Among the new features that Swift offers to Objective-C programmers is one that disguises itself like a boring old man while it offers huge potential for forming elegant solutions to otherwise unwieldy sets of nested branches. I'm of course talking about the switch{.verbatim} statement that many Objective-C programmers probably consider as a clunky syntax device that is most entertaining when used as Duff's Device, but usually offers little to no advantages over multiple if statements.

In this short tutorial, I will try to give you a deeper introduction into the Swift Switch statement. I'll ignore these use cases, where switch is effectively being used just like the Objective-C namesake.

The Switch statement

If you read my explanation of how optionals work, you'll already have seen how the Switch statement can easily be used to unwrap an Optional. If you haven't, you should. Nevertheless, let me start this post with a small reminder of how this works:

var result: String? = secretMethod()
switch result {
case .None:
    println("is nothing")
case let a:
    println("\(a) is a value")
}

As you can see, result{.verbatim} could be a string, but it could also be nil. It's an optional. By switching on result, we can figure out whether it is .None{.verbatim} or whether it is an actual value. Even more, if it is a value, we can also bind this value to variable right away. In this case a{.verbatim}. What's beautiful here, is the clearly visible distinction between the two states, that the variable result{.verbatim} can be in.

Breaking down data

Oftentimes, when you get data from an external source, like a library, or an API, it is not only good practige but usually even required that you check the data for consistency before interpreting it. You need to make sure that all keys exists or that the data is of the correct type, or the arrays have the required length. Not doing so can lead from buggy behaviour (missing key) to crash of the app (indexing non-existent array items). The classic way to do this is by nesting if{.verbatim} statements.

Let's imagine an API that returns a user. However, there're two types of users: System users - like the administrator, or the postmaster - and local users - like "John B", "Bill Gates", etc. Due to the way the system was designed and grew, there're a couple of nuisances that API consumers have to deal with:

• system{.verbatim} and local{.verbatim} users come via the same API call.
• the department{.verbatim} key may not exist, since early versions of the db did not have that field and early employees never had to fill it out.
• the name{.verbatim} array contains either 4 items (username, firstname, middlename, lastname) or 2 items (username, full name) depending on when the user was created.
• age may be a string, in which case it is the date when the user was born, or a number, in which case it is the current age¹.

// Example:
// may return: {type: @"system", department: ..., age: ..., name: [username, firstname, middlename, lastname]}
NSArray *user = [api userWithId:4]; 

Let's try parsing such a dictionary in the traditional way. I'm using Objective-C examples as this will better display just how much more concise the Swift / Switch way is.

// Example:
// may return: {type: @"system", department: ..., age: ..., name: [username, firstname, middlename, lastname]}
NSArray *user = [api userWithId:4]; 
if ([user["type"] isEqualToString: @"system"]) {
}