Storing Constant Values in Swift: Creating a Constants File

Introduction:

This article explores different methods of storing constant values in a shared file in Swift, focusing on using structs and enums.

Using Struct for Constant Values:

To store constant values, we can employ a struct with type properties. In Swift, type properties are accessed using the static keyword. Here’s an example:

struct FontSize {
static let headline = 34
static let subhead = 18
}

Using Caseless Enum for Constant Values:

Alternatively, we can use a caseless enum, which is an enum without cases, solely consisting of static properties. This approach avoids accidental initialization. However, it lacks type safety and allows duplicate values. Example:

enum FontSize {
static let headline = 34
static let subhead = 18
}

Using Enum with Raw Values:

To enhance type safety and prevent duplicate values, an enum with raw values is preferable for storing constants. Example:

enum FontSize: Double {
case headline = 34
case body = 18
}

Leveraging callAsFunction:

Starting from Swift 5.2, the callAsFunction feature allows creating callable values. We can leverage this to access enum values without explicitly using rawValue. Here’s an example:

protocol Functionable {
  associatedtype T
  func callAsFunction() -> T
}

extension Functionable where Self: RawRepresentable {
  func callAsFunction() -> Self.RawValue {
    return rawValue
  }
}

enum FontSize: Double, Functionable {
  case headline = 34
  case body = 18
}

// Accessing enum value without rawValue
let headlineValue = FontSize.headline() 

Conclusion:

By utilizing enums with callAsFunction(), we can create a unique and type-safe approach to store constant values. This helps prevent accidental initialization and duplicates, ensuring code reliability and maintainability.

We welcome your thoughts and suggestions on this approach.

Please refer part 2 of this series where we are using another approach using macros — https://medium.com/@vinodh_36508/part-2-enhancing-constant-value-management-in-swift-enums-using-macros-2f00de2a896f

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