AbstractA.swift

protocol A {
    func doSomething() -> String
}

// AbstractA
extension A {
    func doSomething() -> String {
        return "Hello from AbstractA"
    }
}

class B : A {
    
}

let b = B()
print(b.doSomething())

protocol A {
func value: String { get }
func letSubClassImplement()
func doSomething() -> String
}

// AbstractA
extension A {
func doSomething() -> String {
return "Hello from AbstractA"
}
}

class B : A {
func value: String {
get {
return "some value"
}
}
func letSubClassImplement() {
// sub class implementation
}
}

let b = B()
print(b.doSomething())