Observing property crashes Instruments (Leaks profile) Xcode 9.3 (Swift 4.1)

Question

When I run the following code (a simplified example, created to demonstrate the crash), it executes as expected when I choose Run (both os_log messages print in Console).

However, when I open it in Instruments from the Memory Debug Navigator - by pressing Restart - it crashes (only the first os_log message prints in Console).

The crash occurs at observe(...).

import os
import UIKit

class ObserverCrashingExample: NSObject {

    @objc private var animation: UIViewPropertyAnimator?
    private var observer: NSKeyValueObservation?

    override init() {

        super.init()

        animation = UIViewPropertyAnimator( duration: 1, curve: .linear, animations: { })

        animation!.pauseAnimation()

        os_log("X_AMPLE Executes")

        observer = animation!.observe(\.isRunning, options: [.new, .old]) { _ , _ in }

        os_log("X_AMPLE Does not execute")

    }

    required init?(coder aDecoder: NSCoder) { fatalError("init(coder:) has not been implemented") }

}

I am running Xcode 9.3, Swift 4.1, iOS 11.3 deployment target.

Any ideas? Is this a compiler issue?

Cheers

Answer 1

I tried three word-arounds:

1. Disabling "Record Reference Count". This stopped it from crashing. But the instruments showed me much more leaks than usual, which makes locating real leaks impossible.
2. Commenting all my KVO calls. Didn't work in my case.
3. Downgrading my code to Swift 4.0. I re-installed Xcode 9.2, and it works now.