What is a retain cycle and how do you break it in Obj-C?

Middle PRO

Essence

A retain cycle is two (or more) objects holding strong references to each other; neither count reaches zero, dealloc never runs — a leak. ARC does not detect it: it only counts references, it does not search for unreachable graphs.

Classic cases:

• a delegate declared as strong;
• a block stored in a property of the object that captured self;
• NSTimer with a target (the timer retains the target until invalidate);
• parent ↔ child in a model tree.