Switches & Cases

A few days ago I was reviewing a pull request and came across the following code:

private func updateInsets() {
    if currentImage != nil && currentTitle == nil {
        contentEdgeInsets = Layout.contentEdgeInsets.image
    else if currentImage == nil && currentTitle != nil {
        contentEdgeInsets = Layout.contentEdgeInsets.title
    } else if currentImage != nil && currentTitle != nil {
        titleEdgeInsets = Layout.titleEdgeInsets.imageWithTitle
        contentEdgeInsets = Layout.contentEdgeInsets.imageWithTitle
    }
}

It was a piece of code that was really hard to parse for me. It also could be refactored to be exhaustive, adding some safety, but that was less of a concern as opposed to readability.

I came up with the following:

func switcher(_ a: Int?, _ b: Int?) {
    switch (a, b) {
    case (nil, nil):
        print("nothing")
    case (let thing?, nil):
        print("lhs", thing)
    case (nil, let thing?):
        print("rhs", thing)
    case (let left?, let right?):
        print(left, right)
    }
}

Don’t mind the naming; the point is switching on a tuple, where both arguments are optional. This - in my opinion - improves the readability and gives us some compile-time safety.

As in our use case we didn’t need to use the values, this is what we ended up with:

private func updateInsets() {
    switch (currentImage, currentTitle) {
    case (.some, .none):
        contentEdgeInsets = Layout.contentEdgeInsets.image
    case (.none, .some):
        contentEdgeInsets = Layout.contentEdgeInsets.title
    case (.some, .some):
        titleEdgeInsets = Layout.titleEdgeInsets.imageWithTitle
        contentEdgeInsets = Layout.contentEdgeInsets.imageWithTitle
    case (.none, .none):
        break
    }
}

Neat!

---

And now that we’re talking about switches and cases, there’s also the if case syntax. This can be useful in case (no pun intended) you want to only act differently for a specific case.

This was the example in question:

switch message.transmissionState {
case .failed:
    print("oops!")
default:
    print("all good!")
}

As you can see, this is pretty much like an if-else… without the if and else. To make that intent more clear, we use if case:

func doSomething(with messageState: Message.State) {
    if case .failed = messageState {
        print("oops!")
    } else {
        print("all good!")
    }
}

That’s better.

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I really like these easy wins that improve readability and better convey intent. I would love to hear from you if you have run into something similar recently. You can reach out to me on Twitter: @basthomas.