What is Protocol Oriented Programming in Swift? What added value does it bring?

Preface: POP and OOP are not mutually exclusive. They're design paradigms that are greatly related.

The primary aspect of POP over OOP is that is prefers composition over inheritance. There are several benefits to this.

In large inheritance hierarchies, the ancestor classes tend to contain most of the (generalized) functionality, with the leaf subclasses making only minimal contributions. The issue here is that the ancestor classes end up doing a lot of things. For example, a Car drives, stores cargo, seats passengers, plays music, etc. These are many functionalities that are each quite distinct, but they all get indivisibly lumped into the Car class. Descendants of Car, such as Ferrari, Toyota, BMW, etc. all make minimal modifications to this base class.

The consequence of this is that there is reduced code reuse. My BoomBox also plays music, but it's not a car. Inheriting the music-playing functionality from Car isn't possible.

What Swift encourages instead is that these large monolithic classes be broken down into a composition of smaller components. These components can then be more easily reused. Both Car and BoomBox can use MusicPlayer.

Swift offers multiple features to achieve this, but the most important by far are protocol extensions. They allow implementation of a protocol to exist separate of its implementing class, so that many classes may simply implement this protocol and instantly gain its functionality.

It surprised me that none of the answers mentioned value type in POP.

To understand what is protocol oriented programming, you need to understand what are drawbacks of objected oriented programming.

1. It (Objc) has only one inheritance. If we have very complicated hierarchy of inheritance, the bottom class may have a lot of unnecessary state to hold.
2. It uses class which is a reference type. Reference type may cause code unsafe. e.g. Processing collection of reference types while they are being modified.

While in protocol oriented programming in swift:

1. It can conform multiple protocols.
2. It can be used by not only class, but also structures and enumerations.
3. It has protocol extension which gives us common functionality to all types that conforms to a protocol.
4. It prefers to use value type instead of reference type. Have a look at the standard swift library here, you can find majority of types are structures which is value type. But this doesn't mean you don't use class at all, in some situation, you have to use class.

So protocol oriented programming is nothing but just an another programming paradigm that try to solve the OOP drawbacks.

In Objective C protocol is the same thing as interface in most languages. So in Objective C protocol's usage is limited to SOLID principle "Depend upon Abstractions. Do not depend upon concretions."

In Swift protocols were improved so seriously that since they still could be used as interfaces in fact they are closer to classes (like Abstract classes in C++)

In Objective C the only way to share functionality between classes is an inheritance. And you could inherit the only one parent class. In Swift you could also adopt as many protocols as you want. And since protocols in Swift can have default methods implementation they give us a fully-functional Multiple inheritance. More flexibility, better code reuse - awesome!

Conclusion:

Protocol Oriented Programming is mostly the same as OOP but it pays additional attention to functionality sharing not only via inheritance but also via protocol adoption (Composition over inheritance).

Worth to mention that in C++ abstract classes are very similar to protocols in Swift but no one says C++ supports some specific type of OOP. So in general POP is a one of the versions of OOP if we speak about programming paradigms. For Swift POP is an improved version of OOP.