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29 lines
2.1 KiB
Markdown
29 lines
2.1 KiB
Markdown
---
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layout: docs
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title: Cancellation
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permalink: /cancellation/
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---
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Amp provides primitives to allow the cancellation of operations, namely `CancellationTokenSource` and `CancellationToken`.
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```php
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$tokenSource = new CancellationTokenSource;
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$promise = asyncRequest("...", $tokenSource->getToken());
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Loop::delay(1000, function () use ($tokenSource) {
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$tokenSource->cancel();
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});
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$result = yield $promise;
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```
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Every operation that supports cancellation accepts an instance of `CancellationToken` as (optional) argument. Within a coroutine, `$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 = $token ?? new NullCancellationToken` at the beginning of the method.
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While `throwIfRequested()` works well within coroutines, some operations might want to subscribe with a callback instead. They can do so using `CancellationToken::subscribe()` to subscribe any cancellation requests that might happen.
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If the operation consists of any sub-operations that support cancellation, it passes that same `CancellationToken` instance down to these sub-operations.
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The original caller creates a `CancellationToken` by creating an instance of `CancellationTokenSource` and passing `$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`.
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{:.note}
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> 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.
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