@Observable pessimizes arrays

Using Swift
7

It's not quadratic, it's linear... This is what I am getting for @Observable:

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image1106×738 34.9 KB

It shows time (in seconds) of adding 100K elements depending upon how many elements are in the array already.

Interestingly I'm getting exactly the same behaviour for ObservableObject + Published.
Ditto for this manual code:

class Model2 {
    static var changeCount = 0
    
    var array: [Int] = [] {
        didSet {
            if array != oldValue { Self.changeCount += 1}
        }
    }
}

or this:

        didSet {
            if array.count != oldValue.count { Self.changeCount += 1}
        }

But not this!

        didSet { Self.changeCount &+= array.count }

or simply:

        didSet { Self.changeCount += 1 }

The crucial difference: is "oldValue" being accessed or not. I guess the reason of the slowdown is this: if you are accessing "oldValue" Swift is providing you with a copy, and this is the time to make this copy is what we see in the timing plot.

I hope this finding could bring us closer to the actual fix.

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