Day 10

About 7 min

Day 10 관련

At first, classes seem very similar to structs because we use them to create new data types with properties and methods. However, they introduce a new, important, and complex feature called inheritance – the ability to make one class build on the foundations of another.

This is a powerful feature, there’s no doubt about it, and there is no way to avoid using classes when you start building real iOS apps. But please remember to keep your code simple: just because a feature exists, it doesn’t mean you need to use it. As Martin Fowler wrote, “any fool can write code that a computer can understand, but good programmers write code that humans can understand.”

Today you have eight one-minute videos to watch, and you’ll meet things like method overrides, final classes, and deinitializers. Once you’ve watched each video and optionally gone through the extra reading, there’s a short test to help make sure you’ve understood what was taught.

1. Creating your own classes

100 Days of Swift - Day 10 - 1. Creating your own classes

1. Creating your own classes

Classes are similar to structs in that they allow you to create new types with properties and methods, but they have five important differences and I’m going to walk you through each of those differences one at a time.

The first difference between classes and structs is that classes never come with a memberwise initializer. This means if you have properties in your class, you must always create your own initializer.

For example:

class Dog {
    var name: String
    var breed: String

    init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

Creating instances of that class looks just the same as if it were a struct:

let poppy = Dog(name: "Poppy", breed: "Poodle")

1. Creating your own classes - Additional

Optional: Why does Swift have both classes and structs?
Test: Creating your own classes

2. Class inheritance

100 Days of Swift - Day 10 - 2. Class inheritance

2. Class inheritance

The second difference between classes and structs is that you can create a class based on an existing class – it inherits all the properties and methods of the original class, and can add its own on top.

This is called class inheritance or subclassing, the class you inherit from is called the “parent” or “super” class, and the new class is called the “child” class.

Here’s the Dog class we just created:

class Dog {
    var name: String
    var breed: String

    init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

We could create a new class based on that one called Poodle. It will inherit the same properties and initializer as Dog by default:

class Poodle: Dog {

}

However, we can also give Poodle its own initializer. We know it will always have the breed “Poodle”, so we can make a new initializer that only needs a name property. Even better, we can make the Poodle initializer call the Dog initializer directly so that all the same setup happens:

class Poodle: Dog {
    init(name: String) {
        super.init(name: name, breed: "Poodle")
    }
}

For safety reasons, Swift always makes you call super.init() from child classes – just in case the parent class does some important work when it’s created.

2. Class inheritance - Additional

Optional: Why don’t Swift classes have a memberwise initializer?
Test: Class inheritance

3. Overriding methods

100 Days of Swift - Day 10 - 3. Overriding methods

3. Overriding methods

Child classes can replace parent methods with their own implementations – a process known as overriding. Here’s a trivial Dog class with a makeNoise() method:

class Dog {
    func makeNoise() {
        print("Woof!")
    }
}

If we create a new Poodle class that inherits from Dog, it will inherit the makeNoise() method. So, this will print “Woof!”:

class Poodle: Dog {
}

let poppy = Poodle()
poppy.makeNoise()

Method overriding allows us to change the implementation of makeNoise() for the Poodle class.

Swift requires us to use override func rather than just func when overriding a method – it stops you from overriding a method by accident, and you’ll get an error if you try to override something that doesn’t exist on the parent class:

class Poodle: Dog {
    override func makeNoise() {
        print("Yip!")
    }
}

With that change, poppy.makeNoise() will print “Yip!” rather than “Woof!”.

3. Overriding methods - Additional

Optional: When would you want to override a method?
Test: Overriding methods

4. Final classes

100 Days of Swift - Day 10 - 4. Final classes

4. Final classes

Although class inheritance is very useful – and in fact large parts of Apple’s platforms require you to use it – sometimes you want to disallow other developers from building their own class based on yours.

Swift gives us a final keyword just for this purpose: when you declare a class as being final, no other class can inherit from it. This means they can’t override your methods in order to change your behavior – they need to use your class the way it was written.

To make a class final just put the final keyword before it, like this:

final class Dog {
    var name: String
    var breed: String

    init(name: String, breed: String) {
        self.name = name
        self.breed = breed
    }
}

4. Final classes - Additional

Optional: Which classes should be declared as final?
Test: Final classes

5. Copying objects

100 Days of Swift - Day 10 - 5. Copying objects

5. Copying objects

The third difference between classes and structs is how they are copied. When you copy a struct, both the original and the copy are different things – changing one won’t change the other. When you copy a class, both the original and the copy point to the same thing, so changing one does change the other.

For example, here’s a simple Singer class that has a name property with a default value:

class Singer {
    var name = "Taylor Swift"
}

If we create an instance of that class and prints its name, we’ll get “Taylor Swift”:

var singer = Singer()
print(singer.name)

Now let’s create a second variable from the first one and change its name:

var singerCopy = singer
singerCopy.name = "Justin Bieber"

Because of the way classes work, both singer and singerCopy point to the same object in memory, so when we print the singer name again we’ll see “Justin Bieber”:

print(singer.name)

On the other hand, if Singer were a struct then we would get “Taylor Swift” printed a second time:

struct Singer {
    var name = "Taylor Swift"
}

5. Copying objects - Additional

Optional: Why do copies of a class share their data?
test: Copying objects

6. Deinitializers

100 Days of Swift - Day 10 - 6. Deinitializers

6. Deinitializers

The fourth difference between classes and structs is that classes can have deinitializers – code that gets run when an instance of a class is destroyed.

To demonstrate this, here’s a Person class with a name property, a simple initializer, and a printGreeting() method that prints a message:

class Person {
    var name = "John Doe"

    init() {
        print("\(name) is alive!")
    }

    func printGreeting() {
        print("Hello, I'm \(name)")
    }
}

We’re going to create a few instances of the Person class inside a loop, because each time the loop goes around a new person will be created then destroyed:

for _ in 1...3 {
    let person = Person()
    person.printGreeting()
}

And now for the deinitializer. This will be called when the Person instance is being destroyed:

deinit {
    print("\(name) is no more!")
}

6. Deinitializers - Additional

Optional: Why do classes have deinitializers and structs don’t?
Test: Deinitializers

7. Mutability

100 Days of Swift - Day 10 - 7. Mutability

7. Mutability

The final difference between classes and structs is the way they deal with constants. If you have a constant struct with a variable property, that property can’t be changed because the struct itself is constant.

However, if you have a constant class with a variable property, that property can be changed. Because of this, classes don’t need the mutating keyword with methods that change properties; that’s only needed with structs.

This difference means you can change any variable property on a class even when the class is created as a constant – this is perfectly valid code:

class Singer {
    var name = "Taylor Swift"
}

let taylor = Singer()
taylor.name = "Ed Sheeran"
print(taylor.name)

If you want to stop that from happening you need to make the property constant:

class Singer {
    let name = "Taylor Swift"
}

7. Mutability - Additional

Optional: Why can variable properties in constant classes be changed?
Test: Mutability

8. Classes summary

100 Days of Swift - Day 10 - 8. Classes summary

8. Classes summary

You’ve made it to the end of the eighth part of this series, so let’s summarize:

Classes and structs are similar, in that they can both let you create your own types with properties and methods.
One class can inherit from another, and it gains all the properties and methods of the parent class. It’s common to talk about class hierarchies – one class based on another, which itself is based on another.
You can mark a class with the final keyword, which stops other classes from inheriting from it.
Method overriding lets a child class replace a method in its parent class with a new implementation.
When two variables point at the same class instance, they both point at the same piece of memory – changing one changes the other.
Classes can have a deinitializer, which is code that gets run when an instance of the class is destroyed.
Classes don’t enforce constants as strongly as structs – if a property is declared as a variable, it can be changed regardless of how the class instance was created.

8. Classes summary - Additional

Test: Classes