Cancellation

Amp provides primitives to allow the cancellation of operations, namely CancellationTokenSource and CancellationToken.

$tokenSource = new CancellationTokenSource;
$promise = asyncRequest("...", $tokenSource->getToken());

Loop::delay(1000, function () use ($tokenSource) {
    $tokenSource->cancel();
});

$result = yield $promise;

Every operation that supports cancellation accepts an instance of CancellationToken as (optional) argument. Within a coroutine, $token->throwIfRequested() can be used to fail the operation with a CancelledException. As $token is often an optional parameter and might be null, these calls need to be guared with a if ($token) or similar check. Instead of doing so, it’s often easier to simply set the token to $token = $token ?? new NullCancellationToken at the beginning of the method.

While throwIfRequested() works well within coroutines, some operations might want to subscribe with a callback instead. They can do so using CancellationToken::subscribe() to subscribe any cancellation requests that might happen.

If the operation consists of any sub-operations that support cancellation, it passes that same CancellationToken instance down to these sub-operations.

The original caller creates a CancellationToken by creating an instance of CancellationTokenSource and passing $cancellationTokenSource->getToken() to the operation as shown in the above example. Only the original caller has access to the CancellationTokenSource and can cancel the operation using CancellationTokenSource::cancel(), similar to the way it works with Deferred and Promise.

Cancellations are advisory only. A DNS resolver might ignore cancellation requests after the query has been sent as the response has to be processed anyway and can still be cached. An HTTP client might continue a nearly finished HTTP request to reuse the connection, but might abort a chunked encoding response as it cannot know whether continuing is actually cheaper than aborting.