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WWDC 2025 - SwiftData iOS 26 - Class Inheritance & Migration

#ios #mobile #swift #programming

SwiftData continues to evolve as Apple's premier data persistence framework, and with iOS 26, class inheritance support transforms how we model complex data relationships. This comprehensive guide explores inheritance patterns, migration strategies, and performance optimizations that every iOS developer should master.

Understanding SwiftData Inheritance

When to Use Class Inheritance

Class inheritance in SwiftData works best when models form natural hierarchies with shared characteristics. The key principle is the "is-a" relationship - if one model type naturally extends another, inheritance becomes valuable.

Ideal scenarios for inheritance:

  • Models that share core properties and behaviors
  • Natural subdomain relationships within a broader domain
  • Mixed query patterns (both parent and child types)
  • Hierarchical data structures with common functionality

Avoid inheritance when:

  • Models only share a single property or behavior
  • Subdomains don't form natural hierarchies
  • Only shallow searches (leaf classes) are performed
  • Protocol conformance would better express shared behaviors

Implementation Pattern 

@Model
class Event {
    var title: String
    var location: String
    var scheduledDate: Date
    var duration: TimeInterval

    init(title: String, location: String, scheduledDate: Date, duration: TimeInterval) {
        self.title = title
        self.location = location
        self.scheduledDate = scheduledDate
        self.duration = duration
    }
}

@available(iOS 26, *)
@Model
class WorkEvent: Event {
    var budget: Decimal = 0.0
    var departmentCode: String = ""
}

@available(iOS 26, *)
@Model
class SocialEvent: Event {
    enum Category: String, CaseIterable, Codable {
        case birthday, wedding, celebration
    }
    var category: Category
    var guestCount: Int = 0
}
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Schema Configuration

Update the model container to include all model types:

.modelContainer(for: [Event.self, WorkEvent.self, SocialEvent.self])
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Advanced Querying with Inheritance

Type-Specific Filtering

SwiftData enables sophisticated querying using the is keyword for type checking:

struct EventListView: View {
    @State private var selectedFilter: EventFilter = .all

    enum EventFilter: String, CaseIterable {
        case all = "All Events"
        case social = "Social" 
        case work = "Work"
    }

    @Query var events: [Event]

    init(filter: Binding<EventFilter>) {
        let filterPredicate: Predicate<Event>? = {
            switch filter.wrappedValue {
            case .social:
                return #Predicate { $0 is SocialEvent }
            case .work:
                return #Predicate { $0 is WorkEvent }
            default:
                return nil
            }
        }()

        _events = Query(filter: filterPredicate, sort: \.scheduledDate)
    }
}
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Schema Migration Strategies

Versioned Schema Evolution

Proper migration planning prevents data loss during app updates. Here's the structured approach:

Version 3.0 - Adding Inheritance:

@available(iOS 26, *)
enum EventSchemaV3: VersionedSchema {
    static var versionIdentifier: Schema.Version { 
        Schema.Version(3, 0, 0) 
    }

    static var models: [any PersistentModel.Type] {
        [Event.self, WorkEvent.self, SocialEvent.self]
    }
}
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Lightweight Migration Stage:

@available(iOS 26, *)
static let migrateV2toV3 = MigrationStage.lightweight(
    fromVersion: EventSchemaV2.self,
    toVersion: EventSchemaV3.self
)
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Migration Plan Implementation 

enum EventMigrationPlan: SchemaMigrationPlan {
    static var schemas: [any VersionedSchema.Type] {
        var allSchemas = [EventSchemaV1.self, EventSchemaV2.self]
        if #available(iOS 26, *) {
            allSchemas.append(EventSchemaV3.self)
        }
        return allSchemas
    }

    static var stages: [MigrationStage] {
        var allStages = [migrateV1toV2]
        if #available(iOS 26, *) {
            allStages.append(migrateV2toV3)
        }
        return allStages
    }
}
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Performance Optimization Techniques

Selective Property Fetching

For migration stages and memory-intensive operations, fetch only required properties:

var fetchDescriptor = FetchDescriptor<Event>()
fetchDescriptor.propertiesToFetch = [\.title, \.scheduledDate]
fetchDescriptor.relationshipKeyPathsForPrefetching = [\.venue]
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Fetch Limiting

Optimize widget performance and reduce memory overhead:

var descriptor = FetchDescriptor(sortBy: [SortDescriptor(\Event.scheduledDate)])
descriptor.predicate = #Predicate { $0.scheduledDate >= Date.now }
descriptor.fetchLimit = 5
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Compound Predicate Construction

Combine multiple filtering conditions efficiently:

let searchPredicate = #Predicate<Event> {
    searchQuery.isEmpty || 
    $0.title.localizedStandardContains(searchQuery) ||
    $0.location.localizedStandardContains(searchQuery)
}

let typePredicate = #Predicate<Event> { $0 is SocialEvent }

let combinedPredicate = #Predicate<Event> {
    searchPredicate.evaluate($0) && typePredicate.evaluate($0)
}
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Change Observation Patterns

Local Change Tracking

SwiftData models are Observable by default, enabling reactive UI updates:

func trackEventChanges(for event: Event) {
    withObservationTracking {
        _ = event.scheduledDate
        _ = event.location
    } onChange: {
        showEventUpdateAlert = true
    }
}
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Persistent History Integration

Track external changes using SwiftData's history API:

// Efficient history token retrieval
var historyDescriptor = HistoryDescriptor<DefaultHistoryTransaction>()
historyDescriptor.sortBy = [.init(\.transactionIdentifier, order: .reverse)]
historyDescriptor.fetchLimit = 1

// Targeted change detection
let tokenPredicate = #Predicate<DefaultHistoryTransaction> { 
    $0.token > lastProcessedToken 
}

let entityPredicate = #Predicate<DefaultHistoryTransaction> {
    $0.changes.contains { change in
        ["Event", "WorkEvent", "SocialEvent"].contains(
            change.changedPersistentIdentifier.entityName
        )
    }
}
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Best Practices Summary

Design Considerations

  • Inheritance Hierarchy: Keep inheritance trees shallow and logical
  • Query Patterns: Design inheritance based on actual query needs
  • Migration Planning: Always version schemas before structural changes
  • Performance: Use selective fetching for large datasets

Implementation Guidelines

  • Availability Decorators: Mark new inheritance features with iOS 26+ availability
  • Schema Versioning: Increment version numbers for each significant change
  • Migration Types: Use lightweight migrations when possible, custom for complex transformations
  • Change Observation: Combine local observation with persistent history for complete coverage

Common Pitfalls

  • Over-inheritance: Don't create inheritance just to share single properties
  • Migration Gaps: Ensure continuous migration path through all app versions
  • Query Performance: Balance deep vs shallow searches based on usage patterns
  • Memory Management: Use fetch limits and property selection in resource-constrained environments

SwiftData's inheritance support in iOS 26 opens new possibilities for elegant data modeling.

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