assumeIsolated — Synchronous Actor Access

Purpose: Synchronously access actor-isolated state when you know you're on the correct isolation domain Swift Version: Swift 5.9+ iOS Version: iOS 13+

When to Use This Skill

✅ Use this skill when:

• Testing MainActor code synchronously (avoiding Task overhead)
• Legacy delegate callbacks documented to run on main thread
• Performance-critical code avoiding async hop overhead
• Protocol conformances where callbacks are guaranteed on specific actor

❌ Do NOT use this skill for:

• Uncertain about current isolation (use await instead)
• Already in async context (you have isolation)
• Cross-actor calls needed (use async)

Key Patterns

MainActor.assumeIsolated

static func assumeIsolated<T>(
    _ operation: @MainActor () throws -> T,
    file: StaticString = #fileID,
    line: UInt = #line
) rethrows -> T where T: Sendable

Behavior: Executes synchronously. Crashes if not on MainActor's serial executor.

Legacy Delegate Callbacks

From WWDC 2024-10169 — When documentation guarantees main thread delivery:

@MainActor
class LocationDelegate: NSObject, CLLocationManagerDelegate {
    var location: CLLocation?

    nonisolated func locationManager(
        _ manager: CLLocationManager,
        didUpdateLocations locations: [CLLocation]
    ) {
        MainActor.assumeIsolated {
            self.location = locations.last
        }
    }
}

@preconcurrency Shorthand

// ✅ Using @preconcurrency (cleaner)
extension MyClass: @preconcurrency SomeDelegate {
    func callback() {
        self.updateUI()  // Compiler wraps in assumeIsolated
    }
}

Task vs assumeIsolated

Aspect	Task { @MainActor in }	MainActor.assumeIsolated
Timing	Deferred (next run loop)	Synchronous (inline)
Async support	Yes (can await)	No (sync only)
Failure mode	Runs anyway	Crashes if wrong isolation

Related Skills

• Swift Concurrency — Complete concurrency guide
• Swift Testing — Testing MainActor code

Resources

• WWDC: Migrate your app to Swift 6 (10169)
• Docs: MainActor.assumeIsolated