colorWithAlphaComponent example in Swift

in Swift 3.0

This works for me in xcode 8.2.

yourView.backgroundColor = UIColor.black.withAlphaComponent(0.5)

It may helps you.

Swift 4.0

self.view.backgroundColor = UIColor.black.withAlphaComponent(0.7)

It's not strange, it's behaving exactly as it should. Although many of UIColor's methods are class methods, there are still a few instance methods, and this is one of them. From the UIColor documentation.

colorWithAlphaComponent:

Creates and returns a color object that has the same color space and component values as the receiver, but has the specified alpha component.

So, colorWithAlphaComponent: just changes the alpha value of its receiver. Example:

let purple = UIColor.purpleColor() // 1.0 alpha
let semi = purple.colorWithAlphaComponent(0.5) // 0.5 alpha

And the reason why you're seeing autocompletion for this instance method on the type, is because Swift allows you to use instance methods as curried type methods. In the example you provided, colorWithAlphaComponent actually returns a function that takes a CGFloat as input and returns a UIColor.

let purple = UIColor.purpleColor()
let purpleFunc: (CGFloat -> UIColor) = UIColor.colorWithAlphaComponent(purple)

So, if you wanted to, you could call the type method passing in the instance you want to modify, and then call the resulting function with the alpha that you want to apply, like so.

let purple = UIColor.purpleColor()
let purpleTrans = UIColor.colorWithAlphaComponent(purple)(0.5)

Then as far as the issues you're having with the modal view controller go, you shouldn't be attempting to change the alpha of the view of a modal view controller. See this for more info. Instead, you should be manually creating a view and adding it to the view hierarchy of your existing view controller (if you absolutely have to alter its alpha)