Implement NullCancellationToken

lib/NullCancellationToken.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Amp;
+
+/**
+ * A NullCancellationToken can be used to avoid conditionals to check whether a token has been provided.
+ *
+ * Instead of writing
+ *
+ * ```php
+ * if ($token) {
+ *     $token->throwIfRequested();
+ * }
+ * ```
+ *
+ * potentially multiple times, it allows writing
+ *
+ * ```php
+ * $token = $token ?? new NullCancellationToken;
+ *
+ * // ...
+ *
+ * $token->throwIfRequested();
+ * ```
+ *
+ * instead.
+ */
+class NullCancellationToken implements CancellationToken {
+    /** @inheritdoc */
+    public function subscribe(callable $callback): string {
+        return "null-token";
+    }
+
+    /** @inheritdoc */
+    public function unsubscribe(string $id) {
+        // nothing to do
+    }
+
+    /** @inheritdoc */
+    public function isRequested(): bool {
+        return false;
+    }
+
+    /** @inheritdoc */
+    public function throwIfRequested() {
+        // nothing to do
+    }
+}