/swift-core

name: swift-core
description: Use when implementing Swift 6 fundamentals — concurrency, architecture, testing, i18n, or performance — across any Apple platform.
versions:
  swift: 6.4.2
  ios: 27.2
user-invocable: false
references: references/concurrency.md, references/architecture.md, references/testing.md, references/i18n.md, references/performance.md, references/swift-6-4.md, references/sdk-27-changes.md
related-skills: swiftui-core, solid-swift, ios, macos, ipados, watchos, visionos

<objective> Covers Swift 6 language and platform fundamentals shared across iOS, macOS, iPadOS, watchOS, and visionOS: async/await and actors with strict concurrency checking, Sendable typing, app architecture (MVVM with @Observable as the recommended default, Clean Architecture for complex apps), async testing with XCTest, String Catalog-based localization, and performance profiling with Instruments.

References cover each area in depth: concurrency, architecture/DI, testing, i18n, and performance.

Best practices: prefer actors over classes with locks for shared state, @Observable over ObservableObject, structured concurrency over completion handlers, String Catalogs for all user-facing text, profiling in Release (-O) builds, and value types (structs) over classes where possible. </objective>

Swift Core

Swift 6 fundamentals shared across all Apple platforms.

Agent Workflow (MANDATORY)

Before ANY implementation, spawn 3 agents in parallel, one `Agent` call each with a `name`:

1. **fuse-ai-pilot:explore-codebase** - Analyze existing Swift patterns 2. **fuse-ai-pilot:research-expert** - Verify latest Swift 6 docs via Context7/Exa 3. **mcp__apple-docs__search_apple_docs** - Check Swift concurrency patterns

After implementation, run **fuse-ai-pilot:sniper** for validation.

---

Overview

When to Use

• Implementing async/await, actors, Sendable
• Designing app architecture (MVVM, Clean Architecture)
• Writing async tests with XCTest
• Localizing with String Catalogs
• Profiling with Instruments

Why Swift Core

| Feature | Benefit | |---------|---------| | Actors | Thread-safe shared state without locks | | @Observable | Simple reactive state (replaces ObservableObject) | | String Catalogs | Automatic localization with Xcode 15+ | | Instruments | Built-in performance profiling |

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Key Concepts

Concurrency (Swift 6)

Modern async/await with strict concurrency checking. Actors provide thread-safe state, Sendable marks safe types.

Architecture

MVVM with @Observable is the recommended pattern. Clean Architecture for complex apps with domain separation.

Testing

XCTest with native async/await support. No need for expectations with async tests.

Internationalization

String Catalogs are mandatory. All user-facing text must be localized.

Performance

Profile with Instruments. Use lazy loading, avoid heavy work in view body.

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Reference Guide

| Need | Reference | |------|-----------| | Async/await, actors, Sendable | [concurrency.md](references/concurrency.md) | | MVVM, Clean Architecture, DI | [architecture.md](references/architecture.md) | | XCTest, async tests, mocking | [testing.md](references/testing.md) | | String Catalogs, localization | [i18n.md](references/i18n.md) | | Instruments, optimization | [performance.md](references/performance.md) | | Swift 6.4.x: Swift Build/SwiftPM, Subprocess 1.0, language additions | [swift-6-4.md](references/swift-6-4.md) | | Xcode/OS 27.0-27.2: breaking changes, deprecations, 27.1/27.2 notes | [sdk-27-changes.md](references/sdk-27-changes.md) |

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Best Practices

1. **Actors for shared state** - Prefer actors over classes with locks 2. **@Observable over ObservableObject** - Simpler, better performance 3. **Structured concurrency** - async/await, no completion handlers 4. **String Catalogs** - ALL user-facing text must be localized 5. **Profile in Release** - Always profile with `-O` optimization 6. **Value types** - Prefer structs over classes