iOS Concurrency with GCD & Operations

Sep 12 2023 · Swift 5.8, macOS 13, iOS 16, Xcode 14.3

Part 3: Operations & OperationQueues

22. Cancel Operations

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Previous episode: 21. Challenge: Implement a Dependency Next episode: 23. Conclusion

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You want to be able to cancel unnecessary operations to save your users’ data bandwidth, processing time and battery life. As soon as you add an operation to an operation queue, the only way to cancel it is to call its cancel method. But this only flips the values of its state properties. In particular, isCancelled becomes true. Your job is to code your operations so they check their isCancelled value, then do whatever is necessary: cancel tasks, notify servers, rollback database changes, throw exceptions, or generally clean up.

private var task: URLSessionDataTask?
task = URLSession.shared.dataTask(with: url) { [weak self] ...   // just add task = at the start of line
task = URLSession.shared.dataTask(with: url) { [weak self]
task?.resume()  // this is the new line 
override func cancel() {
guard !self.isCancelled else { return }
guard !isCancelled else { return }
guard !isCancelled else { return }
private var operations: [[Operation]] = [[]]
operations = Array<[Operation]>(repeating: [], count: urls.count)
func cancelOperations(index: Int) {
  guard index < operations.count else { return }
  for operation in operations[index] {
cancelOperations(index: index)
guard index < operations.count else { return }
operations[index] = [tiltShiftOp, downloadOp]
.onDisappear {