Coding Explorer Blog

Welcome to part 2 of the Swift Extensions posts.  If you haven’t already, check out the previous post Swift Extensions Part 1 — Computed Properties and Initializers, to learn about those aspects and get an introduction to what Swift extensions are, and why you might want to use them.  The short reason is they let you extend a type for which you may not have the source code for, and you may want to do so to make working with that type easier in your own code.

Are the initializers in the main type long with a bunch of options that are unnecessary for your code?  Add your own convenience initializer that only takes the parameters you need to change, and assigns defaults to the parameters you don’t care about in the real type’s initializers.  I was making several different UIColor objects in one of my apps, and they never needed alpha changed, so I just hard-coded that to be 1.0 in an Objective-C category (the precursor to Swift extensions).  Of course, this is only really helpful if you call the longer code several times, which was the case for my app.
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So, there has been a reason for my most recent choice of topics to discuss.  This isn’t some grand culmination post, but the topic for today is related to several of the previous topics due to what it can do.  Back in Objective-C, you could extend classes for which you did not have the source code with something called categories.  If you are curious about Objective-C categories, I wrote about them back when this blog was still about Objective-C in my post Objective-C Categories.  This post was much earlier in my blogging career, so it may not be as polished as my current posts, so, you have been warned.

Anyway, Objective-C Categories gave you the ability to add additional methods to another class for which you didn’t have the original source code.  I used this capability in a later Objective-C post Introduction to UIColor, to create a category that made it easier to create a UIColor for my purposes.  I didn’t need to customize alpha (transparency) each time, and I wanted to use color values between 0 and 255 instead of 0.0 and 1.0, so I made a category that translated my 0 to 255 values to their 0.0 to 1.0 equivalents, and hard-coded the alpha to 1.0 (fully opaque).

These are not the same as iOS 8 extensions, such as the Today View Widgets, UIActivityViewController sharing extensions, etc.  We will cover those in a later post.

When you look at the capabilities of Swift Extensions, they are basically doing the same thing as Objective-C categories, adding methods to existing classes.  With one exception, they are all different flavors of methods.
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We touched a bit on this in a previous post Classes In Swift — An Introduction.  To recap what was mentioned in that post, methods are functions that are associated with a type.  In other words, all methods are functions, but not all functions are methods.  Functions can be written outside of the context of a type, especially in playgrounds.  I felt it would be best to state that in a post dedicated to them, instead of having it just nestled in the Classes post.  I also have learned of some nuances about them, that there are a few differences between them, particularly in parameter naming.  While I’m at it, we’ll quickly cover the idea of type methods as well.
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