How do we provide default implementations for protocol methods?

Sources & Resources

Main Source: 🔗 Ace the iOS Interview

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Further Reading:

TL/DR

By leveraging Swift extensions, we can provide a default implementation for methods and properties declared in a protocol.

This helps reduce boilerplate and duplicated code in classes that implement the protocol while still allowing them to easily override the default implementation.

protocol Animal {
func makeNoise()
}

extension Animal {
func makeNoise() {
print("Bark!")
}
}

struct Dog: Animal {}
struct Cat: Animal {
func makeNoise() {
print("Meow!")
}
}

let sparky = Dog()
sparky.makeNoise() // Bark!

let whiskers = Cat()
whiskers.makeNoise() // Meow!

As you can see, we’ve provided the default implementation for makeNoise()in an extension.

Dog is using the default implementation while Cat is free to provide a more specific implementation.

This same approach allows us to make certain functions in our protocol optional. Since we have provided a default implementation in our extension, any entity that implements the protocolis no longer required to implement it.

protocol MyProtocol {
    func doSomething()
}

extension MyProtocol {
    func doSomething() {
        /* Return a default value or just leave empty */
    }
}

struct MyStruct: MyProtocol {
    /* No compile error */
}

Alternatively, you can use @objc optional to make functions within your protocol optional. This would, however, restrict your protocol to only be implemented by class type objects, which would prevent your protocol from being used by structs, enums, etc. You’d also need to explicitly check if that optional method is implemented before you call it.