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115 lines
4 KiB
Markdown
115 lines
4 KiB
Markdown
# ADR-005: Adoption of Swift Observation Framework
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**Status:** Accepted
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**Date:** 2025-01-11
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**Context:**
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After migrating services and repositories from Combine to async/await (ADR-004), the UI layer still used Combine's ObservableObject protocol with @Published properties for view-viewmodel binding. Apple introduced the Observation framework with the @Observable macro as a modern replacement.
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Key considerations:
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1. **Simpler syntax:** @Observable eliminates boilerplate (@Published, objectWillChange)
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2. **Better performance:** Fine-grained observation tracks only accessed properties
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3. **Reduced dependencies:** No need for Combine in view models
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4. **Language-level support:** @Observable is a Swift macro, not a framework feature
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5. **SwiftUI integration:** Native support in SwiftUI for @Observable types
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**Decision:**
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I migrated all view models from ObservableObject + @Published to @Observable. This completed the removal of Combine from the codebase.
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**Migration Pattern:**
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**Before (Combine):**
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```swift
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final class StatusViewModel: ObservableObject {
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@Published var statuses: [Status] = []
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@Published var isLoading = false
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private var cancellables = Set<AnyCancellable>()
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func loadStatuses() {
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isLoading = true
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repository.fetchStatuses()
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.sink { [weak self] completion in
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self?.isLoading = false
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} receiveValue: { [weak self] statuses in
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self?.statuses = statuses
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}
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.store(in: &cancellables)
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}
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}
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```
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**After (Observation):**
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```swift
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@Observable
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final class StatusViewModel {
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var statuses: [Status] = []
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var isLoading = false
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func loadStatuses() async {
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isLoading = true
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defer { isLoading = false }
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do {
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statuses = try await repository.fetchStatuses()
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} catch {
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// handle error
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}
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}
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}
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```
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**Use of @ObservationIgnored:**
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Properties marked with @ObservationIgnored don't trigger view updates:
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- Dependencies (repositories, services) injected via initializer
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- Computed properties
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- Internal state that doesn't affect UI
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- Constants and configuration
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**Consequences:**
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### Positive
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- **Less boilerplate:** No @Published, no Combine imports, no AnyCancellable storage
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- **Better performance:** SwiftUI only observes accessed properties, not the entire object
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- **Clearer code:** Direct property access instead of publisher chains
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- **Natural async integration:** Works seamlessly with async/await in repositories
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- **Compile-time safety:** @Observable provides type-safe observation
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- **Removed Combine dependency:** UI layer no longer needs Combine framework
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### Negative
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- **Migration effort:** Requires rewriting all view models
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- **Learning curve:** Understanding when to use @ObservationIgnored
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- **Breaking changes:** View models are no longer ObservableObject instances
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### Neutral
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- **Different patterns:** Some Combine patterns (like debouncing) need alternative implementations
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- **Observation scope:** Need to understand observation tracking to avoid over-observation
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**Migration Strategy:**
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The migration was done incrementally:
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1. Started with simpler view models with fewer properties
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2. Replaced ObservableObject conformance with @Observable macro
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3. Removed @Published from properties
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4. Converted Combine publisher chains to async/await calls
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5. Removed AnyCancellable storage
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6. Added @ObservationIgnored to dependencies and computed properties
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7. Updated views to work with @Observable (mostly automatic)
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**Related Decisions:**
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- [ADR-004: Migration from Combine to Async/Await](ADR-004-migration-from-combine-to-async-await.md) - Repositories already using async/await enabled smooth integration
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- [ADR-002: Layered Architecture and Dependency Direction](ADR-002-layered-architecture-and-dependency-direction.md) - ViewModels remain in UI layer with same responsibilities
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**Notes:**
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This migration completed the transition away from Combine throughout the entire codebase. The Observation framework provides a modern, performant, and simpler approach to reactive UI updates in SwiftUI.
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