How to cancel DispatchWorkItem and NSOperation

Issue #194

DispatchWorkItem

• https://stackoverflow.com/questions/48844169/swift-ios-dispatchworkitem-is-still-running-even-though-its-getting-cancelled
• https://stackoverflow.com/questions/29492707/how-to-stop-cancel-suspend-resume-tasks-on-gcd-queue

NSOperation

• https://developer.apple.com/documentation/foundation/nsoperation/1411672-cancel?language=objc

Grand Central Dispatch vs NSOperation

• https://stackoverflow.com/questions/43226434/2017-swift-3-1-gcd-vs-nsoperation
• https://medium.com/@johnsundell/a-deep-dive-into-grand-central-dispatch-in-swift-dead7f6e1ca7