In Swift, does resetting the property inside didSet trigger another didSet?

Rob N picture Rob N · Oct 2, 2016 · Viewed 10.1k times · Source

I'm testing this and it appears that if you change the value within didSet, you do not get another call to didSet.

var x: Int = 0 {
    didSet {
        if x == 9 { x = 10 }
    }
}

Can I rely on this? Is it documented somewhere? I don't see it in the Swift Programming Language document.

Answer

FelixSFD picture FelixSFD · Oct 2, 2016

I also thought, that this is not possible (maybe it wasn't in Swift 2), but I tested it and found an example where Apple uses this. (At "Querying and Setting Type Properties")

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
            }
        }
    }
}

And below this piece of code, there is the following 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.