Swiftui

SwiftUI

SwiftUI is Apple's declarative framework for building user interfaces across all Apple platforms (iOS, macOS, watchOS, tvOS, visionOS) with the power of Swift.

When to Use

• Building modern iOS and macOS applications.
• Targeting multiple Apple platforms with shared UI code.
• Implementing complex animations and transitions with less code.
• Utilizing live previews for rapid UI iteration.

Quick Start

import SwiftUI

@main
struct MyApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

// Observation Framework (iOS 17+)
@Observable
class UserSettings {
    var username = "Guest"
    var isLoggedIn = false
}

struct ContentView: View {
    @State private var settings = UserSettings()

    var body: some View {
        NavigationStack {
            VStack(spacing: 20) {
                Text("Hello, \(settings.username)!")
                    .font(.largeTitle)

                Button("Log In") {
                    settings.username = "User"
                    settings.isLoggedIn = true
                }
                .buttonStyle(.borderedProminent)

                NavigationLink("Settings", value: "settings")
            }
            .navigationDestination(for: String.self) { path in
                if path == "settings" {
                    Text("Settings Page")
                }
            }
        }
    }
}

Core Concepts

Declarative Syntax

Instead of imperatively mutating UI views (like UIKit), you describe what the UI should look like for a given state. The system handles the updates.

State & Data Flow (Modern)

• @State: Source of truth for simple, view-local value types.
• @Binding: Two-way connection to a value owned by another view.
• @Observable: (iOS 17+) Macro for creating observable reference types. Replaces @StateObject and @ObservedObject for cleaner data flow.

Modifiers

Methods called on views that wrap the view and return a new view with the modification applied (e.g., .padding(), .background()). Order matters.

Common Patterns

NavigationStack (Path-based)

Replace NavigationView with NavigationStack for robust programmatic navigation.

• Use .navigationDestination(for:) to decouple navigation logic from views.
• Manage navigation state (NavigationPath) in a model for deep linking support.

MVVM with Observation

Bind Views to ViewModels marked with @Observable. The View purely renders the state exposed by the ViewModel.

@Observable class ProfileViewModel {
    var profile: Profile?

    func loadProfile() async { /* ... */ }
}

Best Practices

Do:

• Use @Observable for data models in iOS 17+ targets.
• Break down large views into smaller, reusable subviews (Extract Subview).
• Use Previews with different configurations (Dark Mode, Dynamic Type) to catch UI issues early.
• Use Environment for global dependencies (like themes or user session).

Don't:

• Don't perform heavy work in the body property (it's computed frequently).
• Don't use AnyView unless absolutely necessary (kills performance/diffing).
• Don't force imperative patterns (like trying to "refresh" a view manually); change the state instead.

Troubleshooting

Error	Cause	Solution
Type '...' does not conform to protocol 'View'	The body property is missing or doesn't return some View.	Ensure var body: some View returns a valid view hierarchy.
Modifying state during view update	Changing @State directly inside the body calculation.	Move side effects to .onAppear or buttons/actions.
Trailing closure passed to parameter of type...	Syntax error in view builder structure.	Check braces {} and modifier placement.

References

• Apple SwiftUI Documentation
• WWDC23: Discover Observation
• Hacking with Swift - SwiftUI