Can you explain what the cancel command means in the context of multithreaded programming?

In the context of multithreading in iOS (e.g., using GCD or OperationQueue), the cancel command is used to interrupt the execution of a task or operation.

When cancel is called on an operation or task, it signals that the execution should be stopped if possible. However, cancel does not guarantee immediate termination — the task itself must check its cancellation state and terminate properly.

For example, in Operation, there is a property isCancelled that can be checked within the task:

class MyOperation: Operation {
    override func main() {
        if isCancelled { return }
        // perform task
        for i in 0..<1000 {
            if isCancelled { return }
            // work
        }
    }
}

Thus, cancel is a signal for the task to stop, but the responsibility for proper termination lies with the task itself.