1. SwiftUI uses the concept of a single source of truth where data is stored and managed in one central location to avoid inconsistencies. 2. @State stores data locally for a view and makes it the source of truth for that view, triggering updates when it changes. @Binding provides a reference to a source of truth owned by another view. 3. @ObservedObject and @EnvironmentObject can be used to integrate external data sources into views. @ObservedObject creates a direct dependency while @EnvironmentObject provides data indirectly through the environment.

1. SwiftUI @
State & Data Flow.
@dana.allwhite

2. Contents.
🔎 Basic Concepts - Source of Truth, Property Wrapper
🔎 Internal - @State, @Binding
🔎 External - ObservableObject, EnvironmentObject

3. (Single) Source of Truth
# source of truth - wikipedia
Single source of truth (SSOT) is the practice of structuring information models and associated data schema such that every
data element is mastered (or edited) in only one place. Any possible linkages to this data element (possibly in other areas of the
relational schema or even in distant federated databases) are by reference only. Because all other locations of the data just refer
back to the primary "source of truth" location, updates to the data element in the primary location propagate to the entire system
without the possibility of a duplicate value somewhere being forgotten.

4. Property Wrapper
To wrap a property with some additional behavior when it’s written and read.
# property wrapper - Swift Language Guide

5. @State - Source of Truth of View
struct PlayerView : View {
let episode: Episode
var isPlaying: Bool
var body: some View {
VStack {
Text(episode.title)
Text(episode.showTitle).font(.caption).foregroundColor(.gray)
Button(action: {
self.isPlaying.toggle()
}) {
Image(systemName: isPlaying ? "pause.circle" : "play.circle")
}
}
}
}

6. @State - Source of Truth of View
struct PlayerView : View {
let episode: Episode
@State private var isPlaying: Bool
var body: some View {
VStack {
Text(episode.title)
Text(episode.showTitle).font(.caption).foregroundColor(.gray)
Button(action: {
self.isPlaying.toggle()
}) {
Image(systemName: isPlaying ? "pause.circle" : "play.circle")
}
}
}
}

7. View Update

8. View Update

9. View Update

10. View Update

12. @Binding - Reference to Source of Truth
struct PlayerView : View {
let episode: Episode
@State private var isPlaying: Bool
var body: some View {
VStack {
Text(episode.title)
Text(episode.showTitle).font(.caption).foregroundColor(.gray)
// Button(action: {
// self.isPlaying.toggle()
// }) {
// Image(systemName: isPlaying ? "pause.circle" : "play.circle")
// }
PlayButton()
}
}
}

13. @Binding - Reference to Source of Truth
struct PlayButton: View {
@State var isPlaying: Bool = false
var body: some View {
Button(action: {
self.isPlaying.toggle()
}) {
Image(systemName: isPlaying ? "pause.circle" : "play.circle")
}
}
}
isPlaying
PlayerView
isPlaying
PlayButton

14. @Binding - Reference to Source of Truth
struct PlayButton: View {
@Binding var isPlaying: Bool = false
var body: some View {
Button(action: {
self.isPlaying.toggle()
}) {
Image(systemName: isPlaying ? "pause.circle" : "play.circle")
}
}
}
A property wrapper type that can read and write a value owned by a source of truth.

15. @Binding - Reference to Source of Truth
struct PlayerView : View {
let episode: Episode
@State private var isPlaying: Bool
var body: some View {
VStack {
Text(episode.title)
Text(episode.showTitle).font(.caption).foregroundColor(.gray)
PlayButton(isPlaying: $isPlaying)
}
}
}

16. State Management

17. State Management - SwiftUI

18. SwiftUI Components using @Binding

19. Data from External Source
Property Wrapper Creating Dependency
ObservableObject @ObservedObject Direct
EnvironmentObject @EnviromnetObject Indirect

20. ObservableObject protocol
final class UserData: ObservableObject {
@Published var showFavoritesOnly = false
@Published var landmarks = landmarkData
}
objectWillChange

21. ObservableObject - @ObservedObject
struct LandmarkList: View {
@ObservedObject var userData: UserData
var body: some View {
NavigationView {
List {
Toggle(isOn: $userData.showFavoritesOnly) {
Text("Show Favorites Only")
}
ForEach(userData.landmarks) { landmark in
if !self.userData.showFavoritesOnly || landmark.isFavorite {
// ~~~~
}
}
}
.navigationBarTitle(Text("Landmarks"))
}
}
}
LandmarkList(userData: UserData())

22. EnvironmentObject - .environmentObject(_)
// SceneDelegate
let window = UIWindow(windowScene: windowScene)
window.rootViewController = UIHostingController(rootView:
LandmarkList()
.environmentObject(UserData()))
self.window = window
window.makeKeyAndVisible()

23. EnvironmentObject - @EnvironmentObject
struct LandmarkList: View {
@EnvironmentObject private var userData: UserData
var body: some View {
NavigationView {
List {
Toggle(isOn: $userData.showFavoritesOnly) {
Text("Show Favorites Only”)
}
ForEach(userData.landmarks) { landmark in
if !self.userData.showFavoritesOnly || landmark.isFavorite {
// ~~~
}
}
}
.navigationBarTitle(Text("Landmarks"))
}
}
}

24. When to Use?
@EnvironmentObject
@ObservableObject

25. EnvironmentObject - @Environment

26. .
1. . - single source of truth
2. State source of truth , view function of state .
3. @State framework view change ,
@Binding souce of truth reference
4. @ObservedObject dependency ,
@EnvironmentObject view dependency

27. Reference.
- SwiftUI by Tutorials (Raywenderlich)
- Data Flow Through SwiftUI (WWDC 2019)
- SwiftUI Essential : Handling User Input
- Data in SwiftUI, Part 1: Data
- Single Source of Truth - Wikipedia