Register shutdown function to clear event loop

examples/shutdown.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+require __DIR__ . '/../vendor/autoload.php';
+
+use Revolt\EventLoop;
+
+\register_shutdown_function(function (): void {
+    \register_shutdown_function(function (): void {
+        EventLoop::defer(function (): void {
+            print 'Shutdown function registered within pre-loop-run shutdown function' . PHP_EOL;
+        });
+    });
+
+
+    EventLoop::defer(function (): void {
+        print 'Shutdown function registered before EventLoop::run()' . PHP_EOL;
+    });
+});
+
+EventLoop::run();
+
+\register_shutdown_function(function (): void {
+    \register_shutdown_function(function (): void {
+        EventLoop::defer(function (): void {
+            print 'Shutdown function registered within post-loop-run shutdown function' . PHP_EOL;
+        });
+    });
+
+    EventLoop::defer(function (): void {
+        print 'Shutdown function registered after EventLoop::run()' . PHP_EOL;
+    });
+});
+
+print 'End of script' . PHP_EOL;

src/EventLoop.php

@@ -28,8 +28,12 @@ final class EventLoop
     public static function setDriver(Driver $driver): void
     {
         /** @psalm-suppress RedundantPropertyInitializationCheck, RedundantCondition */
-        if (isset(self::$driver) && self::$driver->isRunning()) {
-            throw new \Error("Can't swap the event loop driver while the driver is running");
+        if (isset(self::$driver)) {
+            if (self::$driver->isRunning()) {
+                throw new \Error("Can't swap the event loop driver while the driver is running");
+            }
+        } else {
+            \register_shutdown_function(self::onShutdown(...), 0);
         }
 
         try {
@@ -67,6 +71,27 @@ protected function now(): float
         }
     }
 
+    private static function onShutdown(int $invocationCount): void
+    {
+        $driver = self::getDriver();
+
+        $pending = false;
+        foreach ($driver->getIdentifiers() as $identifier) {
+            if ($driver->isEnabled($identifier) && $driver->isReferenced($identifier)) {
+                $pending = true;
+                break;
+            }
+        }
+
+        if ($pending) {
+            $driver->run();
+        }
+
+        if (!$invocationCount++ || $pending) {
+            \register_shutdown_function(self::onShutdown(...), $invocationCount);
+        }
+    }
+
     /**
      * Queue a microtask.
      *