SwiftWhen

What is SwiftWhen?

SwiftWhen is a syntax sugar that imitates the When Expression in Kotlin, as an addition to control flow.

Example

let isFreeUser: Bool = ...
let hasEnoughPoints: Bool = ...
let label = when {
    isFreeUser => "Please upgrade to premium user"
    hasEnoughPoints => "Click to redeam"
} ?? "Not enough points"

Installation

SwiftPackageManager

dependencies: [
    .package(url: "https://github.com/lionhylra/SwiftWhen.git", .upToNextMajor(from: "1.0.0"))
]

Documentation

When expression allows you to evaluate multiple confitions(boolean values) and map them to a target value. A custom operator => is used to construct a single mapping.

Boolean Expression => Value

You can put multiple expressions in the when function. The evaluation goes from top to bottom. The first map whose condition is true get selected and its value is returned as the return value of when function. If a mapping is selected, other mappings after that won't be evaluated.

let lastName: String? = ...
let gender: Gender = ...
let greetings = when {
    lastName == nil => "Hi there"
    lastName!.count > 5 => Hi \(lastName!)
    gender == .male => "Hi Mr \(lastName!)"
    gender == .female => "Hi Ms \(lastName!)"
}!

when expression always return an optional value. If no condition is met, when expression returns nil.

let num = -1
let text = when {
    num > 0 => "positive"
    num == 0 => "zero"
}
// text == nil

In a mapping, you can use bool value, bool expression or a function that returns bool as condition. You can also use a closure as a result value.

let n: Int = ...
let text = when {
    n > 0 => "a"
    n == 0 => { "b" }
    n < 0 => { () -> String in 
        let a = "a"
        let b = "b"
        return a + " " + b
    }
}

When to use it

when expression should not be used to replace if statement or switch statement. It can help improve the readability when it requires more lines to write using if of switch. It is convenient you have more than two conditions where swift ternary conditional operator is not enough, especially when you just want to return a value instead of doing a lot.