If you added a task to the queue, can it be canceled later?

Yes, it is possible if the task has not yet started.

For Operation: You can call the cancel() method. If the operation is already running, the isCancelled flag will become true, and the developer must handle its cancellation within the operation logic.

let operation = BlockOperation {
    // Operation logic that should check operation.isCancelled
    if OperationQueue.current?.isCancelled ?? false {
        print("Operation cancelled")
        return
    }
    // Further work
}

let queue = OperationQueue()
queue.addOperation(operation)

// Later, if you need to cancel
operation.cancel()

For DispatchWorkItem: An instance of DispatchWorkItem is created, which is then dispatched to a queue. To cancel, the cancel() method is called. Like with Operation, this is a flag for the developer.

let item = DispatchWorkItem {
    // Task logic that should check item.isCancelled
    if item.isCancelled {
        print("Task cancelled")
        return
    }
    // Further work
}

let queue = DispatchQueue.global()
queue.async(execute: item)

// Later, if you need to cancel
item.cancel()

To cancel a task that has already started, the code inside the task must explicitly check the cancellation flag (isCancelled). If the task does not check this flag, it will continue execution until completion.