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.

Just tested on Xcode 8 Playground (Swift 3) and got the same result. If your change the property from `didSet`, the `didSet` is __not__ called again. — Luca Angeletti, Oct 02 '16 at 16:34
I actually had a `didSet` on my array which at the end also had a `defer` used for popping the *last* element. Guess what happened? It crashed because I was popping every element which made my array empty, had to do an `if else` inside the `defer` so I won't pop if `myArray.isEmpty != true` — mfaani, Jul 18 '17 at 15:04

FelixSFD · Accepted Answer · 2016-10-02T16:50:49.143

18

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.

edited Oct 02 '16 at 16:50

answered Oct 02 '16 at 16:39

FelixSFD

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1

This no longer applies in Swift 4 and will cause a loop – Josh Bernfeld Dec 16 '17 at 04:02
8

@thedeveloper3124 I just tested in Swift 4 and it still works. – thislooksfun Dec 28 '17 at 21:01
1

@thislooksfun I just tested in Swift 5 and it causes a loop – turingtested Jan 21 '21 at 09:58
2

The question is not whether or not it causes a loop. It always causes a loop if your condition is not valid. In Apples case it can't cause a loop because an iteration is only triggered if the value is `>` – Jan 29 '21 at 10:13

score 18 · Answer 2 · answered Mar 08 '18 at 17:54

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.

score 1 · Answer 3 · answered Oct 02 '16 at 16:34

1

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].

answered Oct 02 '16 at 16:34

Alexander

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Did this change on Swift 3? I remember in Swift 2 the recursion call was actually in place. – Luca Angeletti Oct 02 '16 at 16:35
In my case I'm rounding a double to nearest of some other values. It doesn't seem worth it to crash the app if they don't set a pre-rounded value. – Rob N Oct 02 '16 at 16:53

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

3 Answers3

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