Add LoopDriverFactory

The LoopDriverFactory SHOULD be used by driver implementations
to set a default event loop driver.

This can be achieved by having a Composer autoloader like that:

{
    "autoload": {
        "files": ["src/bootstrap.php"]
    }
}

Where src/bootstrap.php contains the following code:

use Interop\Async\EventLoop\LoopDriverFactory;

class MyDriverFactory implements LoopDriverFactory
{
    public function create()
    {
        return new MyDriver();
    }
}

Loop::setFactory(new MyDriverFactory());

This ensures the user doesn't have to care about setting the actual
driver. A user just has to require the specific event loop driver
package he / she wants to use.

Author: kelunik

src/Loop.php

@@ -6,11 +6,24 @@ final class Loop
 {
     use Registry;
 
+    /**
+     * @var LoopDriverFactory
+     */
+    private static $factory = null;
+
     /**
      * @var LoopDriver
      */
     private static $driver = null;
 
+    /**
+     * Set the factory to be used to create a driver if none is passed to
+     * self::execute.
+     */
+    public static function setFactory(LoopDriverFactory $factory = null) {
+        self::$factory = $factory;
+    }
+
     /**
      * Execute a callback within the scope of an event loop driver.
      *
@@ -19,11 +32,12 @@ final class Loop
      *
      * @return void
      */
-    public static function execute(callable $callback, LoopDriver $driver)
+    public static function execute(callable $callback, LoopDriver $driver = null)
     {
-        $previousDriver = self::$driver;
+        $driver = $driver ?: $this->createDriver();
         $previousRegistry = self::$registry;
 
+        $previousDriver = self::$driver;
         self::$driver = $driver;
         self::$registry = [];
 
@@ -37,6 +51,20 @@ public static function execute(callable $callback, LoopDriver $driver)
         }
     }
 
+    /**
+     * Create a new driver if a factory is present, otherwise throw.
+     *
+     * @throws \LogicException if no factory is set
+     */
+    private static function createDriver()
+    {
+        if (self::$factory === null) {
+            throw new \LogicException("Can't create an event loop driver without a factory.");
+        }
+
+        return self::$factory->create();
+    }
+
     /**
      * Retrieve the event loop driver that is in scope.
      *

src/LoopDriverFactory.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Interop\Async\EventLoop;
+
+interface LoopDriverFactory
+{
+	/**
+	 * Create a new event loop driver instance.
+	 *
+	 * @return LoopDriver
+	 */
+	public function create();
+}