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7
Introducing Stacks & Containers Written by Antonio Bello

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In the previous chapter, you learned about common SwiftUI controls, including TextField, Button, Slider and Toggle. In this chapter, you’ll be introduced to container views, which are used to group related views together, as well as to lay them out in respect to each other.

Before starting, though, it’s essential to learn and understand how views are sized.

Preparing the project

Before jumping into views and their sizes, be aware that the starter project for this chapter has some additions compared to the final project of the previous chapter.

If you want to keep working on your own copy, worry not! Just copy these files and add to your project, or drag and drop then directly into Xcode.

  • Practice/ChallengeView.swift
  • Practice/ChallengesViewModel.swift
  • Practice/ChoicesRow.swift
  • Practice/ChoicesView.swift
  • Practice/CongratulationsView.swift
  • Practice/PracticeView.swift
  • Practice/QuestionView.swift
  • StarterView.swift
  • HistoryView.swift

Layout and priorities

In UIKit and AppKit, you were used to using Auto Layout to constrain views. The general rule was to let a parent decide the size of its children, usually obtained by adding constraints, unless their size was statically set using, for example, width and height constraints.

Size expectation
Repa arwuvpotuav

Layout for views with a single child

Open the starter project and go to Practice/ChallengeView.swift, which is a new view created out of the SwiftUI View template. You can see that it contains a single Text:

struct ChallengeView: View {
  var body: some View {
    Text("Hello World!")
  }
}
Blank Hello World
Pdexj Xumcu Zofmg

Text("Hello World!")
  .background(Color.red)
Hello World 1
Mopga Rumqc 6

Text("A great and warm welcome to Kuchi")
  .background(Color.red)
Hello World 2
Ritwi Kormj 0

Text("A great and warm welcome to Kuchi")
  .background(Color.red)
  // fixed frame size
  .frame(width: 150, height: 50, alignment: .center)
  .background(Color.yellow)
Hello World 3
Danxo Dedsm 7

.frame(width: 300, height: 100, alignment: .center)
Hello World 4
Petno Vibbj 4

.frame(width: 100, height: 50, alignment: .center)
Hello World 5
Yufna Wasll 3

Text("A great and warm welcome to Kuchi")
  .background(Color.red)
  .frame(width: 100, height: 50, alignment: .center)
  .minimumScaleFactor(0.5)
  .background(Color.yellow)
Hello World 6
Zidvo Vufvp 4

Image("welcome-background")
  .background(Color.red)
  .frame(width: 100, height: 50, alignment: .center)
  .background(Color.yellow)
Hello World 7
Xavku Mapth 5

Image("welcome-background")
  .resizable()
Hello World 8
Parwi Penjs 7

Stack views

You’ve used stack views in earlier chapters, but you haven’t yet explored container views in any depth. The following section will go into more detail and teach you the logic behind the views.

Layout for container views

In the case of a container view, i.e., a view that contains two or more children views, the rules that determine children’s sizes are:

HStack {
  Text("A great and warm welcome to Kuchi")
    .background(Color.red)
  Text("A great and warm welcome to Kuchi")
    .background(Color.red)
}
.background(Color.yellow)
Hello World 9
Ciwni Guczh 3

Text("A great and warm welcome to Kuchi")
  .background(Color.red)
Text("A great and warn welcome to Kuchi") // <- Replace `m` with
                                          //    `n` in `warm`
  .background(Color.red)
Hello World 10
Ducnu Deqlc 17

Layout priority

A container view sorts its children by restriction degree, going from the control with the most restrictive constraints to the one with the least. In case the restrictions are equivalent, the smallest will take precedence.

Modifier

You can use a modifier to make the view more or less adaptive. Examples include:

Priority

You also have the option of changing the layout priority using the .layoutPriority modifier. With this, you can explicitly alter the control’s weight in the sort order. It takes a Double value, which can be either positive or negative. A view with no explicit layout priority can be assumed to have a value equal to zero.

HStack {
  Text("A great and warm welcome to Kuchi")
    .background(Color.red)

  Text("A great and warm welcome to Kuchi")
    .background(Color.red)

  Text("A great and warm welcome to Kuchi")
    .background(Color.red)
}
.background(Color.yellow)
Hello World 11
Jemyi Jigsj 13

HStack {
  Text("A great and warm welcome to Kuchi")
    .background(Color.red)

  Text("A great and warm welcome to Kuchi")
    .layoutPriority(1)
    .background(Color.red)

  Text("A great and warm welcome to Kuchi")
    .background(Color.red)
}
Hello World 12
Relxu Juvvn 36

HStack {
  Text("A great and warm welcome to Kuchi")
    .layoutPriority(-1)
    .background(Color.red)

  Text("A great and warm welcome to Kuchi")
    .layoutPriority(1)
    .background(Color.red)

  Text("A great and warm welcome to Kuchi")
    .background(Color.red)
}
Hello World 13
Gobxo Nerzf 09

Hello World 14
Zupgu Fotnw 39

Hello World 15
Cahho Ravwr 07

Hello World 16
Kitmi Vehyh 71

Hello World 17
Buzhe Natjd 20

Hello World 18
Meyju Mivrz 70

Hello World 19
Kadvu Wammv 50

The HStack and the VStack

HStack and VStack are both container views, and they behave in the same way. The only difference is the orientation:

// HStack
init(
  alignment: VerticalAlignment = .center,
  spacing: CGFloat? = nil,
  @ViewBuilder content: () -> Content
)

// VStack
init(
  alignment: HorizontalAlignment = .center,
  spacing: CGFloat? = nil,
  @ViewBuilder content: () -> Content
)

A note on alignment

While the VStack alignment can have three possible values — .center, .leading and .trailing — the HStack counterpart is a bit richer. Apart from center, bottom and top, it also has two very useful cases:

var body: some View {
  HStack() {
    Text("Welcome to Kuchi").font(.caption)
    Text("Welcome to Kuchi").font(.title)
    Button(action: {}, label: { Text("OK").font(.body) })
  }
}
HStack center
SWgask riwwuv

HStack(alignment: .bottom) {
HStack bottom
SWgocq dehbom

HStack(alignment: .firstTextBaseline) {
HStack base
HKlesq qole

The ZStack

With no AppKit and UIKit counterpart, the third stack component is ZStack, which stacks children views one on top of the other.

Other container views

It may sound obvious, but any view that can have a one-child view can become a container: simply embed its children in a stack view. So a component, such as a Button, which can have a label view, is not limited to a single Text or Image; instead, you can generate virtually any multi-view content by embedding everything into a Stack view.

Back to Kuchi

So far, this chapter has consisted mostly of theory and freeform examples to demonstrate specific features or behaviors. So, now it’s time to get your hands dirty and make some progress with the Kuchi app.

The Congratulations View

The congratulations view is used to congratulate the user after she gives five correct answers. Open CongratulationsView.swift and take a look at its content.

struct CongratulationsView: View {
  let avatarSize: CGFloat = 120
  let userName: String

  init(userName: String) {
    self.userName = userName
  }

  var body: some View {
    EmptyView()
  }
}


var body: some View {
  VStack {
  }
}

VStack {
  Text("Congratulations!")
    .font(.title)
    .foregroundColor(.gray)
}
Congratulations view
Qocpsedekatiiym miul

Text("You’re awesome!")
  .fontWeight(.bold)
  .foregroundColor(.gray)
Congrats View 2
Yixwpozd Louk 9

Button(action: {
  self.challengesViewModel.restart()
}, label: {
  Text("Play Again")
})
  .padding(.top)

struct CongratulationsView: View {
  // Add this property
  @ObservedObject
  var challengesViewModel = ChallengesViewModel()
  ...
Congrats View 3
Nevdlipt Pues 4

User avatar

But let’s not stop there — surely you can make this look even better! How about adding the user’s avatar and their name on a colored background, but split vertically into two halves of a different color?

Congrats View 4
Numlwepq Zeir 4

Vstack
Tzxezm

// 1
ZStack {
  // 2
  VStack(spacing: 0) {
    Rectangle()
      // 3
      .frame(height: 90)
      .foregroundColor(
        Color(red: 0.5, green: 0, blue: 0).opacity(0.2))
    Rectangle()
      // 3
      .frame(height: 90)
      .foregroundColor(
        Color(red: 0.6, green: 0.1, blue: 0.1).opacity(0.4))
  }

  // 4
  Image(systemName: "person.fill")
    .resizable()
    .padding()
    .frame(width: avatarSize, height: avatarSize)
    .background(Color.white.opacity(0.5))
    .cornerRadius(avatarSize / 2, antialiased: true)
    .shadow(radius: 4)

  // 5
  VStack() {
    Spacer()
    Text(userName)
      .font(.largeTitle)
      .foregroundColor(.white)
      .fontWeight(.bold)
      .shadow(radius: 7)
  }
  .padding()
}
// 6
.frame(height: 180)
Congrats View
Yengbest Wueq

The Spacer view

One thing worth mentioning is how Spacer is used inside the VStack at Step 5. The VStack contains the Spacer and the Text with the username — nothing else. So you might wonder why it’s even necessary?

Congrats View 2
Nuqrlutq Hueh 1

Spacer
Ftetek

Text("You're awesome!")
  .fontWeight(.bold)
  .foregroundColor(.gray)

Spacer() // <== The spacer goes here

Button(action: {
  self.challengesViewModel.restart()
}, label: {
  Text("Play Again")
})
Congrats View 3
Fizbvelg Juoy 8

VStack {
  Spacer() // <== The spacer goes here

  Text("Congratulations!")
  ...
Congrats View 4
Giknling Caaq 8

Completing the challenge view

Earlier you’ve used ChallengeView as a playground to test code shown throughout this chapter. Now you need to fill it with more useful code. The challenge view is designed to show a question and a list of answers.

let challengeTest: ChallengeTest

@State var showAnswers = false

// 1
static let challengeTest = ChallengeTest(
  challenge: Challenge(
    question: "おねがい　します",
    pronunciation: "Onegai shimasu",
    answer: "Please"
  ),
  answers: ["Thank you", "Hello", "Goodbye"]
)

static var previews: some View {
  // 2
  return ChallengeView(challengeTest: challengeTest)
}

ChallengeView(challengeTest: challengeTest!)

var body: some View {
  // 1
  VStack {
    // 2
    Button(action: {
      self.showAnswers.toggle()
    }) {
      // 3
      QuestionView(question: challengeTest.challenge.question)
        .frame(height: 300)
    }

    // 4
    if showAnswers {
      Divider()
      // 5
      ChoicesView(challengeTest: challengeTest)
        .frame(height: 300)
        .padding()
    }
  }
}

Reworking the App Launch

With the challenge view now completed, you still need to work on two other parts of the app in order to run:

@main
struct KuchiApp: App {
  let userManager = UserManager()

  init() {
    userManager.load()
  }

  var body: some Scene {
    WindowGroup {
      StarterView()
        .environmentObject(userManager)
    }
  }
}

@ViewBuilder
var body: some View {
  if self.userViewModel.isRegistered {
    WelcomeView()
  } else {
    #if os(iOS)
    RegisterView(keyboardHandler: KeyboardFollower())
    #endif

    #if os(macOS)
    RegisterView()
    #endif
  }
}

@EnvironmentObject var userManager: UserManager
@ObservedObject var challengesViewModel = ChallengesViewModel()
@State var showPractice = false

struct WelcomeView_Previews: PreviewProvider {
  static var previews: some View {
    WelcomeView()
      .environmentObject(UserManager())
  }
}

// 1
@ViewBuilder
var body: some View {
  if showPractice {
    // 2
    PracticeView(
      challengeTest: $challengesViewModel.currentChallenge,
      userName: $userManager.profile.name
    )
  } else {
    // 3
    ZStack {
      WelcomeBackgroundImage()

      VStack {
        Text(verbatim: "Hi, \(userManager.profile.name)")

        WelcomeMessageView()

        // 4
        Button(action: {
          self.showPractice = true
        }, label: {
          HStack {
            Image(systemName: "play")
            Text(verbatim: "Start")
          }
        })
      }
    }
  }
}
Kuchi
Wumva

Kuchi 2
Bayxe 4

The Lazy Stacks

Stacks are very useful to lay out views in one direction or another. In most cases, they are all you need for that purpose. There’s one exception though, which is when the number of views to stack one after the other is large.

Practice History

To see lazy stacks in action, you’re going to build a history view that displays all the recent challenges. Since we don’t have any tracked history yet, you’ll randomly generate some data.

var body: some View {
  LazyVStack {      
  }
}

init(
  _ data: Data,
  id: KeyPath<Data.Element, ID>,
  content: @escaping (Data.Element) -> Content
)

ForEach(history, id: \.self) { element in
}

ForEach(history, id: \.self) { element in
  getElement(element)
}
Lazy Stack 1
Sorp Mvobj 2

ScrollView {
  LazyVStack {
    ForEach(history, id: \.self) { element in
      getElement(element)
    }
  }
}

Section(header: header) {
  ForEach(history, id: \.self) { element in
    getElement(element)
  }
}
Lazy Stack 2
Takk Zmayt 9

LazyVStack(spacing: 0, pinnedViews: [.sectionHeaders]) {
  Section(header: header) {
    ForEach(history, id: \.self) { element in
      getElement(element)
    }
  }
}

Key points

Another long chapter — but you did a great job of getting through it! A lot of concepts have been covered here, the most important ones being:

Where to go from here?

To know more about container views, the WWDC video that covers them is a must-watch:

8. State & Data Flow — Part I 6. Controls & User Input

Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.

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