I am copy/pasting my comment from this article sumarizing a post on reddit:

I really don't think "Protocol Orientation" is something new... You've got protocols in Obj-C, you've got Interfaces in Java, Abstract Classes in C++, etc.

You can segregate interfaces and avoid inheritance by replacing it with composition in any OO language...

Please help me understand what makes Protocol Orientation different or new. It seems to be more of a buzz-word coined by Apple to maybe push developers into being aware and respecting SOLID principles in their apps... By no means a bad thing, but also not something new.

What is Protocol Oriented Programming?

Is it a new programming paradigm that has the potential to be as revolutionary as structured programming and abolish the dangerous "goto" of OOP?

Or just a way of enforcing good, solid respecting design into OOP code bases?

And since nothing is ever a silver bullet, what are the pitfalls of POP?

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    What is "The dangerous 'goto' of OOP?" If you're speaking rhetorically, I think that functional programming has a better chance of replacing that. – Robert Harvey Aug 12 '16 at 16:33
  • Sounds like design-by-contract to me. – CodesInChaos Aug 12 '16 at 16:34
  • @RobertHarvey By that I mean whatever (anti)patterns you can abuse while doing OOP – Lescai Ionel Aug 12 '16 at 16:35
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    Most so-called "software patterns" are really just workarounds for weaknesses in the programming language, especially OOP patterns. – Robert Harvey Aug 12 '16 at 16:37

Apple Inc. calls interfaces "protocols." When they say "protocol oriented" they mean programming using interfaces instead of inheritance.

From here: http://www.tutorialspoint.com/objective_c/objective_c_protocols.htm

Objective-C allows you to define protocols, which declare the methods expected to be used for a particular situation. Protocols are implemented in the classes conforming to the protocol.

From Objective-C Succintly:

In Objective-C, a protocol is a group of methods that can be implemented by any class. Protocols are essentially the same as interfaces in C#, and they both have similar goals. They can be used as a pseudo-data type, which is useful for making sure that a dynamically-typed object can respond to a certain set of messages. And, because any class can “adopt” a protocol, they can be used to represent a shared API between completely unrelated classes.

From Objective-C for Absolute Beginners:

Apple defines a protocol simply as a list of methods declarations, unattached to a class definition. The methods listed for protocols are suppose to be implemented by you.

So, protocol oriented programming is what Objective-C and Swift programmers call what the rest of us call favoring implementing interfaces over extending classes (inheritance).

Is it a new programming paradigm that has the potential to be as revolutionary as structured programming and abolish the dangerous "goto" of OOP?

No. It's nothing new, just a different name used in the Apple developer community. Composition over inheritance has tremendous advantages though that have been discussed extensively in this forum.

And since nothing is ever a silver bullet, what are the pitfalls of POP?

At first none came to my mind, but I just found this in a forum in CodeRanch, postes by user Jim Yingst:

One disadvantage of interfaces is that one you publish them to other coders outside your own control, they represent a public commitment which can be difficult to change later. You don't know who else is using the interface and you don't know how, so you can't just add a new method to the interface without risking breaking the code of existing clients (if/when they upgrade). In comparison, if you'd used an abstract or concrete class, you could just add a new concrete method with no problems (usually).

It's possible to work around this by creating a new interface with extends or supplements the original one, and changing your concrete implementations to implement the new interface, instead of or in addition to the old one. That way you don't break existing client code. It's generally more work than adding a method to a class would be, but it's possible.

This disadvantage isn't a reason to abandon interfaces, but it is a reason to design your interfaces carefully, and think carefully before you publish a new interface in a new release. Spending some extra time designing things well now can save you many headaches later.

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    That's the same impression I got from reading about it. The OP does also ask for pitfalls of interfaces, do you think you could add a couple or link to a different question about it for good measure? – Ordous Aug 12 '16 at 17:08
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    This is exactly the right answer. I, as an Apple-systems dev, have no trouble finding lots of existing information about "protocol-oriented programming". I just mentally translate interface IDingbat to protocol Dingbat. – Josh Caswell Aug 12 '16 at 18:20
  • @Ordous I added the pitfalls. – Tulains Córdova Aug 12 '16 at 19:00
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    They had to invent new terminology, otherwise it would be too much like C#. – Den Aug 16 '16 at 8:12
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    @Den That reminds me of how people complained in 1936 about lambda calculus for being insufficiently C-like and in 1970 about Pascal's syntax for also being insufficiently C-like, despite C not being invented until 1972. (NeXT, not Apple, added protocols to Objective-C and NeXT ceased to exist two years before Microsoft started forming the C# team.) – 8bittree Sep 6 '16 at 16:28

On big difference between a Swift/Objective-C protocol and a Java-like interface is that the methods in the former can have default implementations. That's not something you can do with a Java Interface. It's almost like having C++ abstract classes and multiple inheritance.

Is POP new? As mentioned already, Objective-C has protocols too, so calling it a "new thing" is a little bit of a stretch. However, what is new to Swift is the ability to implement these protocols using value types rather than reference types and thats the big deal that was featured in Apple's presentation where POP was introduced.

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    Actually Java has default methods in interfaces since v8 and C# is getting it as well. – qbd Oct 8 '18 at 8:18

I did not know the term, being from the Microsoft world. But having read Tulains's excellent answer I can cover the pitfalls part.

It is not like POP (interfaces) are better than OOP (inheritance), they both have their purposes and strengths. An interface is looser and more flexible. Inheritence ties you to the trunc of a particular class tree but it offers you free implementations of the basics, structure and guidance. If inheritance is done well, it is hard for the subclassing programmer to screw things up.

But the most important reason for the rise of POP is probably the relatively new, distributed, networked world with many developers, that do not know each other, potentially working on the same project. It fits the model of a developing community and it can cross process and/or programming language bounderies (SOA). Inheritance is meaningful only within a single project compiled by the same compiler. Which does not invalidate it, it just has a different domain.

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