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What is optional chaining?

Paul Hudson       @twostraws

Optional chaining is a Swift feature that allows execution of a statement to stop and return nil at any point. For example, all views have an optional superview property that stores whichever UIView contains it, all UIView has an optional gestureRecognizer array that stores the gesture recognizers it has, and all arrays have an optional first property that returns the first item.

Optional chaining allows us to put those three optionals together like this:

let firstParentRecognizer = view.superview?.gestureRecognizers?.first

So, superview is optional, gestureRecognizers is optional, and first is optional, but the end result – firstParentRecognizer will be a simple UIGestureRecognizer? rather than a triple optional. The optional chaining – the two question marks – mean that if superview is nil then firstParentRecognizer gets set to nil and the rest of the statement is ignored, and the same is true of gestureRecognizers.

Without optional chaining we’d need to use a pyramid of if let statements, like this:

if let superview = view.superview {
    if let recognizers = superview.gestureRecognizers {
        let firstParentRecognizer = recognizers.first
    }
}

Available from iOS 8.0

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