How to detect gesture on a UIView after it becomes transparent?

Question

I have a UIView on top of some other UI components to detect a long-pressed gesture. When a long press begins, I want to tip the user by changing the background color to gray & alpha = 0.1.

After the long press ends, the UIView has to be changed back as completely transparent. I set its alpha to 0, but the problem is ...

no further guesture can be detected.

mainView = UIView()
mainView.frame = ...
let longPressRecognizer = UILongPressGestureRecognizer(target: self, action:Selector("longPressed:"))
mainView.addGestureRecognizer(longPressRecognizer)

func longPressed(sender: UILongPressGestureRecognizer) {
    let view = sender.view!

    if sender.state == .Began {
        view.backgroundColor = UIColor.grayColor()
        view.alpha = 0.1
    } else if (sender.state == .Ended || sender.state == .Cancelled || sender.state == .Failed) {
        view.backgroundColor = UIColor.whiteColor()
        view.alpha = 0
    }
}

What's the correct way to make this UIView changed back to its original state so that further gestures can be detected as it's initially created?

Answer 1

Setting a UIView's alpha property to 0 will make it stop receiving touches. Instead, set its background to UIColor.clearColor() when you want it invisible.