iOS, Swift

Check if GestureRecognizer added

One of my new projects dynamically generates a UI based on a DSL entered by the user. This leads to a host of defensive coding challenges since there is multiple code paths that can add controls and their associated gesture recognizers.

I found it helpful to add guard statements that stop recognizers from being added twice. The same approach is used for controls, but that is a different topic.

The below utility function can be used to check if the provided recognizer is already in the recognizer collection associated with an object.

func containsGestureRecognizer(recognizers: [UIGestureRecognizer]?, find: UIGestureRecognizer) -> Bool {
if let recognizers = recognizers {
for gr in recognizers {
if gr == find {
return true
}
}
}
return false
}

The following code shows the utility function in action. All you have to do is incorporate containsGestureRecognizer into your guard statements.

/// Example adding it the first time
let recognizer = UITapGestureRecognizer(target: self, action: #selector(self.handleTap(_:)))
view.addGestureRecognizer(recognizer)
/// Test if gesture already added, if added return else add the recognizer
guard containsGestureRecognizer(recognizers: view.gestureRecognizers, find: recognizer) == false else { return }
/// Add the recognizer
view.addGestureRecognizer(recognizer)
view raw example.swift hosted with ❤ by GitHub

Although really an edge case this was a fun bit of code to try out.