Allocations and Leaks Instruments

Allocations Instrument

// Instruments → Allocations template
// Shows live bytes, allocation count, and transient objects

Heap Growth Analysis

// Normal: allocations rise and fall as views appear/disappear
// Leak: allocations only grow, never released

Generation Analysis

// Instruments toolbar → "Mark Generation"
// Perform action (e.g., open and close a screen)
// "Mark Generation" again
// Inspect objects in the new generation that weren't released

Leaks Instrument

// Run Leaks alongside Allocations in the "Leaks" template
// Red bars in the timeline = leaked objects detected
// Click a bar to see the leaked object graph

Reading the Leak Graph

// Example cycle:
// ViewController → ViewModel → Closure → ViewController
// Instruments draws the reference edges visually

Detecting Retain Cycles in Closures

class MyVC: UIViewController {
  var onComplete: (() -> Void)?
  override func viewDidLoad() {
    super.viewDidLoad()
    // LEAK: closure captures self; self holds onComplete
    onComplete = { self.dismiss(animated: true) }
    // FIX:
    onComplete = { [weak self] in self?.dismiss(animated: true) }
  }
}

VM Tracker Instrument

// Instruments → "VM Tracker" in allocation instrument
// Look for: large __DATA dirty pages, high CG raster memory

Zombie Objects

// Instruments → Edit → Record Settings → Enable Zombie Objects
// Accessing a zombie prints: "*** -[MyObject method] sent to deallocated instance"

Heap Shots

// After navigating to and from a screen twice:
// Heap should return to baseline
// Objects remaining in the diff = leaked view controllers or models

MetricKit for Production Memory

import MetricKit
class AppDelegate: UIApplicationDelegate, MXMetricManagerSubscriber {
  func didReceive(_ payloads: [MXMetricPayload]) {
    payloads.forEach { print($0.memoryMetrics as Any) }
  }
}

Quick Check

Lesson Recap