A strong reference cycle is when two instances directly or indirectly hold strong references to each other. This means that neither object can ever be deleted, because both are ensuring that the other will always exist.
This scenario is our first really bad memory management scenario. It is one thing to keep memory around longer than it is needed; it is a whole different level to create memory that can never be freed up to be reused again. This type of memory problem is called a memory leak, because the computer will slowly leak memory until there is no longer any new memory available. This is why you will sometimes see a speed improvement after restarting your device. Upon restart, all of the memory is freed up again. Modern operating systems will sometimes find ways to forcefully free up memory, especially when completely quitting an app, but we cannot rely on this as programmers.
So how can we prevent these strong reference cycles? First, let's take a look at what...