Either Pattern
How do you extend a class to support a new type? In following code, how would you extend class A to take TypeB instead of TypeA?
class A {
let type: TypeA
var title: String {
type.name
}
func ready() {
type.doThing()
}
}
class TypeA {
let name: String
func doThing() {
}
}
One approach is to inject dependency. We can update concrete dependency with a protocol. Now any type confirming to a protocol can be used in class A.
protocol ThingType {
var name: String { get }
func doThing()
}
class A {
let type: ThingType
var title: String {
type.name
}
func ready() {
type.doThing()
}
}
class TypeA: ThingType {
let name: String
func doThing() {
}
}
What if, you can't inject your dependencies? Like you can't construct an object or constructor is dependent on the properties/state of the class A. What other options do you have? Either is very powerful pattern you can use in cases like this.
// This can be extended to as many cases you need.
public enum Either<A, B> {
case a(A)
case b(B)
}
public class A {
let type: Either<EitherTypeA, EitherTypeB>
public init(with rawType: String, name: String) {
// Made up scenario to show dependencies are not injected.
if rawType == "a" {
self.type = Either.a(EitherTypeA(name: name))
} else {
self.type = Either.b(EitherTypeB(name: name))
}
}
public func ready() {
type.doThing()
}
}
public class EitherTypeA {
public let nameA: String
public init(name: String) {
self.nameA = name
}
public func doAThing() {
print("Doing \(nameA) thing")
}
}
public class EitherTypeB {
public let nameB: String
public init(name: String) {
self.nameB = name
}
public func doBThing() {
print("Doing \(nameB) thing")
}
}
/// Property and function signatures doesn't need to be identical. We can map them as needed.
extension Either where A == EitherTypeA, B == EitherTypeB {
var name: String {
switch self {
case .a(let typeA): return typeA.nameA
case .b(let typeB): return typeB.nameB
}
}
func doThing() {
switch self {
case .a(let typeA): typeA.doAThing()
case .b(let typeB): typeB.doBThing()
}
}
}
If you have to work with Class A without Either pattern. Class A will be full of switch-cases/if-else and optional properties.
Here is a playground with all the above scenarios.
Cheers, Stacked Heaps
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