What does an underscore "_" before a swift variable mean?

Question

Here is the code:

var _selected: Bool = false {
    didSet(selected) {
        let animation = CATransition()
        animation.type = kCATransitionFade
        animation.duration = 0.15
        self.label.layer.addAnimation(animation, forKey: "")
        self.label.font = self.selected ? self.highlightedFont : self.font
    }
}

Why the variable is "_selected" instead of "selected"?

Answer 1

It's simply a coding style that shouldn't apply to Swift code.

Developers often mark things with an underscore to indicate that it should be private.

That said, there IS a practical use for the underscore _. Read more about Local and External Parameter Names for Methods.

---

So how do you avoid using _selected? Right now you have two variables when you already have the one you need (selected).

Removing the _ will require you to override the member variable (how this should be done).

override var selected: Bool {
    didSet {
        println("Hello, \(selected)")
    }
}

Additionally, a table view cell will have an overridable method setSelected(selected:animated) that might be worth exploring.