ax-performance

Performance

Quick Patterns

Time Profiler: Find CPU Hotspots

// 1. Launch Instruments > Time Profiler template
// 2. Attach to running app on device (not simulator)
// 3. Record during slow operation for 10-30 seconds
// 4. Stop, examine call tree

// KEY: Check Self Time, not Total Time
// Self Time 80%? Function is doing expensive work
// Self Time 5%, Total Time 80%? Function calls slow code - drill deeper

Memory Leak Detection (5 min)

// 1. Add deinit logging to suspect classes
class MyViewController: UIViewController {
    deinit { print("MyViewController deallocated") }
}

// 2. Navigate to view, navigate away
// See "deallocated"? No leak. Missing? Retained somewhere.

// 3. Debug > Memory Graph Debugger
// Look for purple/red circles with warning badge
// Click to see retain cycle chain

Fix Timer Leak (Most Common - 50% of leaks)

// BAD: Timer never invalidated - RunLoop retains it
progressTimer = Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] _ in
    self?.updateProgress()
}

// GOOD: Combine auto-cleans when released
cancellable = Timer.publish(every: 1.0, tolerance: 0.1, on: .main, in: .default)
    .autoconnect()
    .sink { [weak self] _ in self?.updateProgress() }

Fix Hang: Move Work Off Main Thread

// BAD: Blocks main thread
func loadUserData() {
    let data = try! Data(contentsOf: largeFileURL)  // BLOCKS
    processData(data)
}

// GOOD: Async
func loadUserData() {
    Task.detached {
        let data = try Data(contentsOf: largeFileURL)
        await MainActor.run { self.processData(data) }
    }
}

Enable ProMotion 120Hz (iPhone)

<!-- Info.plist - REQUIRED for >60Hz on iPhone -->
<key>CADisableMinimumFrameDurationOnPhone</key>
<true/>

// MTKView defaults to 60fps - set explicitly
let mtkView = MTKView(frame: frame, device: device)
mtkView.preferredFramesPerSecond = 120
mtkView.isPaused = false
mtkView.enableSetNeedsDisplay = false