How to organize object references to prevent cyclic memory retention?

To prevent retain cycles in the object model on iOS (using Swift or Objective-C), it is important to correctly use strong, weak, and unowned references.

Main idea:

• strong — owning reference, increases the reference count of the object.
• weak — non-owning reference, does not increase the count, automatically set to nil when the object is deallocated.
• unowned — non-owning reference, does not increase the count, but not set to nil, used when the object is guaranteed to outlive the reference.

Cycles often occur, for example, between a parent and a child or between a delegate and an object. To avoid a cycle:

• The parent holds the child via strong.
• The child references the parent via weak or unowned.

Example in Swift:

class Parent {
    var child: Child?
}

class Child {
    weak var parent: Parent?  // weak reference to avoid cycle
}

let parent = Parent()
let child = Child()
parent.child = child
child.parent = parent

Thus, when parent and child are no longer used, memory is released correctly, without leaks caused by cycles.