assumeIsolated — Synchronous Actor Access

Synchronously access actor-isolated state when you know you're already on the correct isolation domain.

When to Use

✅ Use 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

❌ Don't use when:

• Uncertain about current isolation (use await instead)
• Already in async context (you have isolation)
• Cross-actor calls needed (use async)
• Callback origin is unknown or untrusted

API Reference

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.

Custom Actor assumeIsolated

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

Task vs assumeIsolated

Patterns

Pattern 1: Testing MainActor Code

@Test func viewModelUpdates() {
    MainActor.assumeIsolated {
        let vm = ViewModel()
        vm.update()
        #expect(vm.state == .updated)
    }
}

Pattern 2: Legacy Delegate Callbacks

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

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

    // CLLocationManager created on main thread delivers callbacks on main thread
    nonisolated func locationManager(
        _ manager: CLLocationManager,
        didUpdateLocations locations: [CLLocation]
    ) {
        MainActor.assumeIsolated {
            self.location = locations.last
        }
    }
}

Pattern 3: @preconcurrency Shorthand

@preconcurrency is equivalent shorthand — wraps in assumeIsolated automatically:

// ❌ Manual approach (verbose)
extension MyClass: SomeDelegate {
    nonisolated func callback() {
        MainActor.assumeIsolated {
            self.updateUI()
        }
    }
}

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

When protocol adds isolation: @preconcurrency becomes unnecessary and compiler warns.

Pattern 4: Thread Check Before assumeIsolated

When caller context is unknown (e.g., library code):

func getView() -> UIView {
    if Thread.isMainThread {
        return createHostingViewOnMain()
    } else {
        return DispatchQueue.main.sync {
            createHostingViewOnMain()
        }
    }
}

private func createHostingViewOnMain() -> UIView {
    MainActor.assumeIsolated {
        let hosting = UIHostingController(rootView: MyView())
        return hosting.view
    }
}

Pattern 5: Custom Actor Access

actor DataStore {
    var cache: [String: Data] = [:]

    nonisolated func synchronousRead(key: String) -> Data? {
        // Only safe if called from DataStore's executor
        assumeIsolated { isolated in
            isolated.cache[key]
        }
    }
}

Common Mistakes

Mistake 1: Silencing Compiler Errors

// ❌ DANGEROUS: Using assumeIsolated to silence warnings
func unknownContext() {
    MainActor.assumeIsolated {
        updateUI()  // Crashes if not actually on main actor!
    }
}

// ✅ When uncertain, use proper async
func unknownContext() async {
    await MainActor.run {
        updateUI()
    }
}

Mistake 2: Assuming GCD Main Queue == MainActor

They're usually the same, but not guaranteed. Check documentation or use async.

Mistake 3: Using in Async Context

// ❌ Unnecessary — you already have isolation
@MainActor
func updateState() async {
    MainActor.assumeIsolated {  // Pointless
        self.state = .ready
    }
}

// ✅ Direct access
@MainActor
func updateState() async {
    self.state = .ready
}

When @preconcurrency Becomes Unnecessary

If the protocol later adds MainActor isolation:

// Library update:
@MainActor
protocol CaffeineThresholdDelegate: AnyObject {
    func caffeineLevel(at level: Double)
}

// Your code — @preconcurrency now warns:
// "@preconcurrency attribute on conformance has no effect"
extension Recaffeinater: CaffeineThresholdDelegate {
    func caffeineLevel(at level: Double) {
        // Direct access, no wrapper needed
    }
}

Crash Behavior

Per Apple documentation:

"If the current context is not running on the actor's serial executor... this method will crash with a fatal error."

Trapping is intentional: Better to crash than corrupt user data with a race condition.

Resources

WWDC: 2024-10169

Docs: /swift/mainactor/assumeisolated, /swift/actor/assumeisolated

Skills: axiom-swift-concurrency