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

Aspect	Task { @MainActor in }	MainActor.assumeIsolated
Timing	Deferred (next run loop)	Synchronous (inline)
Async support	Yes (can await)	No (sync only)
Context	From any context	Must be sync function
Failure mode	Runs anyway	Crashes if wrong isolation
Use case	Start async work	Verify + access isolated state

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