Objective-C Block Retain Cycles

Overview

Block retain cycles are the #1 cause of Objective-C memory leaks. When a block captures self and is stored on that same object (directly or indirectly through an operation/request), you create a circular reference: self → block → self. Core principle 90% of block memory leaks stem from missing or incorrectly applied weak-strong patterns, not genuine Apple framework bugs.

Red Flags — Suspect Block Retain Cycle

If you see ANY of these, suspect a block retain cycle, not something else:

• Memory grows steadily over time during normal app use
• UIViewController instances not deallocating (verified in Instruments)
• Crash: "Sending message to deallocated instance" from network/async callback
• Network requests or animations prevent view controller from closing
• Weak reference becomes nil unexpectedly in a block
• NSLog, NSAssert, or string formatting hiding self references
• Completion handler fires after the view controller "should be gone"
• ❌ FORBIDDEN Rationalizing as "It's probably normal memory usage"
  • Memory leaks are never "normal"
  • Apps should return to baseline memory after user dismisses a screen