Add CombinedCancellationToken

Implementation has been used in http-client before.

lib/CombinedCancellationToken.php

@@ -0,0 +1,79 @@
+<?php
+
+namespace Amp;
+
+final class CombinedCancellationToken implements CancellationToken
+{
+    private $tokens = [];
+
+    private $nextId = "a";
+    private $callbacks = [];
+    private $exception;
+
+    public function __construct(CancellationToken ...$tokens)
+    {
+        foreach ($tokens as $token) {
+            $id = $token->subscribe(function ($exception) {
+                $this->exception = $exception;
+
+                $callbacks = $this->callbacks;
+                $this->callbacks = [];
+
+                foreach ($callbacks as $callback) {
+                    asyncCall($callback, $this->exception);
+                }
+            });
+
+            $this->tokens[] = [$token, $id];
+        }
+    }
+
+    public function __destruct()
+    {
+        foreach ($this->tokens as [$token, $id]) {
+            /** @var CancellationToken $token */
+            $token->unsubscribe($id);
+        }
+    }
+
+    /** @inheritdoc */
+    public function subscribe(callable $callback): string
+    {
+        $id = $this->nextId++;
+
+        if ($this->exception) {
+            asyncCall($callback, $this->exception);
+        } else {
+            $this->callbacks[$id] = $callback;
+        }
+
+        return $id;
+    }
+
+    /** @inheritdoc */
+    public function unsubscribe(string $id)
+    {
+        unset($this->callbacks[$id]);
+    }
+
+    /** @inheritdoc */
+    public function isRequested(): bool
+    {
+        foreach ($this->tokens as [$token]) {
+            if ($token->isRequested()) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+    /** @inheritdoc */
+    public function throwIfRequested()
+    {
+        foreach ($this->tokens as [$token]) {
+            $token->throwIfRequested();
+        }
+    }
+}
+