Objective-C Block Retain Cycles

Systematic weak-strong pattern diagnosis for Objective-C blocks that capture self. All block memory leaks fit into 4 patterns — learn them once, fix any leak in minutes.

When to Use

Use this skill when:

• View controller never deallocates (deinit doesn't log)
• Instruments shows retain cycle with a block
• Crashes with "message sent to deallocated instance" after using weak self
• Block assigned to property or instance variable
• Network callbacks capturing self

Example Prompts

• "My view controller never deallocates — Instruments shows a block retain cycle"
• "I used __weak self but still getting crashes with deallocated instance"
• "Network callback retains view controller — how do I prevent the leak?"
• "Block assigned to property never releases"

What This Skill Provides

The 4 Mandatory Patterns

All Objective-C block retain cycles fit into 4 patterns:

1. Block passed as parameter — When method stores the block
2. Block assigned to property — Requires cleanup in dealloc
3. Network callbacks — Shared/singleton services retain callbacks
4. Timers and animations — Timer retains block, block retains self

Weak-Strong Pattern

// ❌ Causes retain cycle
[self fetchDataWithCompletion:^(Data *data) {
    [self processData:data];
}];

// ✅ Weak-strong pattern
__weak typeof(self) weakSelf = self;
[self fetchDataWithCompletion:^(Data *data) {
    __strong typeof(weakSelf) strongSelf = weakSelf;
    if (!strongSelf) return;
    [strongSelf processData:data];
}];

Why Weak Alone Isn't Enough

__weak allows nil during block execution. __strong inside block keeps object alive for block's duration — preventing mid-execution crashes.

Mandatory Rules

• Every block referencing self MUST use weak-strong (unless method returns immediately)
• Every block property MUST be nil'd in dealloc
• Every timer with block MUST be invalidated in dealloc
• Network callbacks to singletons MUST use weak-strong

Related

• memory-debugging — General memory leak diagnosis with Instruments
• memory-auditor — Automated scan for leak patterns

Resources

Docs: /objective-c, /automatic-reference-counting