Lifetime of objects in `class` methods

If I use these two classes:

class MotionManager {

    class func doTheThing(completionHandler: @escaping (CMDeviceMotion?)->()
) {

        print("in")

        let manager = CMMotionManager()

        manager.deviceMotionUpdateInterval = 0.01

        manager.startDeviceMotionUpdates(to: .main) { (motion, error) in

            print("out")

            completionHandler(motion)

        }

    }

}

class OtherManager {

    let manager: CMMotionManager

    init() {

        manager = CMMotionManager()

        manager.deviceMotionUpdateInterval = 0.01

    }

    func doTheThing(completionHandler: @escaping (CMDeviceMotion?)->() ) {

        print("in 2")

        manager.startDeviceMotionUpdates(to: .main) { (motion , error) in

            completionHandler(motion)

            print("out 2")

            self.manager.stopDeviceMotionUpdates()

        }

    }

}

let other = OtherManager()

other.doTheThing { (motion) in

    print(motion)

}

MotionManager.doTheThing { (motion) in

    print(motion)

}

The class method on MotionManager prints "in" but never prints "out". The
instance method version on. OtherManager works just fine, printing "in 2"
and "out 2". Clearly the CMMotionManager doesn't survive long enough to
finish its call, but I would have thought it would.

Is this a bug? Or am I just misunderstanding how the class method should
work?

···

--
Nate Birkholz

In the class method case, the scope of your `manager` object is local the the class method and it is deallocated immediately as function returns (before the closure gets a chance to be called).

···

On Jul 24, 2017, at 12:22 PM, Nate Birkholz via swift-users <swift-users@swift.org> wrote:

If I use these two classes:

class MotionManager {
    
    class func doTheThing(completionHandler: @escaping (CMDeviceMotion?)->() ) {
        print("in")
        let manager = CMMotionManager()
        manager.deviceMotionUpdateInterval = 0.01
        
        manager.startDeviceMotionUpdates(to: .main) { (motion, error) in
            print("out")
            completionHandler(motion)
        }
    }
}

class OtherManager {
    let manager: CMMotionManager
    
    init() {
        manager = CMMotionManager()
        manager.deviceMotionUpdateInterval = 0.01
    }
    
    func doTheThing(completionHandler: @escaping (CMDeviceMotion?)->() ) {
        print("in 2")
        manager.startDeviceMotionUpdates(to: .main) { (motion , error) in
            completionHandler(motion)
            print("out 2")
            self.manager.stopDeviceMotionUpdates()
        }
    }
}

let other = OtherManager()

other.doTheThing { (motion) in
    print(motion)
}

MotionManager.doTheThing { (motion) in
    print(motion)
}

The class method on MotionManager prints "in" but never prints "out". The instance method version on. OtherManager works just fine, printing "in 2" and "out 2". Clearly the CMMotionManager doesn't survive long enough to finish its call, but I would have thought it would.

Is this a bug? Or am I just misunderstanding how the class method should work?

--
Nate Birkholz
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