In what cases and systems is task cancellation implemented?

Cancellation of task execution is implemented in systems where it is important to be able to interrupt long or unnecessary operations to free resources and improve application responsiveness. In iOS, this is often seen in:

• URLSessionTasks — you can cancel a network request if it is no longer needed.
• OperationQueue and Operation — operations can be canceled using the cancel() method, allowing you to stop the task if it has not yet completed.
• DispatchWorkItem — you can cancel deferred or asynchronous code execution.

Cancellation is useful, for example, when loading data for a table cell that has been scrolled out of view, or when canceling a network request upon closing a screen.

Example of canceling an operation:

let operation = BlockOperation {
    // Long task
}
operationQueue.addOperation(operation)

// Later
operation.cancel()