Make UIAction handler accessible for testing

ActionSpec.swift

import Quick
import Nimble

class ActionSpec: QuickSpec {
    override func spec() {
        describe("UIAction") {
            it("should invoke handler") {
                waitUntil { done in
                    let action = UIAction(title: "a title") { action in
                        done()
                    }
                    action.handler(action)
                }
            }
        }
    }
}

ActionTest.swift

import XCTest

class ActionTest: XCTestCase {
    func testActionHandler() {
        var handlerWasCalled = false
        let action = UIAction(title: "a title") { action in
            handlerWasCalled = true
        }
        action.handler(action)
        XCTAssertTrue(handlerWasCalled, "should be called")
    }
}

UIAction+Testable.swift

import UIKit

extension UIAction {
    var handler: UIActionHandler {
        get {
            typealias ActionHandlerBlock = @convention(block) (UIAction) -> Void
            let handler = value(forKey: "handler") as AnyObject
            return unsafeBitCast(handler, to: ActionHandlerBlock.self)
        }
    }
}

Shame it's hidden in the first place!