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Classes – The Basics

Classes in Swift are something you will be using a lot if you want to really get to grips with creating applications. I suggest that if you do not know what Object Oriented Programming methodology is that you at least read up on the basics before starting any serious development, there is a highly regarded book by Erich Gamma on the subject titled Design Patterns: Elements of Reusable Object-Oriented Software.

Classes in Swift are simple to create and use, this is I feel an improvement over Objective-C where at times it felt harder than it needed to be due to the syntax involved.

To follow along I suggest creating a playground so you can instantly see the results.

Creating a Class

To create a new Class we use the class keyword followed by the name we want to use for the class (this is also what you should use for the name of the file when creating real classes).

For the Objective-C users out there notice that we do not have to specify a base class and we do not need to use header classes, this will either make you cry or cheer 🙂

Now at the moment the class does absolutely nothing, it’s useless! To store data in our classes we use something called properties, these are really just variables inside the class. To brush up on variables in Swift I suggest reading my post on the subject.

Adding a Property

So lets add a property to our class. We want our class to know how many legs the pet has, so we add what is called a stored property, as the name suggests it simply stores a value. Just for reference we can also use constants by using the let keyword.

There is another kind of property called a computed proerty, this kind of property generates a value when it is called. For our class let’s add a description which is of type String using a computed property. To get the value of description we use the OOP idea of a getter to unsurprisingly get the property. For those new to Object Oriented Programming, a quick note that we get and set properties in a class, we DO NOT access them directly. If you even try and access them directly the programming police will come and arrest you! It’s considered VERY bad Karma 🙂

When a property only has a get and now set it is considered to be read only, this is one way you can protect that property from changing outside of the class. As of this post Swift does not yet have things like private, protected or public properties. You can actually remove the get {} entirely if you wish and the end result would be the same, however I think it is more readable for those learning to clearly see the get and set.

Initialize or ‘Use’ our new Class

Now to use our new class, this is called initializing the class. To do so we simply need to create a variable that will hold the instance of the class, you will immediately realize that this is the same as using any of Swift’s prebuilt classes.

So what happened here is we now have a new instance called somePet that by default has no legs. So don’t expect to play ball unless you want to end up chasing the ball yourself!

A couple of things I want you to take note of here. Firstly, Swift’s type inference is smart enough to realize that our new instance is of a type MyPet and that ARC (Automatic Reference Counting) is taking care of the memory allocation for us, yay for ARC!

Now let us check that description to query how many legs our pet has incase we already forgot

It’s that simple.

But what if you want to change the number of legs, maybe your pet mutated or evolved over night like a Gremlin. So let’s change the number of legs of our existing instance.

Why on Earth does your pet have 5 legs? OK OK, most of us prefer cats or dogs, but whatever you have is your problem to clean up after. But what if we want to set the number of legs when the instance is created? To do that we need to add some parameters to our class initialization. While we are at it, it’s only right our pets are given names so lets add that as a parameter as well. To achieve the goal we add our own init() method (Methods are what we call functions when inside a class) and add the arguments there.

And finally to complete our basic example we will add a method to make our pet go to sleep.

So that’s it for the basics of creating classes in Swift. In a future post we will look at some of the more interesting things you can do like subclasses and observers.