Implement additional cancellation token methods

lib/CancellationToken.php

@@ -29,4 +29,18 @@ interface CancellationToken {
      * @return void
      */
     public function unsubscribe(string $id);
+
+    /**
+     * Returns whether cancellation has been requested yet.
+     *
+     * @return bool
+     */
+    public function isRequested(): bool;
+
+    /**
+     * Throws the `CancelledException` if cancellation has been requested, otherwise does nothing.
+     *
+     * @return void
+     */
+    public function throwIfRequested();
 }

lib/CancellationTokenSource.php

@@ -44,8 +44,13 @@ final class CancellationTokenSource {
 
     public function __construct() {
         $this->token = new class($this->onCancel) implements CancellationToken {
+            /** @var string */
             private $nextId = "a";
+
+            /** @var callable[] */
             private $callbacks = [];
+
+            /** @var \Throwable|null */
             private $exception = null;
 
             public function __construct(&$onCancel) {
@@ -95,6 +100,16 @@ final class CancellationTokenSource {
             public function unsubscribe(string $id) {
                 unset($this->callbacks[$id]);
             }
+
+            public function isRequested(): bool {
+                return isset($this->exception);
+            }
+
+            public function throwIfRequested() {
+                if (isset($this->exception)) {
+                    throw $this->exception;
+                }
+            }
         };
     }