colorWithAlphaComponent example in Swift

Question

What is the correct syntax for this function in Swift?

The following works fine, and colors the background purple:

 self.view.backgroundColor = UIColor.purpleColor()

When I chain the colorWithAlphaComponent function, the view shows the correct alpha for a moment, and then changes to an opaque purple that is relatively dark:

 self.view.backgroundColor = UIColor.purpleColor().colorWithAlphaComponent(0.5)

Is this the recommended function for adding an alpha value to a UIColor?

Furthermore, why does the intellisense popup say that this function expects a UIColor as a parameter? E.g.,

   self.view.backgroundColor = UIColor.colorWithAlphaComponent(<#UIColor#>)

EDIT: The behavior is strange. I am setting the background color on a view controller that is being loaded in a modal. As the modal slides up from the bottom, the alpha is correct. When the modal finishes loading, the background color changes to opaque?!

EDIT 2: The problem was not with the code--both the code above and the suggestion below were properly applying the alpha. The issue is the way that modals are being presented--the underlying view is being removed. See:
Transparent Modal View on Navigation Controller

Answer 1

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)