API Introduce promise resolution middleware

_config/requestprocessors.yml

@@ -11,6 +11,7 @@ SilverStripe\Core\Injector\Injector:
         SessionMiddleware: '%$SilverStripe\Control\Middleware\SessionMiddleware'
         RequestProcessorMiddleware: '%$SilverStripe\Control\RequestProcessor'
         FlushMiddleware: '%$SilverStripe\Control\Middleware\FlushMiddleware'
+        EventsMiddleware: '%$SilverStripe\Control\Middleware\RegisteredEventsMiddleware'
   SilverStripe\Control\Middleware\AllowedHostsMiddleware:
     properties:
       AllowedHosts: '`SS_ALLOWED_HOSTS`'

src/Control/Middleware/RegisteredEventsMiddleware.php

@@ -0,0 +1,80 @@
+<?php
+
+namespace SilverStripe\Control\Middleware;
+
+use InvalidArgumentException;
+use SilverStripe\Control\HTTPRequest;
+use SilverStripe\Control\HTTPResponse;
+use SilverStripe\Core\Injector\Injectable;
+
+/**
+ * Allows events to be registered and passed through middleware.
+ * Useful for event registered prior to the beginning of a middleware chain.
+ */
+class RegisteredEventsMiddleware implements HTTPMiddleware
+{
+    use Injectable;
+
+    /**
+     * List of callable events
+     *
+     * @var array
+     */
+    protected $events = [];
+
+    /**
+     * Register event
+     *
+     * @param callable $event Callback to invoke. This callback will be passed the request object,
+     * but will not be able to invoke subsequent handlers or events.
+     * @param string|null $name Optional name for this event
+     */
+    public function registerEvent($event, $name = null)
+    {
+        if ($name) {
+            $this->events[$name] = $event;
+        } else {
+            $this->events[] = $event;
+        }
+    }
+
+    /**
+     * Check if a named event is registered
+     *
+     * @param string $name
+     * @return bool
+     */
+    public function hasEvent($name)
+    {
+        return !is_numeric($name) && isset($this->events[$name]);
+    }
+
+    /**
+     * Unregister a named event
+     *
+     * @param string $name
+     */
+    public function unregisterEvent($name)
+    {
+        if (is_numeric($name)) {
+            throw new InvalidArgumentException("Only named events can be unregistered");
+        }
+        unset($this->events[$name]);
+    }
+
+    /**
+     * Generate response for the given request
+     *
+     * @param HTTPRequest $request
+     * @param callable $delegate
+     * @return HTTPResponse
+     */
+    public function process(HTTPRequest $request, callable $delegate)
+    {
+        // Invoke all registered events
+        foreach ($this->events as $event) {
+            $event($request);
+        }
+        return $delegate($request);
+    }
+}