UIView subView alpha animation not working

Question

I'm trying to perform alpha animation of subview when UIScrollView is scrolled certain amount. I have UIView which has another UIView as child.

Below variable is changed to trigger the animation

var isExpanded: Bool = false {
    didSet {
        if (oldValue != isExpanded) {
            toggleExpanded()
        }
    }
}

I have tried option 1:

func toggleExpanded() {
    UIView.animate(withDuration: 300 , animations: {
        if (self.isExpanded) {
            self.mySubView.alpha = 1.0
        } else {
            self.mySubView.alpha = 0.0
        }
    })
}

And option 2:

func toggleExpanded() {
    if (isExpanded) {
        mySubView.alpha = 1.0
    } else {
        mySubView.alpha = 0.0
    }

    UIView.animate(withDuration: 300 , animations: {
        self.layoutIfNeeded()
    })
}

Either one seem to work, alpha changes immediately and in option 1 it seems to even be opposite. Can for example auto layout constraints affect animations here? Where to start when encountering this kind of issue?

Thanks.

Answer 1

Your first solution is right, but the problem is with the duration. duration is measured in seconds, try with 1 or 0.5 second and everything will be fine.

Check the documentation of animate(withDuration:animations:completion:) for more information.