Breaking Retain Cycles in Blocks
Diagnose and fix the most common ARC memory leak in legacy apps: retain cycles caused by blocks capturing self, using weak and strong references.
Breaking Retain Cycles in Blocks is a free Objective-C iOS Development for Legacy & Enterprise Apps lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Objective-C iOS Development for Legacy & Enterprise Apps learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What is a Retain Cycle?
A retain cycle happens when two objects strongly reference each other. Neither's count can reach zero, so neither is ever freed — a memory leak ARC cannot break for you.
Blocks Capture Strongly
An Objective-C block captures the variables it uses, including self — and it captures them strongly by default. This is the number one source of cycles.
A Cycle in Action
Here self owns the block (stored in a property), and the block strongly captures self. That mutual ownership leaks.
self.completion = ^{
[self refreshUI];
};The weak Qualifier
Break the cycle by capturing a weak reference to self. A weak reference does not increase the retain count.
__weak typeof(self) weakSelf = self;
self.completion = ^{
[weakSelf refreshUI];
};The weakSelf Pattern
The convention names the capture weakSelf using __weak typeof(self) so the type stays correct even if the class changes.
__weak typeof(self) weakSelf = self;weakSelf Can Become nil
A weak reference becomes nil the instant the object deallocates. If the block runs after that, messages to weakSelf are silently ignored — but partial access can be inconsistent.
The strongSelf Guard
Inside the block, promote weakSelf to a temporary strong reference so it cannot vanish mid-execution.
self.completion = ^{
__strong typeof(weakSelf) strongSelf = weakSelf;
if (strongSelf) {
[strongSelf refreshUI];
}
};When You Do NOT Need weakSelf
Not every block leaks. If self does not own the block — for example a one-shot block passed to a method and not stored — capturing self strongly is fine and even safer.
[UIView animateWithDuration:0.3 animations:^{
self.view.alpha = 0;
}];Delegate Cycles
The same rule applies to delegates: a delegate property should be weak so a view controller and its child do not retain each other.
@property (nonatomic, weak) id<MyDelegate> delegate;Finding Leaks
Tools to hunt cycles:
- Instruments > Leaks flags leaked allocations
- Memory Graph Debugger in Xcode visualizes retain cycles
- Log in
deallocto confirm objects are freed
Rule of Thumb
Ask: does self own this block, and does the block touch self? If both are yes, use weakSelf (plus a strongSelf guard for multi-step work).
Quick Check
Test your retain cycle knowledge.
Recap
You learned to break retain cycles:
- Blocks capture
selfstrongly by default - A cycle leaks when self owns a block that references self
- Use
__weak typeof(self) weakSelfto break it - Add a
strongSelfguard for multi-step work - Mark delegate properties
weak
Instruments and the Memory Graph confirm your fixes.
Frequently asked questions
Is the “Breaking Retain Cycles in Blocks” lesson free?
Yes — the full text of “Breaking Retain Cycles in Blocks” is free to read here on the web, and the Objective-C iOS Development for Legacy & Enterprise Apps course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Objective-C iOS Development for Legacy & Enterprise Apps course, upgrade to CoddyKit PRO.
What will I learn in “Breaking Retain Cycles in Blocks”?
Diagnose and fix the most common ARC memory leak in legacy apps: retain cycles caused by blocks capturing self, using weak and strong references. You practise Objective-C iOS Development for Legacy & Enterprise Apps with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Objective-C iOS Development for Legacy & Enterprise Apps?
No prior experience is required. Objective-C iOS Development for Legacy & Enterprise Apps on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Breaking Retain Cycles in Blocks” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Objective-C iOS Development for Legacy & Enterprise Apps lesson?
Yes. Every Objective-C iOS Development for Legacy & Enterprise Apps lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Manual Retain-Release (MRR) Basics
- Automatic Reference Counting (ARC)
- Weak vs. Strong References
- Breaking Retain Cycles in Blocks