Nil while unwrapping an Optional value Error

Question

fatal error: unexpectedly found nil while unwrapping an Optional value

With following Code:

weak var previewBlurView : UIVisualEffectView?

func blurPreviewWindow() {
    if (self.previewBlurView == nil) {
        var blurEffect: UIVisualEffect
        blurEffect = UIBlurEffect.init(style: UIBlurEffectStyle.Dark)
        self.previewBlurView? = UIVisualEffectView(effect: blurEffect)
        self.previewView.addSubview(self.previewBlurView?)
        self.previewBlurView!.frame = self.previewView.bounds
    }

    self.previewBlurView?.alpha = 0.0
    UIView.animateWithDuration(0.2, delay: 0.0, options: [.BeginFromCurrentState, .CurveEaseOut], animations: {() -> Void in
        self.previewBlurView?.alpha = 1.0
    }, completion: { _ in })
}

I get the crash on the line:

self.previewView.addSubview(self.previewBlurView?)

NOTE

It turned out that all the views were nil due to an external problem of the view controller's instance not referring to the proper one. So in this case, self.previewBlurView turned out to be nil.

Answer 1

Remove the ? in the assignment of self.previewBlurView:

self.previewBlurView = UIVisualEffectView(effect: blurEffect)

What happens otherwise is that the assignment only happens when self.previewBlurView is actually non-nil in the beginning which it is not because you are in the process of assigning something to it.

Compare the following:

var a : Int? = 12
a? = 13
print(a)

var b : Int?
b? = 13
print(b)

Which prints

Optional(13)
nil

The assignment b? = 13 only happens if b is not nil which it unfortunately is.