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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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14. Gestures
Written by Caroline Begbie

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Gestures are the main interface between you and your app. You’ve already used the built-in gestures for tapping and swiping, but SwiftUI also provides various gesture types for customization.

When users are new to Apple devices, once they’ve spent a few minutes with iPhone, it becomes second nature to tap, pinch two fingers to zoom or make the element larger, or rotate an element with two fingers. Your app should use these standard gestures. In this chapter, you’ll explore how to drag, magnify and rotate elements with the built-in gesture recognizers.

Back of the napkin design
Back of the napkin design

In the single card view, you’ll drag around and resize photo and text elements. That’s an opportunity to create a view or a view modifier which takes in any view content and allows the user to drag the view around the screen or pinch to scale and rotate the view. Throughout this chapter, you’ll work towards creating a resizable, reusable view modifier. You’ll be able to use this in any of your future apps.

Creating the Resizable View

To start with, the resizable view will simply show a colored rectangle but, later on, you’ll change it to show any view content.

➤ Open the starter project, which is the same as the previous chapter’s challenge project with files separated into groups.

➤ Create a new SwiftUI View file named ResizableView.swift. Replace ResizableView with this code:

struct ResizableView: View {
  // 1
  private let content = RoundedRectangle(cornerRadius: 30.0)
  private let color = Color.red

  var body: some View {
    // 2
    content
      .frame(width: 250, height: 180)
      .foregroundColor(color)
  }
}

Going through the code:

  1. Create a RoundedRectangle view property. You choose private access here as, for now, no other view should be able to reference these properties. Later on, you’ll change the access to pass in any view.
  2. Use content as the required View in body and apply modifiers to it.

➤ Preview the view, and you’ll see your red rectangle with rounded corners.

Preview rounded rectangle
Preview rounded rectangle

Creating Transforms

Skills you’ll learn in this section: transformation

import SwiftUI

struct Transform {
  var size = CGSize(width: 250, height: 180)
  var rotation: Angle = .zero
  var offset: CGSize = .zero
}

@State private var transform = Transform()

.frame(
  width: transform.size.width,
  height: transform.size.height)
Rounded rectangle with transform sizing
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Creating a Drag Gesture

Skills you’ll learn in this section: drag gesture; operator overloading

var dragGesture: some Gesture {
  DragGesture()
    .onChanged { value in
      transform.offset = value.translation
    }
}
Offset and translation
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.offset(transform.offset)
.gesture(dragGesture)
Dragging the view
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@State private var previousOffset: CGSize = .zero

var dragGesture: some Gesture {
  DragGesture()
    .onChanged { value in
      transform.offset = CGSize(
        width: value.translation.width + previousOffset.width,
        height: value.translation.height + previousOffset.height)
    }
    .onEnded { _ in
      previousOffset = transform.offset
    }
}

Operator Overloading

Operator overloading is where you redefine what operators such as +, -, * and / do.

import SwiftUI

func + (left: CGSize, right: CGSize) -> CGSize {
  CGSize(
    width: left.width + right.width,
    height: left.height + right.height)
}

var dragGesture: some Gesture {
  DragGesture()
    .onChanged { value in
      transform.offset = value.translation + previousOffset
    }
    .onEnded { _ in
      previousOffset = transform.offset
    }
}

Creating a Rotation Gesture

Skills you’ll learn in this section: rotation gesture

@State private var previousRotation: Angle = .zero

var rotationGesture: some Gesture {
  RotationGesture()
    .onChanged { rotation in
      transform.rotation += rotation - previousRotation
      previousRotation = rotation
    }
    .onEnded { _ in
      previousRotation = .zero
    }
}

.rotationEffect(transform.rotation)
.gesture(dragGesture)
.gesture(rotationGesture)
Two fingers to rotate
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Rotation
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ResizableView()

Creating a Scale Gesture

Skills you’ll learn in this section: magnification gesture; simultaneous gestures

@State private var scale: CGFloat = 1.0

var scaleGesture: some Gesture {
  MagnificationGesture()
    .onChanged { scale in
      self.scale = scale
    }
    .onEnded { scale in
      transform.size.width *= scale
      transform.size.height *= scale
      self.scale = 1.0
    }
}

.scaleEffect(scale)

Creating a Simultaneous Gesture

Whereas the drag is a specific gesture with one finger, you can do rotation and scale at the same time with two fingers. To do this, change .gesture(rotationGesture) to:

.gesture(SimultaneousGesture(rotationGesture, scaleGesture))
Completed gestures
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Creating Custom View Modifiers

Skills you’ll learn in this section: creating a ViewModifier; View extension; using a view modifier; advantages of a view modifier

struct ResizableView: ViewModifier {

func body(content: Content) -> some View {

private let content = RoundedRectangle(cornerRadius: 30.0)
private let color = Color.red

struct ResizableView_Previews: PreviewProvider {
  static var previews: some View {
    RoundedRectangle(cornerRadius: 30.0)
      .foregroundColor(Color.blue)
      .modifier(ResizableView())
  }
}

CardsListView()
View Modifier preview
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Using Your Custom View Modifier

In the preview, you used .modifier(ResizableView()). You can improve this by adding a “pass-through” method to View.

extension View {
  func resizableView() -> some View {
    modifier(ResizableView())
  }
}

.resizableView()

var content: some View {
  ZStack {
    Capsule()
      .foregroundColor(.yellow)
      .resizableView()
    Text("Resize Me!")
      .font(.largeTitle)
      .fontWeight(.bold)
      .resizableView()
    Circle()
      .resizableView()
      .offset(CGSize(width: 50, height: 200))
  }
}

content
Resize multiple views
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.font(.system(size: 500))
.minimumScaleFactor(0.01)
.lineLimit(1)
Resize the text
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View Modifier Advantage

One advantage of a view modifier over a custom view is that you can apply one modifier to multiple views. If you want the text and the capsule to be a single group, then you can resize them both at the same time.

Group {
  Capsule()
    .foregroundColor(.yellow)
  Text("Resize Me!")
    .fontWeight(.bold)
    .font(.system(size: 500))
    .minimumScaleFactor(0.01)
    .lineLimit(1)
}
.resizableView()
Grouped Views
Yfeoloc Ruush

Other Gestures

  • Tap gesture

Type Properties

Skills you’ll learn in this section: type properties; type methods

var currentTheme = Color.red

Swift Dive: Stored Property vs Type Property

To create a type property, rather than a stored property, you use the static keyword.

public struct CGPoint {
  public var x: CGFloat
  public var y: CGFloat
}

extension CGPoint {
  public static var zero: CGPoint { 
    CGPoint(x: 0, y: 0)
  }
}

var point = CGPoint(x: 10, y: 10)
point.x = 20

let pointZero = CGPoint.zero  // pointZero contains (x: 0, y: 0)
Type property storage
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Creating Global Defaults for Cards

Going back to your hard coded size values, you’ll now create a file that will hold all your global constants.

import SwiftUI

struct Settings {
  static let cardSize =
    CGSize(width: 1300, height: 2000)
  static let thumbnailSize =
    CGSize(width: 150, height: 250)
  static let defaultElementSize =
    CGSize(width: 250, height: 180)
  static let borderColor: Color = .blue
  static let borderWidth: CGFloat = 5
}

let settings1 = Settings()
let settings2 = Settings()

enum Settings {

extension Settings {
  static let aNewSetting: Int = 0
}

.frame(
  width: Settings.thumbnailSize.width,
  height: Settings.thumbnailSize.height)

var size = CGSize(
  width: Settings.defaultElementSize.width,
  height: Settings.defaultElementSize.height)

Creating Type Methods

As well as static properties, you can also create static methods. To illustrate this, you’ll extend SwiftUI’s built-in Color type. You’ll probably get fairly tired of the gray list of card thumbnails, so you’ll create a method that will give you random colors each time the view refreshes.

import SwiftUI

extension Color {
  static let colors: [Color] = [
    .green, .red, .blue, .gray, .yellow, .pink, .orange, .purple
  ]
}

static func random() -> Color {
  colors.randomElement() ?? .black
}

.foregroundColor(.random())
Random color
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Challenge

Challenge: Make new View Modifiers

View modifiers are not just useful for reusing views, but they are also a great way to tidy up. You can combine modifiers into one custom modifier. Or, as with the toolbar modifier in SingleCardView, if a modifier has a lot of code in it, save yourself some code reading fatigue, and separate it into its own file.

Key Points

  • Custom gestures let you interact with your app in any way you choose. Make sure the gestures make sense. Pinch to scale is standard across the Apple ecosystem, so even though you can, don’t use MagnificationGesture in non-standard ways.
  • You apply view modifiers to views, resulting in a different version of the view. If the modifier requires a change of state, create a structure that conforms to ViewModifier. If the modifier doesn’t require a change of state, you can make code more readable by adding a method to a View extension and use that method to modify a view.
  • static or type properties and methods exist on the type. Stored properties exist per instance of the type. Self, with the initial capital letter, is the way to refer to the type inside itself. self refers to the instance of the type. Apple uses type properties and methods extensively. For example, Color.yellow is a type property.

Where to Go From Here?

By now you should be able to understand a lot of technical jargon. It’s time to check out Apple’s documentation and articles. Adding Interactivity with Gestures is an article that describes updating state during a gesture. Read this article and check your understanding of the topic so far.

13. Outlining a Photo Collage App
15. Structures, Classes & Protocols
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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