add middleware layer support

src/dialogflow-fulfillment.js

@@ -209,6 +209,14 @@ class WebhookClient {
      */
     this.alternativeQueryResults = null;
 
+    /**
+     * List of middlewares defined by the developer
+     *
+     * @private
+     * @type {Object[]}
+     */
+    this.middlewares_ = [];
+
     /**
      * Platform contants, to define platforms, includes supported platforms and unspecified
      * @example
@@ -309,6 +317,10 @@ class WebhookClient {
       ));
     }
 
+    for (const middleware of this.middlewares_) {
+      middleware(this);
+    }
+
     if (handler.get(this.intent)) {
       let result = handler.get(this.intent)(this);
       // If handler is a promise use it, otherwise create use default (empty) promise
@@ -328,6 +340,32 @@ class WebhookClient {
     }
   }
 
+  /**
+   * Add a middleware function
+   *
+   * * @example
+   * const { WebhookClient } = require('dialogflow-webhook');
+   * const agent = new WebhookClient({request: request, response: response});
+   * class Helper {
+   *   constructor(agent) {
+   *     this.agent = agent;
+   *   }
+   *
+   *   func1() {
+   *     this.agent.add(`What's up?`);
+   *   }
+   * }
+   * agent.middleware(a => {
+   *       a.helper = new Helper(a);
+   * });
+   *
+   * @param {requestCallback} handler
+   */
+  middleware(handler) {
+    if (typeof handler === 'function') {
+      this.middlewares_.push(handler);
+    }
+  }
   // --------------------------------------------------------------------------
   //          Deprecated Context methods
   // --------------------------------------------------------------------------

test/webhook-integration-v2-test.js

@@ -94,6 +94,16 @@ test('v2 Integration test', async (t) => {
 function webhookTest(request, callback) {
   let response = new ResponseMock(callback);
   const agent = new WebhookClient({request: request, response: response});
+
+  agent.middleware((agent) => {
+    if (agent.intent === 'Custom Intent') {
+      agent.setContext({
+        name: 'other',
+        lifespan: 2,
+        parameters: {city: 'Toronto'},
+      });
+    }
+  });
   /**
    * Handler function to welcome
    * @param {Object} agent
@@ -255,6 +265,12 @@ const mockSimulatorV2ResponseOther = {
       lifespanCount: 2,
       parameters: {city: 'Rome'},
     },
+    {
+      name:
+        'projects/agent52-3e1ea/agent/sessions/669d7da3-de6f-4b1d-8394-d79c6973e516/contexts/other',
+      lifespanCount: 2,
+      parameters: {city: 'Toronto'},
+    },
   ],
 };
 // Mock webhook request and reponse from Dialogflow for sample v2