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Hey, guys.
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Welcome to another Swift Deep Dive. In this deep dive,
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We're going to be learning all about Swift Classes,
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what they are, what they can do, and how to create them.
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Now, classes like structures are, again, a way of defining a blueprint. They're a way for us to be able to
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plan our properties and methods. And eventually, when we run our app, be able to initialize our class into
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an actual object.
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So in this lesson, I'm going to talk about how to create an object from a class. But in addition, I also
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want to talk about some of the core concepts of object oriented programming, namely inheritance, subClasses,
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SuperClasses,
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what it means to override a method, and what it means to call a method from a SuperClass.
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So let's create a separate project in Xcode to show these concepts in action.
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I want you to go to File, New, and choose a New Project.
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Now, when you see the wizard, I want you to choose macOS and select the Command Line Tool.
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Now, the reason for this is we're not actually creating an entire app, but we are creating a project which
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can have multiple swift files associated in the same project. Our project name is going to be called
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ClassesDemo and then you can go ahead and click Next, and save it anywhere you like.
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So, this is what you should end up with.
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And notice how we have a main.swift file that already gets created for us. And inside, all we have
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is just a print statement.
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So I want you to delete everything that's currently inside your main.swift. And you'll notice that
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when we run our project this time, what will happen is it will look inside the main.swift file and
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run all the lines of code in here as if it were a playground.
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So if I wrote print "hello," well, then that is what will be run and printed into my debug console
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when my program runs. So we can expand this and we can collapse the right-hand pane. And in fact, I'm also
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going to collapse this left-hand pane,
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so we have a full length debug console.
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So now it looks pretty similar to what we have when we create a playground.
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But in this case, I can actually create multiple files inside the same project,
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my ClassesDemo project.
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So how do we write the code for our class? Defining a class is very similar to defining a struct.
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We have the class keyword and then we have the name of the class which is capitalized from the start,
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and then we have a set of curly braces inside of which we can define the implementation of the class.
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Let's create a new Swift file for a class in our project.
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So here I'm going to right-click and I'm going to create a new file in that folder which is going to
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be a new Swift File, and then hit Next.
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And now, I'm going to name my file the name of the class
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that I'm going to create which is going to be called Enemy.
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So let's imagine that we were building a simple game where we have to create enemy characters for our
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game.
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So the first thing I'm gonna do is I'm going to delete all of the code in here.
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We are going to create our code entirely from scratch. In this enemy.swift file.
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I'm going to create my Enemy class. To create my class,
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I'm gonna use the class keyword, and then I provide the name of my class which is going to be called
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Enemy. And it is convention that when you create a class that the name of the file matches exactly the
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name of your class.
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Now, I'm going to open a set of curly braces to define this Enemy class,
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what it can do, and its properties and its methods. For the Enemy characters in my game,
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they might have a number of attributes or properties, right? Things like, maybe they would have a health
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stats, let's say that they start out with 100 health. And they might also have an attack strength, so this
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is how hard they hit.
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Now, these are the properties of my Enemy class. But I can also describe behaviors that the enemies can
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perform.
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So I would do that by creating a function.
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Let's say that each of my enemy characters can move, so I could have a move function. And when this move
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function is triggered, all that happens is that the enemy characters "Walk forwards."
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Now, we can also define a attack function. And when the enemies attack, what should happen is, maybe they
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would "Land a hit" and it will do some damage depending on the attack strength of this enemy character.
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So I'm going to use string interpolation to insert the attack strength value in here.
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So, for example, a default Enemy character with an attack Strength of 10. When it attacks, it's going to
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land a hit, and does 10 damage. Now that we've defined our blueprint that defines an Enemy character, it's
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time to go into our main.Storyboard and we can initialize our Enemy class here and create an object
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from that class. So to initialize a class,
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all we do is write the name of the class, and then a set of parentheses.
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So in our main.swift, I'm going to create a new Enemy.
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Let's say, it's a little skeleton and I'm going to initialize it as a new Enemy object,
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so the name of the class and then a set of parentheses.
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So now that I've got this enemy object which is a skeleton, I can tap into its properties.
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So, for example, I could print the skeleton.health property. And if I run my app by clicking the
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play button,
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you'll see that the skeleton gets created on line two, and on line 3, I'm printing its health which turns
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out to be 100. And that, of course, comes from the default health property for everything that's created
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from the Enemy class.
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Now, in addition to using the properties, I can also get my skeleton to do something, so I can write
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skeleton.move or skeleton.attack.
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So now if I run my code, you can see that the health is printed as 100, the skeleton now walks
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forwards, it lands a hit and does 10 damage. Just by creating this Enemy class, I can now create lots and
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lots of enemies. I could create more skeletons, so skeleton2 is going to be created again from the Enemy
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class, and then maybe we could have a skeleton3. and instead of having to create each one of them with
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a health stat and attack strength, define its behaviors, I've now used that template or that blueprint,
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the Enemy class, to create three entire skeleton objects which each can move, can attack, have all these properties.
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So as you can imagine, it's a lot easier to simply replicate and airdrop in all of these different enemies
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without having to define each of them from scratch. But in the last module, we saw how to use structs
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or structures. And we could do pretty much the same thing with them, right?
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But that's not where it ends.
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The magic of classes really lies within a special capability that they have which is the ability to
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inherit from a SuperClass.
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So what do I mean by the SuperClass?
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And what does it mean to inherit from a SuperClass?
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Well, the key to understanding this is that we can have a relationship between classes.
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Think of the SuperClass as a parent. It knows how to do certain things and has certain properties and
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methods.
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However, what if we want to take its existing capabilities and build on top of it?
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Well, yeah, we could do this by adding more code to our existing class. But there's also another way. We
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can create a SubClass and add our additional functionality there.
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Think of the SubClass as the child of the SuperClass. The SuperClass is taught the SubClass everything
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it knows how to do,
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for example, reading and swimming. And the SubClass can now grow and develop its own capabilities like
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playing with the iPhone, getting addicted to YouTube, or live streaming on Twitch.
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In other words, the SubClass has access to all the properties and methods of the SuperClass.
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That's what we mean by inheritance.
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Now, let's see inheritance in action in our Xcode project.
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So let's say that I want to have these generic sort of simple enemies like skeletons.
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I don't know why skeletons always get a bad rep. They always seem to be the most easy to destroy enemies
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in any game I've played.
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But let's say that we wanted to have a more powerful enemy and we wanted to leverage the code that we've
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created in our Enemy class,
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but we wanted to make it do more and be more specialized.
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Let's say that in my ClassesDemo, I create a new file, again, a Swift File, and I name this file Dragon.
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So I want to create a Dragon class in this file.
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So I do exactly the same thing as before, but this time I'm going to use the class code snippet that
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Xcode auto-suggest gives me, and I'm gonna hit enter
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once I select it.
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Now, this is the default way that we would create a class, whereas before we created a class that was
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just a simple standalone class.
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In this case, I'm going to use the ability of classes to inherit from other classes.
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Firstly, I'm going to give this class a name which is gonna be my Dragon class and dragons are also a
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Enemy,
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so they're going to inherit from the Enemy class.
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So that means when I create a dragon from the Dragon class, I don't actually have to redefine these properties
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or these methods.
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It already has inherited all of those properties methods. And I can show you by simply going into my
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main.swift and I'm going to delete all of this code here. And this time, I'm going to create a dragon.
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So the dragon is gonna be created from the Dragon class.
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And now with my Dragon class being completely empty other than the inheritance from the Enemy class,
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I'm going to use my dragon object, and I'm going to get it to move and I'm going to get it to attack.
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So now if I go ahead and delete all of this code and simply go ahead and press run, you can see that
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when this line 6 runs, the dragon walks forwards, and it also lands a hit when the attack method runs.
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So without having to lift a finger, essentially, in our Dragon class, we've already managed to inherit
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all of the capabilities in the Enemy class just by inheriting it like so. Now that's not where it ends though,
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the whole reason I created this Dragon class is for it to be able to do something different.
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So let's create a property that is unique to our Dragon class. So maybe it has a wingSpan, and let's set
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it to default value of 2 meters.
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So it's a medium-sized dragon. But when I create my dragon object, I could change that.
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So I could say dragon.wingSpan and let's set it to 5.
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Let's make it a giant dragon.
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So this wingSpan property, of course, does not exist in our default sort of Enemy class.
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I can't say skeleton.wingSpan.
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That doesn't make any sense.
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Skeletons don't have wings.
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So that is a custom property that only exists inside the Dragon class.
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But, of course, the Dragon class also has access to all the properties that the Enemy class has because
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it is inheriting from it.
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So, for example, it has the health property, it has the attackStrength property.
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So let's set the attackStrength property to a bit higher than your average enemy.
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Let's make it 15 instead of 10.
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But in this case, I'm creating a new dragon from the Dragon class and I'm tapping into a property from
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the SuperClass, so the class that we're inheriting from. Now, I also want my Dragon class to be able to
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do something that the default Enemy class can't do.
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Now, it seems to me like from all the movies I've watched, dragons always seem to be able to talk.
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So let's give our dragon the capability of speech by adding a method code "talk," and this method is
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going to take a input code "speech", and that's gonna be a string data type, and then all that it's gonna
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do is just print the words it says whatever the speech happened to be.
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So I'm just going to insert that speech input in here. And now we've got a dragon that talks. So back
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in my main.swift.
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I'm going to say dragon.talk. And the speech that it's going to give is "My teeth are swords! My claws
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are spears! My wings are a hurricane."
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So now we've got a dragon that talks, that moves, that attacks.
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And if I run my code as it is, you can see that my dragon is talking, it's walking for some reason, and
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it's landing a hit doing a 15-point damage. So you can see we've now got this dragon object created from
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the Dragon class which can do everything that the enemy can do, but it can do a lot more.
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So there's some custom things that only the dragon can do that a bog-standard enemy can't.
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But if you look at our code, when the dragon moves, it probably shouldn't walk.
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I think it should probably fly instead of walk,
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right?
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So in addition to defining custom properties and methods, we can also tap into the methods that are available
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to us from the SuperClass,
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so the class that we're inheriting from, and we can override it.
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So if we tap into the move method, you can see that this override keyword goes in front of our normal
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function declaration.
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And this means now that when we trigger the move method, we can say it does something completely different.
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In my case,
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I think that when a dragon moves, instead of walking, it probably should "fly forwards."
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So now if I go back to my main.swift without changing any of this code, I run my code,
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you can see now when my dragon moves, it flies forwards, instead of walking forwards. And I've managed
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to achieve that by overriding this method that I got from my SuperClass and providing a custom implementation,
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so doing something completely different from the move method that the dragon inherited.
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Now sometimes, however, you actually want the behavior of the method from the SuperClass.
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So in our case of our dragon, what if I want it to land a hit, do some damage, but also to maybe be
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able to spit some fire, in addition.
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Well, in that case, I can do this.
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I can override the attack method from my SuperClass and I can do something customized to the Dragon
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class, namely maybe dragons should be able to spits fire and it does 10 damage. But this is not all
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that I wanted to do,
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I want it to perform the method that came from the SuperClass, the Enemy class, which I'm inheriting from,
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but then do some custom stuff as well.
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So I want both the functionality of the attack method from the SuperClass, as well as some functionality
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that I've added on as custom.
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So to achieve this, what I do is I use the super keyword which refers to the Enemy class which I'm inheriting
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from,
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and then I write ".attack."
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So now I'm triggering the attack method from my SuperClass which will print "Land a hit, does X amount
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of damage," and then I want to "Spits fire and do an extra 10 damage."
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So now if I go back to my main.swift and I run my code, when dragon.attack gets triggered, my
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dragon will, first, land a hit,
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does 15 damage, which comes from super.attack, and then it's going to spit fire and do another 10 damage,
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which comes from my custom additional implementation of the attack method. So this really is the superpower
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of classes.
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You can see that by inheriting from the Enemy class, I saved myself a lot of work and a lot of repetition.
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When we create functions, we know that they help us reduce the amount of repeated code by packaging a whole
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bunch of instructions inside the same package.
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Now, when we create classes and inherit from other classes, we're, again, saving ourselves from creating
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repeated bits of code,
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and instead, we are just layering functionality upon a SuperClass.
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So in fact, without this capability of inheritance that comes from classes, we wouldn't have been able
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to build any of the apps that we have created so far.
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If you go to help and develop a documentation, you can see how inheritance is used amongst the components
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that are part of Apple's UIKit. And that includes almost everything we've shown on screen like
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UILabels, UISliders, UIImageViews, everything with a UI in front of it, basically.
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So for example, when we create a button, we're actually creating a UIButton.
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So if you search for a UIButton and click on it, you can see that this is a UIButton Class and it allows
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you to respond to user interactions. But this class actually inherits from the UIControl Class,
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so if we click on UIControl, we get taken to this class' documentation and you can see that this is
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the base class for all of the controls which convey a specific action or intention in response to user
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interactions.
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So we're inheriting some of these capabilities
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when we use a UIButton. Now, a UIControl itself then inherits from a UIView which is an object that manages
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the content for a rectangular area on the screen. And then a UIView itself inherits from UIResponder,
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and UIResponder inherits from NSObject.
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So what does that mean?
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Why is it structured like that?
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Well, let's think about it in reverse.
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Apple's most basic class is the NSObject.
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This is the simplest and most generic component
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and it's how Apple started out. The NS, in fact, stands for NeXTSTEP which was the company that Steve
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Jobs started when he got kicked out of Apple, initially.
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Now, the UIResponder can do everything an NS object can do, but it has its own additional capabilities.
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The UIView in turn builds on top of the capabilities of the UIResponder, and the UIControl builds
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on top of that again. At each level.
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more code and more capabilities are added so that components become less generic and more specialized.
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Until finally, we get a UIButton that the user can press. So now that we've chased it all the way up
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the tree,
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we can see how it would take so much longer if we had to create everything from scratch, and create all
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of our view controllers from scratch, all of our UILabels from scratch, rather than simply just inheriting
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and using the capabilities that Apple has already created within our UIKit.
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And this is all possible because of how classes work and how they're able to inherit from other classes.
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So in the next lesson, we're going to take a look at the difference between struct and classes and when
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we would use each.
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So for all of that and more, I'll see you there.