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

Question

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 1

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 .

Answer 2

From Apple docs (emphasis mine):

Similarly, if you implement a didSet observer, it's passed a constant parameter containing the old property value. You can name the parameter or use the default parameter name of oldValue. If you assign a value to a property within its own didSet observer, the new value that you assign replaces the one that was just set .

So, assigning a value in didSet is officially OK and won't trigger an infinite recursion.

Answer 3

It'll work just fine, but it seems pretty like a pretty bad idea from the standpoint of a consumer of your API.

It doesn't recurse, the way I suspected it might, so that's good at least.

I can think of few cases in which it would be acceptable for a setter to change what i'm setting. One such example might be a variable that's set to an angle, which is automatically normalized to be [0, 2π] .