Swift Correct Use of Getters and Setters

Swift correct use of getters and setters

Swift provides a much more structured approach to getters and setters than Java.

You can, but you should not, write setters and getters as you did in your code.

Instead (if you are using stored properties) just declare the property with a visibility non private (e.g. internal in my example). This way callers outside of your class will be able to see the property and change it.

class Person {
    
    var name: String {
        willSet(newValue) {
            print("\(self.name) is going to be renamed as \(newValue)")
        }
        didSet(oldValue) {
            print("\(oldValue) has been renamed as \(self.name)")
        }
    }
    
    init(name: String) {
        self.name = name
    }
}

Ok but in java getter and setters do allow me to add custom logic to be executed before or after the value is changed.

Right! In Swift, you can do this using the willSet and didSet observers.

willSet(newValue)

You write here the code you want to run before a new value is written in the property.
Here you can access the current value (that is going to be overwritten) with self.name while the new value is available with newValue.

didSet(oldValue)

You write here the code you want to run after a new value is written in the property.
Here you can access the old value (that has been overwritten) with oldValue while the new value is available in self.name.

Both willSet and didSet are optional [I am not talking about Optional Type! I mean you are not forced to write them :)].

If you don't need to run some code just before or after the property has been changed, just omit them.

Example

let aVerySmartPerson = Person(name: "Walter White")
aVerySmartPerson.name = "Heisenberg"

// > Walter White is going to be renamed as Heisenberg
// > Walter White has been renamed as Heisenberg

Can I make my own getters and setters and should I?

I've written an article for exactly this. I'll paste it here.

Stop writing getters and setters in Swift

I see this time and time again, and it's about time I write an article in one place to consolidate all my thoughts. If you find yourself writing code that looks like this, listen up:

public class C {
    private var _i: Int = 0
    public var i: Int {
        get {
            return self._i
        }
        set {
            self._i = newValue
        }
    }
}

This pattern* is completely pointless in Swift, and I'll explain why, but firstly we need to take a short detour through Java land. Why Java? Because most of the people I run into who write Swift like this have some sort of Java background, either

because it was taught in their computer sceince courses, or
because they're coming over to iOS development, from Android

What's the point of getters and setters?

Suppose we have the following class in Java:

public class WeatherReport {
    public String cityName;
    public double temperatureF;

    public WeatherReport(String cityName, double temperatureF) {
        this.cityName = cityName;
        this.temperatureF = temperatureF;
    }
}

If you showed this class to any CS prof, they're surely going to bark at you for breaking encapsulation. But what does that really mean? Well, imagine how a class like this would be used. Someone would write some code that looks something like this:

WeatherReport weatherReport = weatherAPI.fetchWeatherReport();
weatherDisplayUI.updateTemperatureF(weatherReport.temperatureF);

Now suppose you wanted to upgrade your class to store data in a more sensible temperature unit (beating the imperial system dead horse, am I funny yet?) like Celcius or Kelvin. What happens when you update your class to look like this:

public class WeatherReport {
    public String cityName;
    public double temperatureC;

    public WeatherReport(String cityName, double temperatureC) {
        this.cityName = cityName;
        this.temperatureC = temperatureC;
    }
}

You've changed the implementation details of your WeatherReport class, but you've also made an API breaking change. Because temperatureF was public, it was part of this class' API. Now that you've removed it, you're going to cause compilation errors in every consumer that depended on the exitense of the temperatureF instance variable.

Even worse, you've changed the semantics of the second double argument of your constructor, which won't cause compilation errors, but behavioural errors at runtime (as people's old Farenheit based values are attemped to be used as if they were celcius values). However, that's not an issue I'll be discussing in this article.

The issue here is that consumers of this class will be strongly coupled to the implementation details of your class. To fix this, you introduce a layer of seperation between your implementation details and your interface. Suppose the Farenheit version of our class was implemented like so:

public class WeatherReport {
    private String cityName;
    private double temperatureF;

    public WeatherReport(String cityName, double temperatureF) {
        this.cityName = cityName;
        this.temperatureF = temperatureF;
    }

    public String getCityName() {
        return this.cityName;
    }

    public void setCityName(String cityName) {
        this.cityName = cityName;
    }

    public double getTemperatureF() {
        return this.temperatureF;
    }

    public void setTemperatureF(double temperatureF) {
        this.temperatureF = temperatureF;
    }
}

The getters and setters are really basic methods that access or update our instance variables. Notice how this time, our instance variables are private, and only our getters and setters are public. A consumer would use this code, as so:

WeatherReport weatherReport = weatherAPI.fetchWeatherReport();
weatherDisplayUI.updateTemperatureF(weatherReport.getTemperatureF());

This time, when we make the upgrade to celcius, we have the freedom to change our instance variables, and tweak our class to keep it backwards compatible:

public class WeatherReport {
    private String cityName;
    private double temperatureC;

    public WeatherReport(String cityName, double getTemperatureC) {
        this.cityName = cityName;
        this.temperatureC = temperatureC;
    }

    public String getCityName() {
        return this.cityName;
    }

    public void setCityName(String cityName) {
        this.cityName = cityName;
    }

    // Updated getTemperatureF is no longer a simple getter, but instead a function that derives
    //  its Farenheit value from the Celcius value that actuallyed stored in an instance variable.
    public double getTemperatureF() {
        return this.getTemperatureC() * 9.0/5.0 + 32.0;
    }

    // Updated getTemperatureF is no longer a simple setter, but instead a function
    // that updates the celcius value stored in the instance variable by first converting from Farenheit
    public void setTemperatureF(double temperatureF) {
        this.setTemperatureC((temperatureF - 32.0) * 5.0/9.0);
    }

    // Mew getter, for the new temperatureC instance variable
    public double getTemperatureC() { 
        return this.temperatureC;
    }

    // New setter, for the new temperatureC instance variable
    public void setTemperatureC(double temperatureC) {
        this.temperatureC = temperatureC;
    }
}

We've added new getters and setters so that new consumers can deal with temperatures in Celcius. But importantly, we've re-implemented the methods that used to be getters and setters for temperatureF (which no longer exists), to do the appropraite conversions and forward on to the Celcius getters and setters. Because these methods still exist, and behave identically as before, we've successfully made out implementation change (storing F to storing C), without breaking our API. Consumers of this API won't notice a difference.

So why doesn't this translate into Swift?

It does. But simply put, it's already done for you. You see, stored properties in Swift are not instance variables. In fact, Swift does not provide a way for you to create or directly access instance variables.

To understand this, we need to have a fuller understanding of what properties are. There are two types, stored and computed, and neither of them are "instance variables".

Stored properties: Are a combination of a comiler-synthesized instance variable (which you never get to see, hear, touch, taste, or smell), and the getter and setter that you use to interact with them.
Computed proepties: Are just a getter and setter, without any instance variable to act as backing storage. Really, they just behave as functions with type () -> T, and (T) -> Void, but have a pleasant dot notation syntax:
```
print(weatherReport.temperatureC)
weatherReport.temperatureC = 100
```
rather than a function calling synax:
```
print(weatherReport.getTemperatureC())
weatherReport.setTemperatureC(100)
```

So in fact, when you write:

class C {
    var i: Int
}

i is the name of the getter and setter for an instance variable the compiler created for you. Let's call the instance variable $i (which is not an otherwise legal Swift identifier). There is no way to directly access $i. You can only get its value by calling the getter i, or update its value by calling its setter i.

So lets see how the WeatherReport migration problem looks like in Swift. Our initial type would look like this:

public struct WeatherReport {
    public let cityName: String
    public let temperatureF: Double
}

Consumers would access the temperature with weatherReport.temperatureF. Now, this looks like a direct access of an isntance variable, but remember, that's simply not possible in Swift. Instead, this code calls the compiler-syntehsized getter temperatureF, which is what accesses the instance variable $temperatureF.

Now let's do our upgrade to Celcius. We will first update our stored property:

public struct WeatherReport {
    public let cityName: String
    public let temperatureC: Double
}

This has broken our API. New consumers can use temperatureC, but old consumers who depended on temperatureF will no longer work. To support them, we simply add in a new computed property, that does the conversions between Celcius and Fahenheit:

public struct WeatherReport {
    public let cityName: String

    public let temperatureC: Double
    public var temperatureF: Double {
        get { return temperatureC * 9/5 + 32 }
        set { temperatureC = (newValue - 32) * 5/9 }
    }
}

Because our WeatherReport type still has a getter called temperatureF, consumers will behave just as before. They can't tell whether a property that they access is a getter for a stored property, or a computed property that derives its value in some other way.

So lets look at the original "bad" code. What's so bad about it?

public class C {
    private var _i: Int = 0
    public var i: Int {
        get {
            return self._i
        }
        set {
            self._i = newValue
        }
    }
}

When you call c.i, the following happens:

You access the getter i.
The getter i accesses self._i, which is yet another getter
The getter _i access the "hidden" instance variable $i

And it's similar for the setter. You have two layers of "getterness". See what that would look like in Java:

public class C {
    private int i;

    public C(int i) {
        this.i = i;
    }

    public int getI1() {
        return this.i;
    }

    public void setI1(int i) {
        this.i = i;
    }

    public int getI2() {
        return this.getI1();
    }

    public void setI2(int i) {
        this.setI1(i);
    }
}

It's silly!

But what if I want a private setter?

Rather than writing this:

public class C {
    private var _i: Int = 0

    public var i: Int {
        get {
            return self._i
        }
    }
}

You can use this nifty syntax, to specify a seperate access level for the setter:

public class C {
    public private(set) var i: Int = 0
}

Now isn't that clean?

Getters and Setters in Swift

I understand that getter brings back String value and converts it to Double and assigns this value to variable newValue.

This isn't correct. The getter just returns the double. There is no newValue in the getter.

In the setter, newValue is a shortcut for "the implied argument of the setter." The explicit syntax is this:

var doubleValue: Double {
    ...
    set(newValue) {
        display.text = "\(newValue)"
    }
}

Property getters and setters

Setters and Getters apply to computed properties; such properties do not have storage in the instance - the value from the getter is meant to be computed from other instance properties. In your case, there is no x to be assigned.

Explicitly: "How can I do this without explicit backing ivars". You can't - you'll need something to backup the computed property. Try this:

class Point {
  private var _x: Int = 0             // _x -> backingX
  var x: Int {
    set { _x = 2 * newValue }
    get { return _x / 2 }
  }
}

Specifically, in the Swift REPL:

 15> var pt = Point()
pt: Point = {
  _x = 0
}
 16> pt.x = 10
 17> pt
$R3: Point = {
  _x = 20
}
 18> pt.x
$R4: Int = 10

What is the correct way to use getters/setters

This works well, does what I need, but I do then have the problem of
two empty IBActions:

You don't need these empty @IBActions. They aren't doing anything for you. The segues are wired from the buttons in the Storyboard, so you don't need the @IBActions. If you delete the code, you also need to remove the connection from the Storyboard or your app will crash when it tries to call the removed @IBActions. To remove, control-click on the buttons in the Storyboard, and then click on the little x next to Touch Up Inside to remove the connection. Then you can delete @IBAction in your code.

if I remove them I have no way to differentiate between which segue
should be used

Not true. You have two different segues wired from two different buttons. The segue's identifiers are how you differentiate between the two segues.

I tried doing: myClass: ViewControllerTwo! = ViewControllerTwo();

When using segues, the segue creates the destination viewController for you.
This doesn't work because you are creating an entirely new ViewControllerTwo here. You are passing the values to this new instance, but this isn't the ViewControllerTwo that the Storyboard segue created for you.

When using segues, you should pass the data in prepare(for:sender:), just like you showed above. Get the destination ViewController and fill in the data you want to pass.

And now for the question you didn't ask:

How could I have done this using the @IBActions to trigger the
segue?

Remove the segues that are wired from your buttons.
Wire the segue from viewController1 to viewController2. Click on the segue arrow in the Storyboard and assign the identifier slideShow.
Assign unique tags to your two buttons. You can set this in Interface Builder. Give the first button tag 1, and the second button tag 2.

Your buttons can share the same @IBAction:

@IBAction func goToSlideShow(button: UIButton) {
    self.performSegue(withIdentifier: "slideShow", sender: button)
}

In prepare(for:sender:):

override func prepare(for segue: UIStoryboardSegue, sender: Any?)
{
    let passedPic = segue.destination as! ViewControllerTwo

    if let button = sender as? UIButton {
        if button.tag == 1 {
            passedPic.picsChosen = sLS1
        } else if button.tag == 2 {
            passedPic.picsChosen = sLS2
        }
    }
}

Different between getters and func getSomething()?

The getter of the variable is always executed. The only exception to that is when you get or set variables inside of init. This is the swift way of implementing getters and setters. Languages like Java do not have this option so you have to create getter and setter functions.

IMO providing getters and setters on your variable is safer. This way you ensure that they are always called. If you have getter and setter functions and modify a variable directly and forget to call the setters and getters it can lead to weird errors.

Why doesn't Swift use getter and setter for properties as much as Java or C#?

Java historically had different syntax for direct property access versus a method call that gets the value (a "getter"). Since you might someday want to override a property with a method, for consistency it is common to create a method in all cases.

Swift avoids this problem by having the same syntax for direct property access and "getters" (computed properties). This means you can change your mind later without impacting callers, and so there is no reason to create a method "just in case."

A computed property is defined as one with a "getter" (a get method) in Swift.

Swift Correct Use of Getters and Setters