Updated for Xcode 13.3
Extensions let us add functionality to classes, structs, and more, which is helpful for modifying types we don’t own – types that were written by Apple or someone else, for example. Methods added using extensions are indistinguishable from methods that were originally part of the type, but there is a difference for properties: extensions may not add new stored properties, only computed properties.
Extensions are also useful for organizing our own code, and although there are several ways of doing this I want to focus on two here: conformance grouping and purpose grouping.
Conformance grouping means adding a protocol conformance to a type as an extension, adding all the required methods inside that extension. This makes it easier to understand how much code a developer needs to keep in their head while reading an extension – if the current extension adds support for
Printable, they won’t find printing methods mixed in with methods from other, unrelated protocols.
On the other hand, purpose grouping means creating extensions to do specific tasks, which makes it easier to work with large types. For example, you might have an extension specifically to handle loading and saving of that type.
It’s worth adding here that many folks realize they have a large class and try to make it smaller by splitting it into extensions. To be quite clear: the type is exactly the same size it was before, it’s just neatly split up. This does mean it’s likely to be a little easier to understand, but it doesn’t mean the class is any smaller.
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