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.

Practice

1. What is the main purpose of using Instruments to debug memory leaks in Swift apps?

easy

A. To check the app's network connections

B. To improve the app's user interface design

C. To write new Swift code faster

D. To find parts of the code where memory is not released properly

Solution

1. Step 1: Understand Instruments' role

   Instruments is a tool that helps find memory leaks by showing where memory is not freed.

2. Step 2: Identify the main goal

   The main goal is to find code parts causing memory leaks, not UI or network issues.

3. Final Answer:

   To find parts of the code where memory is not released properly -> Option D

4. Quick Check:

   Instruments finds memory leaks = A [OK]

Hint: Instruments shows where memory stays allocated [OK]

Common Mistakes:

• Thinking Instruments improves UI design
• Confusing memory leaks with network issues
• Believing Instruments helps write code faster

2. Which of the following is the correct way to start a memory leak analysis session in Instruments for a Swift app?

easy

A. Open Instruments, select the Leaks template, then run your app

B. Open Xcode, write code, then press Cmd+S

C. Use Terminal to run 'swift leak-check' command

D. Restart the device and open the app

Solution

1. Step 1: Know how to use Instruments

   Instruments has a Leaks template specifically for memory leak detection.

2. Step 2: Correct procedure

   You open Instruments, choose Leaks, then run your app to monitor memory usage.

3. Final Answer:

   Open Instruments, select the Leaks template, then run your app -> Option A

4. Quick Check:

   Start Instruments with Leaks template = D [OK]

Hint: Use Instruments Leaks template to start analysis [OK]

Common Mistakes:

• Trying to use a non-existent 'swift leak-check' command
• Confusing saving code with leak detection
• Restarting device does not start Instruments

3. Given this Instruments snapshot showing a persistent memory block, what does it most likely indicate?
Leaked Object: MyViewController
Size: 512 KB
Reference Count: 1

medium

A. MyViewController is properly released after use

B. MyViewController is still retained and causing a memory leak

C. The app is running out of CPU resources

D. The snapshot shows a network error

Solution

1. Step 1: Analyze the snapshot details

   The snapshot shows a leaked object with a reference count of 1, meaning it is still held in memory.

2. Step 2: Understand what a leak means

   If the object is not released when expected, it causes a memory leak.

3. Final Answer:

   MyViewController is still retained and causing a memory leak -> Option B

4. Quick Check:

   Leaked object with ref count 1 = A [OK]

Hint: Leaked object with ref count means memory not freed [OK]

Common Mistakes:

• Assuming ref count 1 means object is released
• Confusing memory leaks with CPU or network issues
• Ignoring the meaning of leaked objects in Instruments

4. You notice your Swift app leaks memory because a closure captures self strongly. How can you fix this leak?

medium

A. Remove the closure entirely

B. Increase the reference count of self inside the closure

C. Use [weak self] in the closure capture list

D. Call the closure on the main thread

Solution

1. Step 1: Identify the cause of the leak

   A closure capturing self strongly creates a retain cycle causing a memory leak.

2. Step 2: Apply weak capture to break retain cycle

   Using [weak self] in the closure capture list prevents strong reference cycles and fixes the leak.

3. Final Answer:

   Use [weak self] in the closure capture list -> Option C

4. Quick Check:

   Weak capture breaks retain cycles = B [OK]

Hint: Use [weak self] to avoid retain cycles in closures [OK]

Common Mistakes:

• Removing closure without replacing functionality
• Increasing reference count worsens leaks
• Thinking thread choice fixes memory leaks

5. You have a Swift app with multiple view controllers. Instruments shows a leak in a view controller that uses a delegate property declared as var delegate: MyDelegate?. What is the best way to fix this leak?

hard

A. Change the delegate property to weak var delegate: MyDelegate?

B. Make the delegate property a strong reference

C. Remove the delegate property completely

D. Assign delegate to nil only in deinit

Solution

1. Step 1: Understand delegate retain cycles

   Delegates should be weak to avoid retain cycles between view controllers and delegates.

2. Step 2: Fix the property declaration

   Changing delegate to weak breaks the strong reference cycle and prevents leaks.

3. Final Answer:

   Change the delegate property to weak var delegate: MyDelegate? -> Option A

4. Quick Check:

   Delegates must be weak to avoid leaks = C [OK]

Hint: Declare delegates as weak to prevent retain cycles [OK]

Common Mistakes:

• Making delegate strong causes leaks
• Removing delegate loses communication
• Only nil assignment in deinit does not fix leaks