Swift: Overriding didSet results in a recursion

Question

When overriding the didSet observer of a property results in recursion, why?

class TwiceInt {
    var value:Int  = 0 {
        didSet {
            value *= 2
           


        

Answer 1

Putting a println() in both didSet blocks, you can see that it repeatedly calls the super-implementation first, then the override, then super, then override... until it explodes.

I can only image that this is a bug in Swift. I get the same issue in Swift 1.2 (bundled with the Xcode 6.3 beta).

---

It should definitely function, at least as I read it. From https://developer.apple.com/library/mac/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html#//apple_ref/doc/uid/TP40014097-CH14-ID254:

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.

and after their AudioChannel sample (quoted below the note):

NOTE

In the first of these two checks, the didSet observer sets currentLevel to a different value. This does not, however, cause the observer to be called again.

struct AudioChannel {
    static let thresholdLevel = 10
    static var maxInputLevelForAllChannels = 0
    var currentLevel: Int = 0 {
        didSet {
            if currentLevel > AudioChannel.thresholdLevel {
                // cap the new audio level to the threshold level
                currentLevel = AudioChannel.thresholdLevel
            }
            if currentLevel > AudioChannel.maxInputLevelForAllChannels {
                // store this as the new overall maximum input level
                AudioChannel.maxInputLevelForAllChannels = currentLevel
            }
        }
    }
}