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How to make gesture recognizers work together using require(toFail:)

Swift version: 5.1

Paul Hudson    @twostraws   

It’s common to have multiple gesture recognizers attached to a single view, all doing different things depending on the user’s taps on your screen. By default, iOS lets them fight for control, but usually you want to execute them in some sort of particular order: one gesture should only be matched if another failed.

For example, here are two gesture recognizers that exist on the same view:

let swipe = UISwipeGestureRecognizer(target: self, action: #selector(executeSwipe))
let tap = UITapGestureRecognizer(target: self, action: #selector(executeTap))

view.addGestureRecognizer(swipe)
view.addGestureRecognizer(tap)

A swipe gesture is a tap followed by a linear movement, whereas a tap is just a tap – we need to make sure the swipe gesture has definitely not been recognizer before the tap gesture is checked.

iOS often does a fairly good job of this, but there’s no need to leave it up to chance: if you call require(toFail:) on the tap gesture recognizer, passing in the swipe recognizer, iOS will definitely make sure they don’t compete:

tap.require(toFail: swipe)

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Available from iOS 3.2

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