The app is…

Our first complete Swift program is a Guess the Number app, a classic educational game for children where the player must guess a number generated randomly.

For each guess, the game tells the player whether the guess is greater or lower than the generated number (also called the secret number).

It is worth to remember that the goal here is not to build an App Store ready app with a perfect software architecture, but to show how to use Xcode to build apps for the iOS platform, so forgive me if the code is not exactly Clean Code, and if the game is trivial.

Before ping into the code, we must define the interface of the app and the expected workflow.

This game presents only one screen, which is shown in the following screenshot:

At the top of the screen, a label reports the name of the app, Guess a Number.

In the next row, another static label with the word, between, connects the title with a dynamic label that reports the current range. The text inside the label must change every time a new number is inserted. A text field at the center of the screen is where the player will insert their guess.

A big button, with OK written on it, is the command that confirms that the player has inserted the chosen number.

The last two labels give feedback to the player:

• Your last guess was too low is displayed if the number inserted is lower than the secret number
• Your last guess was too high is displayed if it's greater than the secret number

The last label reports the current number of guesses. The workflow is straightforward:

1. The app selects a random number.
2. The player inserts their guess.
3. If the number is equal to the secret number, a popup tells the player that they have won, and shows them the number of guesses.
4. If the number is lower than the secret number but greater than the lower bound, it becomes the new lower bound. Otherwise, it is silently discarded.
5. If the number is greater and lower than the upper bound, it becomes the new upper bound. Otherwise, it's, again, silently discarded.

Building a skeleton app

Let's start building the app.

There are two different ways to create a new project in Xcode: using a wizard or selecting a new project from the menu.

When Xcode starts, it presents a wizard showing the recently used projects and a shortcut to create a new project, as shown in the following screenshot:

If you have already Xcode open, you can select a new project by going to File | New | Project…, as shown in this screenshot:

Either way you choose, Xcode will ask for the type of app to be created. Since our app is going to be really simple, let's choose the Single View Application:

Before starting to write code, we need to complete the configuration by adding the Organization Identifier, using the reverse domain name notation like co.uk.effectivecode, and Product Name. Together, they produce a Bundle Identifier, which is the unique identifier of the app.

Pay attention to the selected language, which must obviously be Swift. Following is the screenshot that shows you how to fill in the form:

Once done with this data, we are ready to run the app by going to Product | Run, as shown in this screenshot:

After the simulator finishes loading the app, we can see our magnificent creation, a shiny, brilliant white page!

We can stop the app by going to Product | Stop, as shown in the following screenshot:

Given that a white page is not what we want, let's fix that writing code and adding content.

Adding the graphics components

When we are developing an iOS app, it is good practice to implement the app outside-in, starting from the graphics.

By taking a look at the files generated by the Xcode template, we can identify the two files that we'll use to implement Guess the Number app:

• Main.storyboard: This contains the graphics components
• ViewController.swift: This handles all of the logic of the app

Here is a screenshot that presents the structure of the files in an Xcode project:

Let's start by selecting the storyboard file to add the labels.

The first thing we notice is that the canvas is not the same size or ratio as an iPhone and an iPad. To handle different sizes and different devices, Apple (since iOS 5) have added a constraints system, called Auto Layout, as a system to connect the graphics components in relative way, regardless of the actual size of the running device.

As Auto Layout is beyond the scope of this chapter, we'll implement the created app only for iPhone 6.

After deciding our target device, we need to resize the canvas as per the real size of the device. From the tree structure at the right, we select View Controller, as shown here:

After having done that, we move to the right-hand side, where there are the properties of the View Controller. There, we select the tab containing Simulated Metrics, in which we can insert the requested size. The following screenshot will help you locate the correct tab:

Selecting the iPhone 4.7-inch size, we selected the appropriate size for iPhone 6 and 6S; after implementing the app, you could run it in different simulators to understand what this means.

Now the size is the expected size, we can proceed to add labels, text fields, and the buttons from the list in the bottom-right corner of the screen.

To add a component, we must choose it from the list of components. Then, we drag it onto the screen, where we can place it at the expected coordinates.

This screenshot shows the list of UI components, called Object library:

As it is usually difficult to find, it's worth mentioning that we select the circle icon with a square inside to select the Object library.

When you add the text field, pay attention to selecting Number Pad as the value for Keyboard Type, as illustrated in the following screenshot:

As mentioned, to add a component to the storyboard, it must be selected from the object library and with the left button of the mouse clicked and dragged onto the View Controller:

When the components are in the storyboard, you'll notice that moving the blue lines appear to help you to align them to the already set components:

After selecting values for all the components, the app should appear as shown in the mockup we had drawn earlier, which this screenshot can confirm:

Connecting the dots

If we run the app, the screen is the same as the one in the storyboard, but if we try to insert a number into the text field and then press the button, nothing happens.

This is because the storyboard is still detached from the View Controller, which handles all of the logic.

To connect the labels to the View Controller, we need to create instances of a label prepended with the @IBOutlet keyword. Using this signature, Interface Builder, the graphic editor inside Xcode can recognize the instances available for connection to the components:

class ViewController: UIViewController {
    @IBOutlet weak var rangeLbl: UILabel!
    @IBOutlet weak var numberTxtField: UITextField!
    @IBOutlet weak var messageLbl: UILabel!
    @IBOutlet weak var numGuessesLbl: UILabel!

    @IBAction func onOkPressed(sender: AnyObject) {
    }
}

We have also added a method with the @IBAction prefix, which will be called when the button is pressed.

Now, let's move on to Interface Builder to connect the labels and outlets.

First of all, we need to select View Controller from the tree of components, as shown in this screenshot:

In the tabs to the right, select the outlet views, the last one with an arrow as a symbol. The following screenshot will help you find the correct symbol:

This shows all the possible outlets to which a component can be connected.

Upon moving the cursor onto the circle beside the rangeLbl label, we see that it changes to a cross. Now, we must click-and-drag a line to the label in the storyboard, as shown in this screenshot:

After doing the same for all the labels, the following screenshot shows the final configurations for the outlets:

For the action of the button, the process is similar: select the circle close to the onOkPressed action, and drag a line to the OK button, as shown in this screenshot:

When the button is released, a popup appears with the list of possible events to connect the action to.

In our case, we connect the action to the Touch Up Inside event, which is triggered when we release the button without moving from its area. The following screenshot presents the list of the events raised by the UIButton component:

Now, suppose we added a log command like this one:

@IBAction func onOkPressed(sender: AnyObject) {
    print(numberTxtField.text)
}

Then, we can see the value of the text field we insert printed on the debug console.

Now that all the components are connected to their respective outlets, we can add the simple code required to create the app.

Adding the code

First of all, we need to add a few instance variables to handle the state:

private var lowerBound = 0
private var upperBound = 100
private var numGuesses = 0
private var secretNumber = 0

Just for the sake of clarity, and the separation of responsibilities, we create two extensions to the View Controller. An extension in Swift is similar to a category in Objective-C, a distinct data structure that adds a method to the class it extends.

Because we don't need the source of the class that the extension extends, we can use this mechanism to add features to third-party classes, or even to CocoaTouch classes.

Given this original purpose, extensions can also be used to organize the code inside a source file. This could seem a bit unorthodox, but if it doesn't hurt and is useful, why not use it?

The first extension contains the logic of the game:

private extension ViewController{
    enum Comparison{
        case Smaller
        case Greater
        case Equals
    }

    func selectedNumber(number: Int){
    }

    func compareNumber(number: Int, otherNumber: Int) -> Comparison {
    }
}

Note

The private keyword is added to the extension, making the methods inside private. This means that other classes that hold a reference to an instance of View Controller can't call these private methods.

Also, this piece of code shows that it is possible to create enumerations inside a private extension.

The second extension is for rendering all the labels:

private extension ViewController{
    func extractSecretNumber() {
    }
    
    func renderRange() {
    }
    
    func renderNumGuesses() {
    }
    func resetData() {
    }
    func resetMsg() {
    }
    func reset(){
        resetData()
        renderRange()
        renderNumGuesses()
        extractSecretNumber()
        resetMsg()
    }
}

Let's start from the beginning, which is the viewDidLoad method in the case of the View Controller:

override func viewDidLoad() {
    super.viewDidLoad()
    numberTxtField.becomeFirstResponder()
    reset()
}

When the becomeFirstResponder method is called, the component called is numberTxtField, in our case it gets the focus, and the keyboard appears.

After that, reset() is called:

func reset(){
    resetData()
    renderRange()
    renderNumGuesses()
    extractSecretNumber()
    resetMsg()
}

This basically calls the reset method of each component:

func resetData() {
    lowerBound = 0
    upperBound = 100
    numGuesses = 0
}

func resetMsg() {
    messageLbl.text = ""
}

Then, the method is called and is used to render the two dynamic labels:

func renderRange() {
    rangeLbl.text = "\(lowerBound) and \(upperBound)"
}

func renderNumGuesses() {
    numGuessesLbl.text = "Number of Guesses: \(numGuesses)"
}

It also extracts the secret number using the arc4random_uniform function, and performs some typecast magic to align to the expected numeric type:

func extractSecretNumber() {
    let diff = upperBound - lowerBound
    let randomNumber = Int(arc4random_uniform(UInt32(diff)))
    secretNumber = randomNumber + Int(lowerBound)
}

Now, all the action is in the onOkPressed action (pun intended):

@IBAction func onOkPressed(sender: AnyObject) { 
    guard let number = Int(numberTxtField.text!) else {
        let alert = UIAlertController(title: nil, message: "Enter a number", preferredStyle: UIAlertControllerStyle.Alert)
        alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: nil))
        self.presentViewController(alert, animated: true, completion: nil)
        return
    }
        selectedNumber(number)
}

Here, we retrieve the inserted number. Then, if it is valid (that is, it's not empty, not a word, and so on), we call the selectedNumber method. Otherwise, we present a popup asking for a number. This code uses the Swift 2.0 keyword guard that permits to create a really clear code flow; in this way, if the number is not a valid one, we return from the function and we don't need to check in the selectecNumber() function if the parameter is valid or not.

Note

The text property of a UITextField is an optional, but because we are certain that is present, we can safely unwrap it.

Also, the handy Int(String) constructor converts a string in a number only if the strings is a valid number.

All the juice is in selectedNumber, where there is a switch case:

func selectedNumber(number: Int){
    switch compareNumber(number, otherNumber: secretNumber){
//....

The compareNumber basically transforms a compare check into an enumeration:

func compareNumber(number: Int, otherNumber: Int) -> Comparison{
    if number < otherNumber {
        return .Smaller
    } else if number > otherNumber {
        return .Greater
    }

    return .Equals
}

Back to the switch statement of selectedNumber, it first checks whether the number inserted is the same as the secret number:

case .Equals:
    let alert = UIAlertController(title: nil, message: "You won in \(numGuesses) guesses!", preferredStyle: UIAlertControllerStyle.Alert)
    alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: { cmd in self.reset()
            self.numberTxtField.text = ""}))
            self.presentViewController(alert, 
            animated: true, completion: nil)

If this is the case, a popup with the number of guesses is presented, and when it is dismissed, all of the data is cleaned and the game starts again.

If the number is smaller, we calculate the lower bound again, and then we render the feedback labels:

    case .Smaller:
        lowerBound = max(lowerBound, number)
        messageLbl.text = "Your last guess was too low"
        numberTxtField.text = ""
        numGuesses++
        renderRange()
        renderNumGuesses()

If the number is greater, the code is similar, but instead of the lower bound, we calculate the upper bound:

    case .Greater:
        upperBound = min(upperBound, number)
        messageLbl.text = "Your last guess was too high"
        numberTxtField.text = ""
        numGuesses++
        renderRange()
        renderNumGuesses()
}

Et voilà! With this simple code, we have implemented our app.

Note

You can download the code of the app from https://github.com/gscalzo/Swift2ByExample/tree/1_GuessTheNumber.