Merge pull request #8308 from woocommerce/issue/2228-docs-final-classes

Document that classes should be final by default

Author: iamgabrielma

docs/choosing-between-structs-and-classes.md

@@ -10,3 +10,11 @@ But consider using `class` instead if:
 - You need to manage mutable states. Especially if there are more than a few `mutating` functions, the `struct` becomes harder to reason about.
 - You have to set a `struct` property declaration as `var` because it has a `mutating` function. In this case, a constant (`let`) `class` property may be easier to reason about.
 
+### Classes should be marked as `Final` by default
+
+When using classes, these should be marked as `final` by default so their behavior cannot be altered by subclassing and/or overriding. There's a few reasons for that:
+
+- It favors composition over inheritance
+- The class could be missused or abused when subclassed or overriden, which could be problematic if the class is doing anything important. By making it `final` we disallow this at compiler time, and enforce that the class has to be used as it was written.
+- Marking a class as `final` tells the Swift compiler that the class methods should be called directly rather than looking them up in a method table (static vs dynamic dispatch), which [reduces function call overhead and increases performance](https://developer.apple.com/swift/blog/?id=27)
+- If inheritance is necessary later on, it can always be allowed, but the `final` modifier will create a reminder of its necessity and refactoring