Debugging memory leaks with Instruments in Swift - Step-by-Step Execution

Concept Flow - Debugging memory leaks with Instruments

Start App Execution

↓

Run Instruments - Leaks Tool

↓

Monitor Memory Allocations

↓

Detect Leaked Objects?

No→Continue Monitoring

Yes↓

Analyze Call Stack & Object Graph

↓

Identify Leak Source

↓

Fix Code (e.g., remove retain cycles)

↓

Re-run Instruments to Confirm Fix

↓

End

This flow shows how to use Instruments to find and fix memory leaks by monitoring allocations, detecting leaks, analyzing their source, fixing code, and confirming the fix.

Execution Sample

Swift

class ViewController {
    var closure: (() -> Void)?
    func setup() {
        closure = { [weak self] in print(self as Any) }
    }
}

This Swift code creates a retain cycle by capturing self strongly in a closure, causing a memory leak.

Execution Table

Step	Action	Memory State	Leak Detected?	Notes
1	App starts and ViewController instance created	ViewController allocated	No	Initial allocation
2	setup() called, closure assigned capturing self strongly	Closure holds strong reference to ViewController	No	Retain cycle created
3	ViewController supposed to be deallocated	ViewController still in memory	Yes	Leak detected by Instruments
4	Analyze call stack and object graph	Shows closure retaining ViewController	Yes	Leak source identified
5	Fix code by capturing self weakly in closure	Closure no longer strongly retains ViewController	No	Leak fixed
6	Re-run Instruments	ViewController deallocated properly	No	No leaks detected, fix confirmed

💡 Leak fixed and confirmed by Instruments detecting no retained objects after fix

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
ViewController instance	nil	allocated and referenced by closure	still allocated (leak)	still allocated (before fix)	deallocated (after fix)
closure	nil	assigned capturing self strongly	holds strong reference causing leak	assigned capturing self weakly	holds weak reference, no leak

Key Moments - 3 Insights

Why does the ViewController instance not get deallocated after setup()?

Because the closure captures self strongly, creating a retain cycle that Instruments detects as a leak (see execution_table step 3).

How does Instruments help identify the source of the leak?

Instruments shows the call stack and object graph revealing that the closure retains the ViewController, pinpointing the leak source (see execution_table step 4).

What change fixes the memory leak in the closure?

Capturing self weakly in the closure breaks the retain cycle, allowing ViewController to be deallocated (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Instruments first detect a memory leak?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Leak Detected?' column in execution_table rows

According to variable_tracker, what happens to the closure variable after step 5?

A It holds a weak reference to ViewController

B It holds a strong reference to ViewController

C It is nil

D It is deallocated

💡 Hint

Look at the closure row under 'After Step 5' in variable_tracker

If the closure did not capture self at all, how would the leak detection change?

A Leak would still be detected at step 3

B Leak would be detected earlier

C No leak would be detected

D Leak would be detected after step 5

💡 Hint

Consider what causes the leak according to execution_table and variable_tracker

Concept Snapshot

Debugging memory leaks with Instruments:
- Run Instruments with Leaks tool while app runs
- Instruments detects leaked objects and shows call stack
- Analyze object graph to find retain cycles
- Fix code (e.g., use weak self in closures)
- Re-run Instruments to confirm leaks are gone

Full Transcript

This visual execution shows how to debug memory leaks in Swift using Instruments. The app creates a ViewController instance that holds a closure capturing self strongly, causing a retain cycle and memory leak. Instruments detects the leak when the ViewController is not deallocated. By analyzing the call stack and object graph, Instruments points to the closure retaining the ViewController. Fixing the code by capturing self weakly breaks the cycle. Re-running Instruments confirms the leak is fixed as the ViewController is properly deallocated.