Book Image

SwiftUI Essentials – iOS 14 Edition

By : Neil Smyth
Book Image

SwiftUI Essentials – iOS 14 Edition

By: Neil Smyth

Overview of this book

Do you want to create iOS apps with SwiftUI, Xcode 12, and Swift 5.3, and want to publish it on the app store? This book helps you achieve these skills with a step-by-step approach. This course first walks you through the steps necessary to set up an iOS development environment together and introduces Swift Playgrounds to learn and experiment with Swift—specifically, the Swift 5.3 programming language. After establishing key concepts of SwiftUI and project architecture, this course provides a guided tour of Xcode in SwiftUI development mode. The book also covers the creation of custom SwiftUI views and explains how these views are combined to create user interface layouts, including the use of stacks, frames, and forms. One of the more important skills you’ll learn is how to integrate SwiftUI views into existing UIKit-based projects and explain the integration of UIKit code into SwiftUI. Finally, the book explains how to package up a completed app and upload it to the app store for publication. Along the way, the topics covered in the book are put into practice through detailed tutorials, the source code for which is also available for download. By the end of this course, you will be able to build your own apps for iOS 14 using SwiftUI and publish it on the app store. The code files for the book can be found here: https://www.ebookfrenzy.com/retail/swiftui-ios14/

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Table of Contents (56 chapters)
1
1. Start Here
1. Start Here
1.1 For Swift Programmers
1.2 For Non-Swift Programmers
1.3 Source Code Download
1.4 Feedback
1.5 Errata
Free Chapter
2
2. Joining the Apple Developer Program
2. Joining the Apple Developer Program
2.1 Downloading Xcode 12 and the iOS 14 SDK
2.2 Apple Developer Program
2.3 When to Enroll in the Apple Developer Program?
2.4 Enrolling in the Apple Developer Program
2.5 Summary
3
3. Installing Xcode 12 and the iOS 14 SDK
3. Installing Xcode 12 and the iOS 14 SDK
3.1 Identifying Your macOS Version
3.2 Installing Xcode 12 and the iOS 14 SDK
3.3 Starting Xcode
3.4 Adding Your Apple ID to the Xcode Preferences
3.5 Developer and Distribution Signing Identities
3.6 Summary
4
4. An Introduction to Xcode 12 Playgrounds
4. An Introduction to Xcode 12 Playgrounds
4.1 What is a Playground?
4.2 Creating a New Playground
4.3 A Swift Playground Example
4.4 Viewing Results
4.5 Adding Rich Text Comments
4.6 Working with Playground Pages
4.7 Working with SwiftUI and Live View in Playgrounds
4.8 Summary
5
5. Swift Data Types, Constants and Variables
5. Swift Data Types, Constants and Variables
5.1 Using a Swift Playground
5.2 Swift Data Types
5.3 Swift Variables
5.4 Swift Constants
5.5 Declaring Constants and Variables
5.6 Type Annotations and Type Inference
5.7 The Swift Tuple
5.8 The Swift Optional Type
5.9 Type Casting and Type Checking
5.10 Summary
6
6. Swift Operators and Expressions
6. Swift Operators and Expressions
6.1 Expression Syntax in Swift
6.2 The Basic Assignment Operator
6.3 Swift Arithmetic Operators
6.4 Compound Assignment Operators
6.5 Comparison Operators
6.6 Boolean Logical Operators
6.7 Range Operators
6.8 The Ternary Operator
6.9 Nil Coalescing Operator
6.10 Bitwise Operators
6.11 Compound Bitwise Operators
6.12 Summary
7
7. Swift Control Flow
7. Swift Control Flow
7.1 Looping Control Flow
7.2 The Swift for-in Statement
7.3 The repeat ... while loop
7.4 Breaking from Loops
7.5 The continue Statement
7.6 Conditional Control Flow
7.7 Using the if Statement
7.8 Using if ... else … Statements
7.9 Using if ... else if ... Statements
7.10 The guard Statement
7.11 Summary
8
8. The Swift Switch Statement
8. The Swift Switch Statement
8.1 Why Use a switch Statement?
8.2 Using the switch Statement Syntax
8.3 A Swift switch Statement Example
8.4 Combining case Statements
8.5 Range Matching in a switch Statement
8.6 Using the where statement
8.7 Fallthrough
8.8 Summary
9
9. Swift Functions, Methods and Closures
9. Swift Functions, Methods and Closures
9.1 What is a Function?
9.2 What is a Method?
9.3 How to Declare a Swift Function
9.4 Implicit Returns from Single Expressions
9.5 Calling a Swift Function
9.6 Handling Return Values
9.7 Local and External Parameter Names
9.8 Declaring Default Function Parameters
9.9 Returning Multiple Results from a Function
9.10 Variable Numbers of Function Parameters
9.11 Parameters as Variables
9.12 Working with In-Out Parameters
9.13 Functions as Parameters
9.14 Closure Expressions
9.15 Shorthand Argument Names
9.16 Closures in Swift
9.17 Summary
10
10. The Basics of Swift Object-Oriented Programming
10. The Basics of Swift Object-Oriented Programming
10.1 What is an Instance?
10.2 What is a Class?
10.3 Declaring a Swift Class
10.4 Adding Instance Properties to a Class
10.5 Defining Methods
10.6 Declaring and Initializing a Class Instance
10.7 Initializing and De-initializing a Class Instance
10.8 Calling Methods and Accessing Properties
10.9 Stored and Computed Properties
10.10 Lazy Stored Properties
10.11 Using self in Swift
10.12 Understanding Swift Protocols
10.13 Opaque Return Types
10.14 Summary
11
11. An Introduction to Swift Subclassing and Extensions
11. An Introduction to Swift Subclassing and Extensions
11.1 Inheritance, Classes and Subclasses
11.2 A Swift Inheritance Example
11.3 Extending the Functionality of a Subclass
11.4 Overriding Inherited Methods
11.5 Initializing the Subclass
11.6 Using the SavingsAccount Class
11.7 Swift Class Extensions
11.8 Summary
12
12. An Introduction to Swift Structures and Enumerations
12. An Introduction to Swift Structures and Enumerations
12.1 An Overview of Swift Structures
12.2 Value Types vs. Reference Types
12.3 When to Use Structures or Classes
12.4 An Overview of Enumerations
12.5 Summary
13
13. An Introduction to Swift Property Wrappers
13. An Introduction to Swift Property Wrappers
13.1 Understanding Property Wrappers
13.2 A Simple Property Wrapper Example
13.3 Supporting Multiple Variables and Types
13.4 Summary
14
14. Working with Array and Dictionary Collections in Swift
14. Working with Array and Dictionary Collections in Swift
14.1 Mutable and Immutable Collections
14.2 Swift Array Initialization
14.3 Working with Arrays in Swift
14.4 Random Items and Shuffling
14.5 Appending Items to an Array
14.6 Array Iteration
14.7 Creating Mixed Type Arrays
14.8 Swift Dictionary Collections
14.9 Swift Dictionary Initialization
14.10 Sequence-based Dictionary Initialization
14.11 Dictionary Item Count
14.12 Accessing and Updating Dictionary Items
14.13 Adding and Removing Dictionary Entries
14.14 Dictionary Iteration
14.15 Summary
15
15. Understanding Error Handling in Swift 5
15. Understanding Error Handling in Swift 5
15.1 Understanding Error Handling
15.2 Declaring Error Types
15.3 Throwing an Error
15.4 Calling Throwing Methods and Functions
15.5 Accessing the Error Object
15.6 Disabling Error Catching
15.7 Using the defer Statement
15.8 Summary
16
16. An Overview of SwiftUI
16. An Overview of SwiftUI
16.1 UIKit and Interface Builder
16.2 SwiftUI Declarative Syntax
16.3 SwiftUI is Data Driven
16.4 SwiftUI vs. UIKit
16.5 Summary
17
17. Using Xcode in SwiftUI Mode
17. Using Xcode in SwiftUI Mode
17.1 Starting Xcode 12
17.2 Creating a SwiftUI Project
17.3 Xcode in SwiftUI Mode
17.4 The Preview Canvas
17.5 Preview Pinning
17.6 The Preview Toolbar
17.7 Modifying the Design
17.8 Editor Context Menu
17.9 Previewing on Multiple Device Configurations
17.10 Running the App on a Simulator
17.11 Running the App on a Physical iOS Device
17.12 Managing Devices and Simulators
17.13 Enabling Network Testing
17.14 Dealing with Build Errors
17.15 Monitoring Application Performance
17.16 Exploring the User Interface Layout Hierarchy
17.17 Summary
18
18. SwiftUI Architecture
18. SwiftUI Architecture
18.1 SwiftUI App Hierarchy
18.2 App
18.3 Scenes
18.4 Views
18.5 Summary
19
19. The Anatomy of a Basic SwiftUI Project
19. The Anatomy of a Basic SwiftUI Project
19.1 Creating an Example Project
19.2 Project Folders
19.3 The DemoProjectApp.swift File
19.4 The ContentView.swift File
19.5 Assets.xcassets
19.6 Info.plist
19.7 Summary
20
20. Creating Custom Views with SwiftUI
20. Creating Custom Views with SwiftUI
20.1 SwiftUI Views
20.2 Creating a Basic View
20.3 Adding Additional Views
20.4 Working with Subviews
20.5 Views as Properties
20.6 Modifying Views
20.7 Working with Text Styles
20.8 Modifier Ordering
20.9 Custom Modifiers
20.10 Basic Event Handling
20.11 Building Custom Container Views
20.12 Working with the Label View
20.13 Summary
21
21. SwiftUI Stacks and Frames
21. SwiftUI Stacks and Frames
21.1 SwiftUI Stacks
21.2 Spacers, Alignment and Padding
21.3 Container Child Limit
21.4 Text Line Limits and Layout Priority
21.5 Traditional vs. Lazy Stacks
21.6 SwiftUI Frames
21.7 Frames and the Geometry Reader
21.8 Summary
22
22. SwiftUI State Properties, Observable, State and Environment Objects
22. SwiftUI State Properties, Observable, State and Environment Objects
22.1 State Properties
22.2 State Binding
22.3 Observable Objects
22.4 State Objects
22.5 Environment Objects
22.6 Summary
23
23. A SwiftUI Example Tutorial
23. A SwiftUI Example Tutorial
23.1 Creating the Example Project
23.2 Reviewing the Project
23.3 Adding a VStack to the Layout
23.4 Adding a Slider View to the Stack
23.5 Adding a State Property
23.6 Adding Modifiers to the Text View
23.7 Adding Rotation and Animation
23.8 Adding a TextField to the Stack
23.9 Adding a Color Picker
23.10 Tidying the Layout
23.11 Summary
24
24. SwiftUI Lifecycle Event Modifiers
24. SwiftUI Lifecycle Event Modifiers
24.1 Creating the LifecycleDemo Project
24.2 Designing the App
24.3 The onAppear and onDisappear Modifiers
24.4 The onChange Modifier
24.5 ScenePhase and the onChange Modifier
24.6 Summary
25
25. SwiftUI Observable and Environment Objects – A Tutorial
25. SwiftUI Observable and Environment Objects – A Tutorial
25.1 About the ObservableDemo Project
25.2 Creating the Project
25.3 Adding the Observable Object
25.4 Designing the ContentView Layout
25.5 Adding the Second View
25.6 Adding Navigation
25.7 Using an Environment Object
25.8 Summary
26
26. SwiftUI Data Persistence using AppStorage and SceneStorage
26. SwiftUI Data Persistence using AppStorage and SceneStorage
26.1 The @SceneStorage Property Wrapper
26.2 The @AppStorage Property Wrapper
26.3 Creating and Preparing the StorageDemo Project
26.4 Using Scene Storage
26.5 Using App Storage
26.6 Storing Custom Types
26.7 Summary
27
27. SwiftUI Stack Alignment and Alignment Guides
27. SwiftUI Stack Alignment and Alignment Guides
27.1 Container Alignment
27.2 Alignment Guides
27.3 Using the Alignment Guides Tool
27.4 Custom Alignment Types
27.5 Cross Stack Alignment
27.6 ZStack Custom Alignment
27.7 Summary
28
28. SwiftUI Lists and Navigation
28. SwiftUI Lists and Navigation
28.1 SwiftUI Lists
28.2 SwiftUI Dynamic Lists
28.3 SwiftUI NavigationView and NavigationLink
28.4 Making the List Editable
28.5 Hierarchical Lists
28.6 Summary
29
29. A SwiftUI List and Navigation Tutorial
29. A SwiftUI List and Navigation Tutorial
29.1 About the ListNavDemo Project
29.2 Creating the ListNavDemo Project
29.3 Preparing the Project
29.4 Adding the Car Structure
29.5 Loading the JSON Data
29.6 Adding the Data Store
29.7 Designing the Content View
29.8 Designing the Detail View
29.9 Adding Navigation to the List
29.10 Designing the Add Car View
29.11 Implementing Add and Edit Buttons
29.12 Adding the Edit Button Methods
29.13 Summary
30
30. An Overview of List, OutlineGroup and DisclosureGroup
30. An Overview of List, OutlineGroup and DisclosureGroup
30.1 Hierarchical Data and Disclosures
30.2 Hierarchies and Disclosure in SwiftUI Lists
30.3 Using OutlineGroup
30.4 Using DisclosureGroup
30.5 Summary
31
31. A SwiftUI List, OutlineGroup and DisclosureGroup Tutorial
31. A SwiftUI List, OutlineGroup and DisclosureGroup Tutorial
31.1 About the Example Project
31.2 Creating the OutlineGroupDemo Project
31.3 Adding the Data Structure
31.4 Adding the List View
31.5 Testing the Project
31.6 Using the Sidebar List Style
31.7 Using OutlineGroup
31.8 Working with DisclosureGroups
31.9 Summary
32
32. Building SwiftUI Grids with LazyVGrid and LazyHGrid
32. Building SwiftUI Grids with LazyVGrid and LazyHGrid
32.1 SwiftUI Grids
32.2 GridItems
32.3 Creating the GridDemo Project
32.4 Working with Flexible GridItems
32.5 Adding Scrolling Support to a Grid
32.6 Working with Adaptive GridItems
32.7 Working with Fixed GridItems
32.8 Using the LazyHGrid View
32.9 Summary
33
33. Building Tabbed and Paged Views in SwiftUI
33. Building Tabbed and Paged Views in SwiftUI
33.1 An Overview of SwiftUI TabView
33.2 Creating the TabViewDemo App
33.3 Adding the TabView Container
33.4 Adding the Content Views
33.5 Adding View Paging
33.6 Adding the Tab Items
33.7 Adding Tab Item Tags
33.8 Summary
34
34. Building Context Menus in SwiftUI
34. Building Context Menus in SwiftUI
34.1 Creating the ContextMenuDemo Project
34.2 Preparing the Content View
34.3 Adding the Context Menu
34.4 Testing the Context Menu
34.5 Summary
35
35. Basic SwiftUI Graphics Drawing
35. Basic SwiftUI Graphics Drawing
35.1 Creating the DrawDemo Project
35.2 SwiftUI Shapes
35.3 Using Overlays
35.4 Drawing Custom Paths and Shapes
35.5 Drawing Gradients
35.6 Summary
36
36. SwiftUI Animation and Transitions
36. SwiftUI Animation and Transitions
36.1 Creating the AnimationDemo Example Project
36.2 Implicit Animation
36.3 Repeating an Animation
36.4 Explicit Animation
36.5 Animation and State Bindings
36.6 Automatically Starting an Animation
36.7 SwiftUI Transitions
36.8 Combining Transitions
36.9 Asymmetrical Transitions
36.10 Summary
37
37. Working with Gesture Recognizers in SwiftUI
37. Working with Gesture Recognizers in SwiftUI
37.1 Creating the GestureDemo Example Project
37.2 Basic Gestures
37.3 The onChange Action Callback
37.4 The updating Callback Action
37.5 Composing Gestures
37.6 Summary
38
38. Creating a Customized SwiftUI ProgressView
38. Creating a Customized SwiftUI ProgressView
38.1 ProgressView Styles
38.2 Creating the ProgressViewDemo Project
38.3 Adding a ProgressView
38.4 Using the Circular ProgressView Style
38.5 Declaring an Indeterminate ProgressView
38.6 ProgressView Customization
38.7 Summary
39
39. An Overview of SwiftUI DocumentGroup Scenes
39. An Overview of SwiftUI DocumentGroup Scenes
39.1 Documents in Apps
39.2 Creating the DocDemo App
39.3 The DocumentGroup Scene
39.4 Declaring File Type Support
39.5 Configuring File Type Support in Xcode
39.6 The Document Structure
39.7 The Content View
39.8 Running the Example App
39.9 Summary
40
40. A SwiftUI DocumentGroup Tutorial
40. A SwiftUI DocumentGroup Tutorial
40.1 Creating the ImageDocDemo Project
40.2 Modifying the Info.plist File
40.3 Adding an Image Asset
40.4 Modifying the ImageDocDemoDocument.swift File
40.5 Designing the Content View
40.6 Filtering the Image
40.7 Testing the App
40.8 Summary
41
41. An Introduction to SiriKit
41. An Introduction to SiriKit
41.1 Siri and SiriKit
41.2 SiriKit Domains
41.3 Siri Shortcuts
41.4 SiriKit Intents
41.5 How SiriKit Integration Works
41.6 Resolving Intent Parameters
41.7 The Confirm Method
41.8 The Handle Method
41.9 Custom Vocabulary
41.10 The Siri User Interface
41.11 Summary
42
42. A SwiftUI SiriKit Messaging Extension Tutorial
42. A SwiftUI SiriKit Messaging Extension Tutorial
42.1 Creating the Example Project
42.2 Enabling the Siri Entitlement
42.3 Seeking Siri Authorization
42.4 Adding the Intents Extension
42.5 Supported Intents
42.6 Trying the Example
42.7 Specifying a Default Phrase
42.8 Reviewing the Intent Handler
42.9 Summary
43
43. Customizing the SiriKit Intent User Interface
43. Customizing the SiriKit Intent User Interface
43.1 Adding the Intents UI Extension
43.2 Modifying the UI Extension
43.3 Using the configure Method
43.4 Using the configureView Method
43.5 Designing the Siri Snippet
43.6 Implementing a configureView Method
43.7 Testing the Extension
43.8 Summary
44
44. A SwiftUI SiriKit NSUserActivity Tutorial
44. A SwiftUI SiriKit NSUserActivity Tutorial
44.1 About the SiriKit Photo Search Project
44.2 Creating the SiriPhoto Project
44.3 Enabling the Siri Entitlement
44.4 Seeking Siri Authorization
44.5 Adding an Image Asset
44.6 Adding the Intents Extension to the Project
44.7 Reviewing the Default Intents Extension
44.8 Modifying the Supported Intents
44.9 Modifying the IntentHandler Implementation
44.10 Implementing the Resolve Methods
44.11 Implementing the Confirmation Method
44.12 Handling the Intent
44.13 Testing the App
44.14 Adding a Data Class to SiriPhoto
44.15 Designing the Content View
44.16 Adding Supported Activity Types to SiriPhoto
44.17 Handling the NSUserActivity Object
44.18 Testing the Completed App
44.19 Summary
45
45. An Overview of Siri Shortcut App Integration
45. An Overview of Siri Shortcut App Integration
45.1 An Overview of Siri Shortcuts
45.2 An Introduction to the Intent Definition File
45.3 Automatically Generated Classes
45.4 Donating Shortcuts
45.5 The Add to Siri Button
45.6 Summary
46
46. A SwiftUI Siri Shortcut Tutorial
46. A SwiftUI Siri Shortcut Tutorial
46.1 About the Example App
46.2 App Groups and UserDefaults
46.3 Preparing the Project
46.4 Running the App
46.5 Enabling Siri Support
46.6 Seeking Siri Authorization
46.7 Adding the Intents Extension
46.8 Adding the SiriKit Intent Definition File
46.9 Adding the Intent to the App Group
46.10 Configuring the SiriKit Intent Definition File
46.11 Adding Intent Parameters
46.12 Declaring Shortcut Combinations
46.13 Configuring the Intent Response
46.14 Configuring Target Membership
46.15 Modifying the Intent Handler Code
46.16 Adding the Confirm Method
46.17 Donating Shortcuts to Siri
46.18 Testing the Shortcuts
46.19 Designing the Intent UI
46.20 Summary
47
47. Building Widgets with SwiftUI and WidgetKit
47. Building Widgets with SwiftUI and WidgetKit
47.1 An Overview of Widgets
47.2 The Widget Extension
47.3 Widget Configuration Types
47.4 Widget Entry View
47.5 Widget Timeline Entries
47.6 Widget Timeline
47.7 Widget Provider
47.8 Reload Policy
47.9 Relevance
47.10 Forcing a Timeline Reload
47.11 Widget Sizes
47.12 Widget Placeholder
47.13 Summary
48
48. A SwiftUI WidgetKit Tutorial
48. A SwiftUI WidgetKit Tutorial
48.1 About the WidgetDemo Project
48.2 Creating the WidgetDemo Project
48.3 Building the App
48.4 Adding the Widget Extension
48.5 Adding the Widget Data
48.6 Creating Sample Timelines
48.7 Adding Image and Color Assets
48.8 Designing the Widget View
48.9 Modifying the Widget Provider
48.10 Configuring the Placeholder View
48.11 Previewing the Widget
48.12 Summary
49
49. Supporting WidgetKit Size Families
49. Supporting WidgetKit Size Families
49.1 Supporting Multiple Size Families
49.2 Adding Size Support to the Widget View
49.3 Summary
50
50. A SwiftUI WidgetKit Deep Link Tutorial
50. A SwiftUI WidgetKit Deep Link Tutorial
50.1 Adding Deep Link Support to the Widget
50.2 Adding Deep Link Support to the App
50.3 Testing the Widget
50.4 Summary
51
51. Adding Configuration Options to a WidgetKit Widget
51. Adding Configuration Options to a WidgetKit Widget
51.1 Modifying the Weather Data
51.2 Configuring the Intent Definition
51.3 Modifying the Widget
51.4 Testing Widget Configuration
51.5 Customizing the Configuration Intent UI
51.6 Summary
52
52. Integrating UIViews with SwiftUI
52. Integrating UIViews with SwiftUI
52.1 SwiftUI and UIKit Integration
52.2 Integrating UIViews into SwiftUI
52.3 Adding a Coordinator
52.4 Handling UIKit Delegation and Data Sources
52.5 An Example Project
52.6 Wrapping the UIScrolledView
52.7 Implementing the Coordinator
52.8 Using MyScrollView
52.9 Summary
53
53. Integrating UIViewControllers with SwiftUI
53. Integrating UIViewControllers with SwiftUI
53.1 UIViewControllers and SwiftUI
53.2 Creating the ViewControllerDemo project
53.3 Wrapping the UIImagePickerController
53.4 Designing the Content View
53.5 Completing MyImagePicker
53.6 Completing the Content View
53.7 Testing the App
53.8 Summary
54
54. Integrating SwiftUI with UIKit
54. Integrating SwiftUI with UIKit
54.1 An Overview of the Hosting Controller
54.2 A UIHostingController Example Project
54.3 Adding the SwiftUI Content View
54.4 Preparing the Storyboard
54.5 Adding a Hosting Controller
54.6 Configuring the Segue Action
54.7 Embedding a Container View
54.8 Embedding SwiftUI in Code
54.9 Summary
55
55. Preparing and Submitting an iOS 14 Application to the App Store
55. Preparing and Submitting an iOS 14 Application to the App Store
55.1 Verifying the iOS Distribution Certificate
55.2 Adding App Icons
55.3 Assign the Project to a Team
55.4 Archiving the Application for Distribution
55.5 Configuring the Application in App Store Connect
55.6 Validating and Submitting the Application
55.7 Configuring and Submitting the App for Review
56
Index
Index

2.2 Apple Developer Program

Membership in the Apple Developer Program currently costs $99 per year to enroll as an individual developer. Organization level membership is also available.

Prior to the introduction of iOS 9 and Xcode 7, one of the key advantages of the developer program was that it permitted the creation of certificates and provisioning profiles to test your applications on physical iOS devices. Fortunately, this is no longer the case and all that is now required to test apps on physical iOS devices is an Apple ID.

Clearly much can be achieved without the need to pay to join the Apple Developer program. There are, however, areas of app development which cannot be fully tested without program membership. Of particular significance is the fact that Siri integration, iCloud access, Apple Pay, Game Center and In-App Purchasing can only be enabled and tested with Apple Developer Program membership.

Of further significance is the fact that Apple Developer Program members have access to technical support from Apple’s iOS support engineers (though the annual fee initially covers the submission of only two support incident reports, more can be purchased). Membership also includes access to the Apple Developer forums; an invaluable resource both for obtaining assistance and guidance from other iOS developers, and for finding solutions to problems that others have encountered and subsequently resolved.

Program membership also provides early access to the pre-release Beta versions of Xcode, macOS and iOS.

By far the most important aspect of the Apple Developer Program is that membership is a mandatory requirement in order to publish an application for sale or download in the App Store.

Clearly, program membership is going to be required at some point before your application reaches the App Store. The only question remaining is when exactly to sign up.

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