Why no Infinite loop in didSet?

问题:

In my FirstViewController I have a button directing to my SecondViewController, passing data to a property in the SecondViewController. This property has a property observer, creating a new instance of the SecondViewController when set.

While it's working as I want, I wonder why it's not getting stuck in an infinite loop, creating an instance of the SecondViewController forever. And is it good practice to do it this way?

FirstViewController:

class FirstViewController: UIViewController {
    @IBAction func something(sender: UIButton) {
        let destination = storyboard?.instantiateViewControllerWithIdentifier("secondViewController") as SecondViewController
        destination.selected = 1
        showViewController(destination, sender: self)
    }
}

SecondViewController:

class SecondViewController: UIViewController {
    var selected: Int = 0 {
        didSet {
            let destination = storyboard?.instantiateViewControllerWithIdentifier("secondViewController") as SecondViewController
            destination.selected = selected
            showViewController(destination, sender: self)
        }
    }

    @IBAction func something(sender: UIButton) {
        selected = 2
    }
}

回答1:

If you check Apple's documentation for Swift in The Swift Programming Language - Properties, Apple says that:

Note:

If you assign a value to a property within its own didSet observer, the new value that you assign will replace the one that was just set.

So if you put a breakpoint in the first line of your didSet block, I believe it should only be called once.