add doc for middleware usage in "dispatch" event

Author: samsonasik

doc/book/middleware.md

@@ -27,7 +27,7 @@ a "controller" in the routing defaults, you provide "middleware":
 ```php
 // Via configuration:
 return [
-    'router' => 
+    'router' =>
         'routes' => [
             'home' => [
                 'type' => 'literal',
@@ -106,3 +106,45 @@ causing the application to short-circuit and return the response immediately.
 You can, however, return arbitrary values. If you do, the result is pushed into
 the `MvcEvent` as the event result, allowing later dispatch listeners to
 manipulate the results.
+
+## Using Middleware in 'dispatch' Event
+
+During Mvc Workflow, you can use the Middleware in 'dispatch' event by provide `Psr7Bridge`. For example, you have `AuthorizationMiddleware`:
+
+```php
+namespace Application\Middleware;
+
+class AuthorizationMiddleware
+{
+    public function __invoke($request, $response, $next = null)
+    {
+        // handle authorization here...
+    }
+}
+```
+
+As the request and response in 'dispatch' event is a `Zend\Http` Request and Response object, we need the bridge to convert into PSR-7 Request and Response. To do that, you can do the following:
+
+```php
+namespace Application;
+
+use Application\Middleware\AuthorizationMiddleware;
+use Zend\Psr7Bridge\Psr7ServerRequest;
+use Zend\Psr7Bridge\Psr7Response;
+
+class Module
+{
+    public function onBootstrap($e)
+    {
+        $app          = $e->getApplication();
+        $eventManager = app->getEventManager();
+        $services     = app->getServiceManager();
+
+        $eventManager->attach('dispatch', function ($e) use ($services) {
+            $request  = Psr7ServerRequest::fromZend($e->getRequest());
+            $response = Psr7Response::fromZend($e->getResponse());
+            $result   = ($services->get(AuthorizationMiddleware::class))($request, $response);
+        }, 2);
+    }
+}
+```