In Swift, value types are preferred over classes. However, object-oriented concepts don’t work well with structs and enums: a struct cannot inherit from another struct, neither can an enum inherit from another enum.
On the other hand, value types can inherit from protocols, even multiple protocols. Thus, with POP, value types have become first-class citizens in Swift.
Pillars of POPs
Protocol Extensions
Protocols serve as blueprints: they tell us what adopters shall implement, but you can’t provide implementation within a protocol. What if we need to define default behavior for conforming types? We need to implement it in a base class, right? Wrong! Having to rely on a base class for default implementation would eclipse the benefits of protocols. Besides, that would not work for value types. Luckily, there is another way: protocol extensions are the way to go! In Swift, you can extend a protocol and provide a default implementation for methods, computed properties, subscripts, and convenience initializers. In the following example, I provided default implementation for the type method uid().
extension Entity {
static func uid() -> String {
return UUID().uuidString
}
}
swift
Now types that adopt the protocol need not implement the uid() method anymore.
struct Order: Entity {
var name: String
let uid: String = Order.uid()
}
let order = Order(name: "My Order")
print(order.uid)
// 4812B485-3965-443B-A76D-72986B0A4FF4
Protocol Inheritance
A protocol can inherit from other protocols and then add further requirements on top of the requirements it inherits. In the following example, the protocol Persistable inherits from the Entity protocol I introduced earlier. It adds the requirement to save an entity to file and load it based on its unique identifier.
protocol Persistable: Entity {
func write(instance: Entity, to filePath: String)
init?(by uid: String)
}
swift
The types that adopt the Persistable protocol must satisfy the requirements defined in both the Entity and the Persistable protocol.
If your type requires persistence capabilities, it should implement the Persistable protocol.
struct PersistableEntity: Persistable {
var name: String
func write(instance: Entity, to filePath: String) { // ...
}
init?(by uid: String) {
// try to load from the filesystem based on id
}
}
swift
Whereas types that do not need to be persisted shall only implement the Entity protocol:
struct InMemoryEntity: Entity {
var name: String
}
Protocol inheritance is a powerful feature that allows for more granular and flexible designs.
Protocol Composition
Swift does not allow multiple inheritances for classes. However, Swift types can adopt multiple protocols. Sometimes you may find this feature useful.
Here’s an example: let’s assume that we need a type that represents an Entity.
We also need to compare instances of a given type. And we want to provide a custom description, too.
We have three protocols that define the mentioned requirements:
Entity
Equatable
CustomStringConvertible
If these were base classes, we’d have to merge the functionality into one superclass; however, with POP and protocol composition, the solution becomes:
struct MyEntity: Entity, Equatable, CustomStringConvertible {
var name: String
// Equatable
public static func ==(lhs: MyEntity, rhs: MyEntity) -> Bool {
return lhs.name == rhs.name
}
// CustomStringConvertible
public var description: String {
return "MyEntity: \(name)"
}
}
let entity1 = MyEntity(name: "42")
print(entity1)
let entity2 = MyEntity(name: "42")
assert(entity1 == entity2, "Entities shall be equal")
This design not only is more flexible than squeezing all the required functionality into a monolithic base class but also works for value types.
