Unowned variables are similar to weak variables in that they provide a way to reference data without having ownership. However, weak variables can become
nil – they are effectively optional. In comparison, unowned variables must never be set to nil once they have been initialized, which means you don't need to worry about unwrapping optionals.
The most common place you'll see unowned variables is with closures that declare
[unowned self] – this means "I want to reference
self inside this closure but I don't want to own it." Why
unowned rather than
weak? Both would work, but let's face it: if
self is nil inside a closure, something has gone wrong!
Available from iOS 7.0
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