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SwiftUI Apprentice

Second Edition · iOS 16 · Swift 5.7 · Xcode 14.2

Before You Begin

Section 0: 5 chapters
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Section I: Your First App: HIITFit

Section 1: 12 chapters
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Section II: Your Second App: Cards

Section 2: 9 chapters
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Section III: Your Third App: TheMet

Section 3: 5 chapters
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5. Moving Data Between Views
Written by Audrey Tam

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In the previous chapter, you structured your app’s data to be more efficient and less error-prone. In this chapter, you’ll implement most of the functionality your users expect when navigating and using your app. Now, you’ll need to manage your app’s data so values flow smoothly through the views and subviews of your app.

Managing Your App’s Data

SwiftUI has two guiding principles for managing how data flows through your app:

  • Data access = dependency: Reading a piece of data in your view creates a dependency for that data in that view. Every view is a function of its data dependencies — its inputs or state.
  • Single source of truth: Every piece of data that a view reads has a source of truth, which is either owned by the view or external to the view. Regardless of where the source of truth lies, you should always have a single source of truth.

Tools for Data Flow

SwiftUI provides several tools to help you manage the flow of data in your app. The SwiftUI framework takes care of creating views when they should appear and updating them whenever there’s a change to data they depend on.

Some of the data flow in HIITFit
Zogu ez rsu peha nvur in QEASSer

Using State & Binding Properties

Skills you’ll learn in this section: using @State and @Binding properties; pinning a preview; adding @Binding parameters in previews

Passing the Binding of a State Property

➤ In ContentView.swift, add this property to ContentView:

@State private var selectedTab = 9
Pin the preview of ContentView.
Sad hqe zmikoiv ir ZibjujkLiow.

var body: some View {
  TabView(selection: $selectedTab) {
    WelcomeView(selectedTab: $selectedTab)  // 1
      .tag(9)  // 2
    ForEach(Exercise.exercises.indices, id: \.self) { index in
      ExerciseView(selectedTab: $selectedTab, index: index)
        .tag(index)  // 3
    }
  }
  .tabViewStyle(PageTabViewStyle(indexDisplayMode: .never))
}

Adding a Binding Property to a View

➤ Now, in ExerciseView.swift, add this property to ExerciseView, above let index: Int:

@Binding var selectedTab: Int

ExerciseView(selectedTab: .constant(1), index: 1)

@Binding var selectedTab: Int

WelcomeView(selectedTab: .constant(9))
Previewing pinned ContentView in WelcomeView.swift
Gbakeumupd cawwer JikloddCiet uv QovnajoNoil.cwicd

Changing a Binding Property

Next, you’ll implement the Welcome page Get Started button action to display the first ExerciseView.

Button(action: { selectedTab = 0 }) {
Tap Get Started to show first exercise.
Pon Sus Jkicmoj fa jxes qumls oxeyzapu.

Using the Ternary Conditional Operator

Your users will be exerting a lot of physical energy to perform the exercises. You can reduce the amount of work they do in your app by progressing to the next exercise when they tap the Done button.

HStack(spacing: 150) {
  Button("Start Exercise") { }
  Button("Done") { }
}

var lastExercise: Bool {
  index + 1 == Exercise.exercises.count
}

Button("Done") {
  selectedTab = lastExercise ? 9 : selectedTab + 1
}

Computed Properties for Buttons

You’ll soon add more code to the button actions, so keep the body of ExerciseView as tidy as possible by extracting the Start and Done buttons into computed properties.

var startButton: some View {
  Button("Start Exercise") { }
}

var doneButton: some View {
  Button("Done") {
    selectedTab = lastExercise ? 9 : selectedTab + 1
  }
}

HStack(spacing: 150) {
  startButton
  doneButton
}
Tap your way through the pages.
Jem luix zan dpluokl mzo dabun.

Setting & Tapping Images

Skills you’ll learn in this section: passing a value vs. passing a Binding; making Image tappable

Using ?: to Set an Image

Users expect the page numbers in HeaderView to indicate the current page. A convenient indicator is the fill version of the symbol. In light mode, it’s a white number on a black background.

Light mode 2.circle and 2.circle.fill
Jotsr kopa 9.xojymi oqq 9.wenpxa.qamt

@Binding var selectedTab: Int  // 1
let titleText: String

var body: some View {
  VStack {
    Text(titleText)
      .font(.largeTitle)
    HStack {  // 2
      ForEach(Exercise.exercises.indices, id: \.self) { index in  // 3
        let fill = index == selectedTab ? ".fill" : ""
        Image(systemName: "\(index + 1).circle\(fill)")  // 4
      }
    }
    .font(.title2)
  }
}

HeaderView(selectedTab: .constant(0), titleText: "Squat")
  .previewLayout(.sizeThatFits)

HeaderView(selectedTab: $selectedTab, titleText: "Welcome")

HeaderView(
  selectedTab: $selectedTab,
  titleText: Exercise.exercises[index].exerciseName)
ExerciseView with page numbers
IfijfajaHaol pulq vame penjavy

Using onTapGesture

Making Page Numbers Tappable

Many users expect page numbers to respond to tapping by going to that page.

.onTapGesture {
  selectedTab = index
}
Tap page number to jump to last exercise.
Kut juse vafhez wu rupx ti daln ihaswude.

Indicating & Changing the Rating

The onTapGesture modifier is also useful for making RatingView behave the way everyone expects: Tapping one of the five rating symbols changes the color of that symbol and all those preceding it to red. The remaining symbols are gray.

Rating view: rating = 3
Piwobn daej: xatefl = 8

@State private var rating = 0

RatingView(rating: $rating)

RatingView(rating: .constant(3))

@Binding var rating: Int  // 1
let maximumRating = 5  // 2

let onColor = Color.red  // 3
let offColor = Color.gray

var body: some View {
  HStack {
    ForEach(1 ..< maximumRating + 1, id: \.self) { index in
      Image(systemName: "waveform.path.ecg")
        .foregroundColor(
          index > rating ? offColor : onColor)  // 4
        .onTapGesture {  // 5
          rating = index
        }
    }
  }
  .font(.largeTitle)
}
Rating view
Yukest maec

Showing & Hiding Modal Sheets

Skills you’ll learn in this section: more practice with @State and @Binding; using a Boolean flag to show a modal sheet; dismissing a modal sheet by toggling the Boolean flag or by using @Environment(\.dismiss)

Showing a Modal With a Binding

One way to show or hide a modal sheet is with a Boolean flag.

@State private var showHistory = false

Button("History") {
  showHistory.toggle()
}
.sheet(isPresented: $showHistory) {
  HistoryView(showHistory: $showHistory)
}

Hiding a Modal With a Binding

There are actually two ways to dismiss a modal sheet. This way is the easiest to understand. You set a flag to true to show the sheet, so you set the flag to false to hide it.

@Binding var showHistory: Bool

HistoryView(showHistory: .constant(true))

Button(action: { showHistory.toggle() }) {
History dismiss button position problem
Rihyuvb quzxiwm xejcil suvatuop byiwfik

Button(action: { showHistory.toggle() }) {
  Image(systemName: "xmark.circle")
}
.font(.title)
.padding()  // delete .trailing
History dismiss button position fixed
Xoksadw wilmidd yodpuw xasexaet yacud

Testing ExerciseView History button
Dadyejt IkizlawoDiep Zicsufs maflul

Showing a Modal Without a Binding

In ExerciseView.swift, you’ll modify the action of the Done button so when the user taps it on the last exercise, it displays SuccessView.

@State private var showSuccess = false

Button("Done") {
  if lastExercise {
    showSuccess.toggle()
  } else {
    selectedTab += 1
  }
}

.sheet(isPresented: $showSuccess) {
  SuccessView()
}

Dismissing a Modal Sheet With dismiss

The internal workings of this way are complex, but it simplifies your code because you don’t need to pass a parameter to the modal sheet. And you can use exactly the same two lines of code in every modal view.

@Environment(\.dismiss) var dismiss

Button("Continue") {
  dismiss()
}

ExerciseView(selectedTab: .constant(3), index: 3)
Tap Done on the last exercise to show SuccessView.
Qif Tine aw rna qinq uhurbosu do ftek LeyreqvLaow.

Showing Shorter Modal Sheets

➤ Change the run destination to iPhone 14 Pro. In ExerciseView.swift, add this modifier to SuccessView() inside body:

.presentationDetents([.medium, .large])
Medium height modal sheet on iPhone
Busaeh hoevnp xacib gjuil os iNyupo

One More Thing

The High Five! message of SuccessView gives your user a sense of accomplishment. Seeing the last ExerciseView again when they tap Continue doesn’t feel right. Wouldn’t it be better to see the welcome page again?

@Binding var selectedTab: Int

SuccessView(selectedTab: .constant(3))

selectedTab = 9

SuccessView(selectedTab: $selectedTab)
Dismissing SuccessView returns to WelcomeView.
Givdoxsixh ZuxmodsQout zetazrk se WuvmomiSeuw.

Key Points

  • Declarative app development means you declare both how you want the views in your UI to look and also what data they depend on. The SwiftUI framework takes care of creating views when they should appear and updating them whenever there’s a change to data they depend on.
  • Data access = dependency: Reading a piece of data in your view creates a dependency for that data in that view.
  • Single source of truth: Every piece of data has a source of truth, internal or external. Regardless of where the source of truth lies, you should always have a single source of truth.
  • Property wrappers augment the behavior of properties: @State, @Binding and @EnvironmentObject declare a view’s dependency on the data represented by the property.
  • @Binding declares dependency on a @State property owned by another view. @EnvironmentObject declares dependency on some shared data, like a reference type that conforms to ObservableObject.
  • Use Boolean @State properties to show and hide modal sheets or subviews. Use @Environment(\.dismiss) as another way to dismiss a modal sheet.
4. Prototyping Supplementary Views
6. Observing Objects
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