A Kotlin Style .copy Function for Swift Structs

Working in Android using Kotlin, we tend to create classes with immutable fields. This is quite nice when creating state objects, as it prevents parts of the code that interpret state (for rendering purposes, etc) from modifying the state. This lends to better clarity about where values originate from, less bugs, and easier focused testing.

We use Kotlin’s data class to create immutable objects. If we need to overwrite existing field values in one of our immutable objects, we use the data class’s .copy function to set a new value for the desired field while preserving the rest of the values. Then we’d store this new copy of the object as the source of truth.

While trying to bring this immutable object concept to our iOS codebase, I discovered that Swift’s struct isn’t quite as convenient as Kotlin’s data class because Swift’s struct doesn't have a similar copy function. To adopt this immutability pattern in Swift, you’ll have to write quite a lot of boilerplate code. 

Initializing a New Copy of the Struct

If you want to change one or more properties for a given struct, but preserve the other property values (as Kotlin’s data class provides), you’ll need an initializer that allows you to specify all the struct’s properties. The default initializer gives you this ability… until you set a default value for a property in the struct or define your own init. Once you do either you lose that default init provided by the compiler.

So the first step is defining an init that captures every field value.

Overriding Specific Property Values

Using the init function above, you take your current struct and set every field to the current value, except the values you want to overwrite. This can get cumbersome, especially when your struct has numerous properties, or contains properties that are also structs.

So the next step is to define a .copy function that accepts new values for its properties, but defaults to using the current values unless specified. The copy function takes optional parameter values and defaults all params to nil unless specified. If the param is non-nil, it sets that value in the new copy of the struct, otherwise it defaults to the current state’s value for the field.

Not So Fast, What About Optional Properties?

That works pretty well… until you have a struct that has optional fields. Then things don’t work as expected. What about the case when you have a non-nil value set for an optional property, and you want to set it nil. Uh-oh, the .copy function will always default to the current value when it receives nil for a param.

What if rather than make the params in the copy function optional, we just set the default value to the struct’s current value? That’s how Kotlin solves this problem in its data class, in Swift it looks like this:

Unfortunately in Swift you can’t reference self in default parameter values, so that’s not an option. I needed an alternate solution. 

An Alternate Solution: Using a Builder

I found a good solution on Stack Overflow: using a functional builder pattern to capture the override values for the new copy of the struct, while using the original struct’s values as input for the rest of the properties.

This works a little differently, as instead of a simple copy function that accepts params for our fields, we instead define a closure that receives the builder as the sole argument, and allows you to set overrides for selected properties.

And voilà, it’s not quite as convenient as Kotlin’s data class and its copy function, but it’s pretty close.

Sourcery—Automating All the Boilerplate Code

Using the Sourcery code generator for Swift, I wrote a stencil template that generates an initializer for the copy function, as well as the builder for a given struct:

Scott Birksted is a Senior Development Manager for the Deliver Mobile team that focuses on Order and Inventory Management features in the Shopify Mobile app for iOS and Android. Scott has worked in mobile development since its infancy (pre-iOS/Android) and is passionate about writing testable extensible mobile code and first class mobile user experiences.

We're always on the lookout for talent and we’d love to hear from you. Visit our Engineering career page to find out about our open positions.