Understanding Protocol Oriented programming in Swift

WWDC2015 was all about Protocol oriented programming. How is it different?

Lets see.

Inheritance

Inheritance is widely used and loved by lot. But people have started talking against Inheritance

Consider a simple example:

class Shape {
    var color: String!
    func area() -> Double {
        fatalError("This method is abstract & must be overridden before using")
    }
}

class Circle: Shape {
    var radius: Double! = 10.4
    override func area() -> Double {
        return radius * radius * M_PI
    }
}

class Square: Shape {
    var sideLength: Double! = 10.0
    override func area() -> Double {
        return sideLength * sideLength
    }
}

Protocol Oriented Solution

Lets reimplement above code using Protocols

protocol Shape {
    var color: String { get set}
    func area() -> Double
}

struct Circle: Shape {
    var color: String
    var radius: Double
    func area() -> Double {
        return radius * radius * M_PI
    }
}

struct Square: Shape {
    var color: String
    var sideLength: Double
    func area() -> Double {
        return sideLength * sideLength
    }
}

var arr = [Shape]()
arr.append(Circle(color: "Red", radius: 10.0))
arr.append(Square(color: "Green", sideLength: 50))

Benefits of protocol oriented code

Code reuse in Value types

One of the basic difference between Class and struct is that classes can inherit while structs cannot. Swift clearly is promoting value type over reference type. So protocols can be used to make your code more reusable in Value-type environment.

Clarity

The structures in above example more clearly state their properties and methods. There is no hidden property/method.

But if you conform to a lot of protocols, it makes your class definition very long. But you could always use Object composition

Conform to multiple protocols

One of the drawbacks of inheritance is there cannot be multiple superclasses(This design is discouraged although). But on the contrary a Struct/Class can conform to multiple protocols