SwiftUI Architecture

SwiftUI Architecture

• There are many
• Reduced simply to Model-View
• With a data Store to drive the view

State

• Declare with @State var
• Views are a function of state
• Change the state (input) -> View will react accordingly

Binding

• State is the source-of-truth
• Binding is the "reference type" to the state

StateObject & ObservedObject

• Equivalent to State & Binding
• But for custom object types

Create your observable object

class Book: ObservableObject {
    @Published var title = "Great Expectations"
}

View observe the object

struct BookView: View {
    @ObservedObject var book: Book
    
    var body: some View {
        Text(book.title)
    }
}

Where is the source of truth?

struct MyApp: App {
    @StateObject private var book = Book()

    @SceneBuilder var body: some Scene {
        ...
        BookView(book: book)
        ...
    }
}

Passing via Environment

• Another way to pass the Book to BookView
• Environment is like a global scope, passing to any child views
• No need to pass during view init BookView(book: book)

Passing via Environment

struct MyApp: App {
    @StateObject private var book = Book()

    @SceneBuilder var body: some Scene {
        BookView()
            .environmentObject(book)
        // Previously
        // BookView(book: book)
    }
}

Passing via Environment

struct BookView: View {
    @EnvironmentObject var book: Book

    // Previously
    // @ObservedObject var book: Book
    
    var body: some View {
        Text(book.title)
    }
}