/modern-swift

Migration Patterns

Common patterns for migrating legacy Swift code to modern best practices.

Delegate → AsyncStream

✅ Modern Pattern

// Before: Delegate pattern
protocol LocationManagerDelegate: AnyObject {
    func locationManager(_ manager: LocationManager, didUpdateLocation: Location)
}

class LocationManager {
    weak var delegate: LocationManagerDelegate?
}

// After: AsyncStream
class LocationManager {
    var locations: AsyncStream<Location> {
        AsyncStream { continuation in
            // Setup location updates
            self.onLocationUpdate = { location in
                continuation.yield(location)
            }
            continuation.onTermination = { _ in
                // Cleanup
            }
        }
    }
}

// Usage
for await location in locationManager.locations {
    updateUI(with: location)
}

Effort: ~3 hours per delegate Risk: Medium - changes API surface

UIKit → SwiftUI

✅ Modern Pattern

// Before: UIKit
class ProfileViewController: UIViewController {
    @IBOutlet weak var nameLabel: UILabel!
    @IBOutlet weak var avatarImageView: UIImageView!

    override func viewDidLoad() {
        super.viewDidLoad()
        nameLabel.text = user.name
        avatarImageView.load(url: user.avatarURL)
    }
}

// After: SwiftUI
struct ProfileView: View {
    let user: User

    var body: some View {
        VStack {
            AsyncImage(url: user.avatarURL) { image in
                image.resizable()
                    .aspectRatio(contentMode: .fill)
            } placeholder: {
                ProgressView()
            }
            .frame(width: 100, height: 100)
            .clipShape(Circle())

            Text(user.name)
                .font(.headline)
        }
    }
}

Effort: ~8 hours per view controller Risk: High - requires understanding of both frameworks

Common UIKit → SwiftUI Mappings

Migration Workflow

1. Analyze

• Identify all occurrences of the pattern using Grep
• Map dependencies and call sites
• Estimate effort and risk

2. Plan

• Create migration checklist with TodoWrite
• Identify test points
• Plan rollback strategy if needed

3. Execute

• Migrate one component at a time
• Add compatibility shims if needed
• Update call sites progressively

4. Verify

• Run existing tests after each change
• Test edge cases
• Check performance impact

Effort & Risk Table

Deprecated API Replacements

Always check Sosumi MCP server for current API status and replacements:

Use Sosumi to verify deprecation status and find migration guides for 2025.