axiom-assume-isolated

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
        }
    }
}