A Practical Guide to Computed Variables in Swift Code

How do you make your Swift code smarter and more efficient?

One of the key concepts in Swift programming is the computed property, which is a type of property used to encapsulate logic for dynamically calculating a value instead of directly storing it.

This blog delves deep into computed variables in Swift, explains the mechanics of computed properties and stored properties, and demonstrates how these tools can enhance your programming experience.

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What Are Properties in Swift?

In Swift, properties associate values with instances of a class, structure, or enumeration. Properties are divided into two types:

1. Stored properties: These directly store values within a class or structure.

2. Computed properties: These dynamically compute their value using custom logic rather than storing it directly.

Here’s how they differ in functionality:

• A stored property holds a constant or variable value.

• A computed property calculates its value when accessed and can include optional setters to modify other properties or values indirectly.

Exploring Stored Properties

A stored property is the simplest kind of property. A stored property holds a default or assigned value directly in the memory associated with an instance.

Example of Stored Properties

Swift

1struct Person {
2    var firstName: String
3    var lastName: String
4    var age: Int
5}
6
7let person = Person(firstName: "John", lastName: "Doe", age: 30)
8print(person.firstName) // Outputs: John

​

In this example, firstName, lastName, and age are stored properties. They store values directly for each new instance of the Person structure.

Understanding Computed Properties

A computed property differs from a stored property because it doesn’t directly store a value. Instead, it provides a getter to calculate and return a value, and optionally includes a setter to modify related properties.

Syntax of a Computed Property

A computed property is defined using the var keyword and a getter block. Optionally, it can include a setter block for updating other values.

Swift

1struct Circle {
2    var radius: Double
3    var circumference: Double {
4        get {
5            return 2 * .pi * radius
6        }
7        set(newCircumference) {
8            radius = newCircumference / (2 * .pi)
9        }
10    }
11}
12
13var circle = Circle(radius: 5)
14print(circle.circumference) // Outputs: 31.415
15circle.circumference = 62.83
16print(circle.radius) // Updates radius based on the new value assigned to circumference

​

Here:

• circumference is a computed property.

• It has a getter to calculate the circumference.

• The optional setter updates the radius.

One Computed Property Example

If you only need a read-only computed property, you can omit the set block.

Swift

1struct Square {
2    var sideLength: Double
3    var area: Double {
4        return sideLength * sideLength
5    }
6}
7
8let square = Square(sideLength: 4)
9print(square.area) // Outputs: 16

​

The area is a read-only computed property because it lacks a set block.

Combining Stored and Computed Properties

Often, stored and computed properties coexist in the same class or structure, enabling developers to manage data flexibly. You might have a stored property for input values and a computed property to derive a new result.

Example: Struct Person with Computed Properties

Swift

1struct Person {
2    var firstName: String
3    var lastName: String
4    var fullName: String {
5        return "\(firstName) \(lastName)"
6    }
7}
8
9let person = Person(firstName: "Jane", lastName: "Doe")
10print(person.fullName) // Outputs: Jane Doe

​

Here, fullName is a computed property that derives its value from the firstName and lastName stored properties.

Key Concepts Around Computed Properties

Read-Only Computed Properties

A read-only computed property doesn’t have a setter. It’s ideal for scenarios where you only need to retrieve a derived value without modifying other properties.

Example:

Swift

1struct Rectangle {
2    var width: Double
3    var height: Double
4    var area: Double {
5        return width * height
6    }
7}

​

Getter and Optional Setter

In Swift, a computed property can optionally include a get keyword to define its logic explicitly. This allows for precise control over how the property calculates and returns values.

Using Property Observers

Property observers, such as willSet and didSet, track value changes in stored properties. They don’t apply to computed properties since these don’t directly store values.

Example:

Swift

1struct Counter {
2    var count: Int = 0 {
3        willSet {
4            print("About to set count to \(newValue)")
5        }
6        didSet {
7            print("Count changed from \(oldValue) to \(count)")
8        }
9    }
10}
11
12var counter = Counter()
13counter.count = 10
14// Outputs:
15// About to set count to 10
16// Count changed from 0 to 10

​

Best Practices for Using Computed Properties

1. Minimize complexity: Avoid overly complex calculations in your computed properties to maintain readability.

2. Avoid side effects: A computed property should ideally not alter the state of an instance unexpectedly.

3. Use clear property names: Choose a descriptive property name to make your code intuitive.

Example: A Filename Property

Here’s an example demonstrating a filename property that combines a stored property with a computed property:

Swift

1struct File {
2    var name: String
3    var extensionType: String
4    var filename: String {
5        return "\(name).\(extensionType)"
6    }
7}
8
9let file = File(name: "document", extensionType: "txt")
10print(file.filename) // Outputs: document.txt

​

Wrapping Up!

Understanding a computed variable in Swift can simplify your coding process and improve readability. Comparing how Kotlin and Golang approach similar concepts helps you pick the best tools for your projects. Keep practicing, experimenting, and building amazing code! 💻

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