Convenience Initializer for UIGestureRecognizer that takes a closure

UIGestureRecognizer+Closure.swift

import UIKit
import ObjectiveC

private typealias GestureClosure = (UIGestureRecognizer) -> Void
private var handle: UInt8 = 0

extension UIGestureRecognizer {

    /**
     Initialize a UIGestureRecognizer (or subclass) with a closure

     - Parameter closure: Closure to be executed when the recognizer fires
     - Warning: Likely leads to retain cycles if a view controller is captured
     */
    public convenience init(closure: @escaping (UIGestureRecognizer) -> Void) {
        self.init()
        self.closure = closure
        addTarget(self, action: #selector(executeGestureClosure))
    }

    // Private

    @objc private func executeGestureClosure() {
        closure?(self)
    }

    private var closure: GestureClosure? {
        get { return loadClosure() }
        set { newValue.map(storeClosure) }
    }

    private struct ClosureWrapper {
        let closure: GestureClosure
    }

    private func loadClosure() -> GestureClosure? {
        guard let wrapper = objc_getAssociatedObject(self, &handle)
            as? ClosureWrapper else { return nil }
        return wrapper.closure
    }

    private func storeClosure(_ closure: @escaping GestureClosure) {
        let wrapper = ClosureWrapper(closure: closure)
        objc_setAssociatedObject(self,
                                 &handle,
                                 wrapper,
                                 .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    }
}