Main Thread Checker and Hangs

Main Thread Checker

// Enabled by default in Xcode Debug scheme:
// Product → Scheme → Edit → Diagnostics → ☑ Main Thread Checker
// Violating calls log a purple runtime warning in Xcode

Common MTC Violations

URLSession.shared.dataTask(with: url) { data, _, _ in
  self.label.text = "Done"  // VIOLATION: background thread
}.resume()
// Fix:
DispatchQueue.main.async { self.label.text = "Done" }

MTC with async/await

@MainActor
final class ProfileViewModel: ObservableObject {
  @Published var name = ""
  func load() async {
    name = try? await fetchName()  // always on main thread
  }
}

What Are Hangs?

// Common causes:
// - Synchronous network calls on main thread
// - Heavy computation blocking main run loop
// - Deadlocks between DispatchQueues

Instruments Hangs Template

// Instruments → Hangs template
// Shows hang duration (ms), call stack, and responsible frame
// Look for synchronous I/O or heavy loops

Main Thread Checker in Tests

// XCTestScheme Diagnostics → ☑ Main Thread Checker
// UI test violation = instant fail in CI

MetricKit Hang Reports

func didReceive(_ payloads: [MXDiagnosticPayload]) {
  payloads.compactMap { $0.hangDiagnostics }.flatMap { $0 }.forEach {
    print($0.callStackTree)
  }
}

Detecting Hangs with Instruments

// Time Profiler: look for long CPU bursts on main thread
// Core Animation: FPS drops to 0 during hang
// Hangs instrument: highlights exact frames where hang occurred

Async Fixes for Common Hangs

// HANG:
let processed = heavyProcessing(image)  // blocks main thread
// FIX:
let processed = await Task.detached(priority: .userInitiated) {
  heavyProcessing(image)
}.value

Avoiding Synchronous APIs on Main Thread

// HANG: synchronous file read on main thread
let data = try! Data(contentsOf: largeFileURL)
// FIX:
let data = try await Task.detached { try Data(contentsOf: largeFileURL) }.value

Main Run Loop and Timers

// AVOID: heavy work in main-thread timer callback
Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) { _ in
  processAllItems()  // slow!
}
// PREFER: dispatch to background
Timer.scheduledTimer(withTimeInterval: 0.1, repeats: true) { _ in
  Task.detached { processAllItems() }
}

Quick Check

Lesson Recap