Overriding methods in Swift extensions

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Extensions cannot/should not override.

It is not possible to override functionality (like properties or methods) in extensions as documented in Apple’s Swift Guide.

Extensions can add new functionality to a type, but they cannot override existing functionality.

Swift Developer Guide

The compiler is allowing you to override in the extension for compatibility with Objective-C. But it’s actually violating the language directive.

😊That just reminded me of Isaac Asimov’s “Three Laws of Robotics” 🤖

Extensions (syntactic sugar) define independent methods that receive their own arguments. The function that is called for i.e. layoutSubviews depends on the context the compiler knows about when the code is compiled. UIView inherits from UIResponder which inherits from NSObject so the override in the extension is permitted but should not be.

So there’s nothing wrong with grouping but you should override in the class not in the extension.

Directive Notes

You can only override a superclass method i.e. load() initialize()in an extension of a subclass if the method is Objective-C compatible.

Therefore we can take a look at why it is allowing you to compile using layoutSubviews.

All Swift apps execute inside the Objective-C runtime except for when using pure Swift-only frameworks which allow for a Swift-only runtime.

As we found out the Objective-C runtime generally calls two class main methods load() and initialize() automatically when initializing classes in your app’s processes.

Regarding the dynamic modifier

From the Apple Developer Library (archive.org)

You can use the dynamic modifier to require that access to members be dynamically dispatched through the Objective-C runtime.

When Swift APIs are imported by the Objective-C runtime, there are no guarantees of dynamic dispatch for properties, methods, subscripts, or initializers. The Swift compiler may still devirtualize or inline member access to optimize the performance of your code, bypassing the Objective-C runtime. 😳

So dynamic can be applied to your layoutSubviews -> UIView Class since it’s represented by Objective-C and access to that member is always used using the Objective-C runtime.

That’s why the compiler allowing you to use override and dynamic.

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