

struct User {
  let user = User(firstName: "Joe", lastName: "Schmuck", age: 42)
  print(user.fullName)  // Prints "Joe Schmuck"
}

To explain this further, we would now like to know if our user is an adult. A user can be considered an adult if they are 18 years or above. Sure, the caller can compute this on their end, but what if this was available as a computed property on the User itself since we already know the age of the user. Let’s add another computed property isAdult in our User struct.

struct User {
  let firstName: String
  let lastName: String
  let age: Int
  var fullName: String { firstName + " " + lastName }
  var isAdult: Bool { return age >= 18}
}
struct User {
  let user = User(firstName: "Joe", lastName: "Schmuck", age: 42)
  print(user.isAdult)  // Prints "true"
}

To sum it up, computed properties are a powerful tool that give us the ability to add additional logic to our code. It makes our code concise and easier to follow along.