Things.swift

func debug() {
    let manager = ThingManager()
    let thing1 = Thing(name: "thing1")
    manager.addThing(thing1)
    manager.sawSomeThing(named: thing1.name)
    print("Manager has this many things: ", manager.things.count)
    Timer.scheduledTimer(withTimeInterval: 10.0, repeats: false, block: { (timer) in
        // By now, the manager should have forgotten about the thing
        print("Manager has this many things: ", manager.things.count)
    })
}

struct Thing {
    var name: String
}

class ThingManager {
    var things: [String: Thing] = [:]
    fileprivate var thingWatchingQueue = DispatchQueue.global()
    fileprivate var thingWatchingTimers: [String: Timer] = [:]
    
    func addThing(_ thing: Thing) {
        self.things[thing.name] = thing
    }
    
    func sawSomeThing(named name: String) {
        self.thingWatchingQueue.async {
            // re-up the timer so we don't forget about that thing
            if let timer = self.thingWatchingTimers[name] {
                timer.invalidate()
            }
            self.thingWatchingTimers[name] = Timer.scheduledTimer(withTimeInterval: 5.0, repeats: false, block: { (timer) in
                self.removeThing(named: name)
            })
        }
    }
    
    func removeThing(named name: String) {
        self.things.removeValue(forKey: name)
    }
}