Memory and ARC interview questions

How ARC counts references, how retain cycles form and break, weak vs unowned, and how to find leaks.

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unowned(unsafe)
Like unowned but without the runtime check, so access after deallocation is undefined behaviour. Almost never needed.
weak references
Always optional vars that ARC sets to nil when the object is freed.
Timer retain
A scheduled Timer is retained by the run loop and retains its target; invalidate it, or use the block API with [weak self].
withExtendedLifetime
Keeps an object alive until the closure ends, when the optimizer might otherwise free it early.