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.
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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!")
}
}
Text("Hello World!")
.background(Color.red)
Text("A great and warm welcome to Kuchi")
.background(Color.red)
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Text("A great and warm welcome to Kuchi")
.background(Color.red)
// fixed frame size
.frame(width: 150, height: 50, alignment: .center)
.background(Color.yellow)
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.frame(width: 300, height: 100, alignment: .center)
.frame(width: 100, height: 50, alignment: .center)
Text("A great and warm welcome to Kuchi")
.background(Color.red)
.frame(width: 100, height: 50, alignment: .center)
// Add this scale factor
.minimumScaleFactor(0.5)
.background(Color.yellow)
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Image("welcome-background")
.background(Color.red)
.frame(width: 100, height: 50, alignment: .center)
.background(Color.yellow)
Image("welcome-background")
.resizable()
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 child views, the rules that determine children’s sizes are:
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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)
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)
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.
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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(spacing: 2) {
Text("A great welcome to Kuchi")
.background(Color.red)
Text("A great welcome to Kuchi")
.background(Color.red)
Text("A great welcome to Kuchi")
.background(Color.red)
}
.background(Color.yellow)
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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)
}
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)
}
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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
)
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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(alignment: .bottom) {
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HStack(alignment: .firstTextBaseline) {
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.
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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 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)
}
Text("You’re awesome!")
.fontWeight(.bold)
.foregroundColor(.gray)
Button(action: {
challengesViewModel.restart()
}, label: {
Text("Play Again")
})
.padding(.top)
struct CongratulationsView: View {
// Add this property
@ObservedObject
var challengesViewModel = ChallengesViewModel()
...
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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?
// 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)
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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?
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Text("You're awesome!")
.fontWeight(.bold)
.foregroundColor(.gray)
Spacer() // <== The spacer goes here
Button(action: {
self.challengesViewModel.restart()
}, label: {
Text("Play Again")
})
VStack {
Spacer() // <== The spacer goes here
Text("Congratulations!")
...
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.
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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: {
showAnswers.toggle()
}) {
// 3
QuestionView(question: challengeTest.challenge.question)
.frame(height: 300)
}
// 4
if showAnswers {
Divider()
// 5
ChoicesView(challengeTest: challengeTest)
.frame(height: 300)
.padding()
}
}
}
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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)
}
}
}
struct KuchiApp_Previews: PreviewProvider {
static let userManager = UserManager(name: "Ray")
static var previews: some View {
StarterView()
.environmentObject(userManager)
}
}
var body: some View {
if self.userViewModel.isRegistered {
WelcomeView()
} else {
RegisterView()
}
}
@EnvironmentObject var userManager: UserManager
@ObservedObject var challengesViewModel = ChallengesViewModel()
@State var showPractice = false
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struct WelcomeView_Previews: PreviewProvider {
static var previews: some View {
WelcomeView()
.environmentObject(UserManager())
}
}
var body: some View {
if showPractice {
// 1
PracticeView(
challengeTest: $challengesViewModel.currentChallenge,
userName: $userManager.profile.name
)
} else {
// 2
ZStack {
WelcomeBackgroundImage()
VStack {
Text(verbatim: "Hi, \(userManager.profile.name)")
WelcomeMessageView()
// 3
Button(action: {
self.showPractice = true
}, label: {
HStack {
Image(systemName: "play")
Text(verbatim: "Start")
}
})
}
}
}
}
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.
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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
)
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ForEach(history, id: \.self) { element in
}
ForEach(history, id: \.self) { element in
getElement(element)
}
ScrollView {
LazyVStack {
ForEach(history, id: \.self) { element in
getElement(element)
}
}
}
Section(header: header) {
ForEach(history, id: \.self) { element in
getElement(element)
}
}
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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: