feat(tailor): make possible to add custom event during template fetching

- by adding tailor object (event emitter) as third parameter of  function

fix zalando#347

README.md

@@ -46,8 +46,9 @@ server.listen(process.env.PORT || 8080);
 ## Options
 
 * `fetchContext(request)` - Function that returns a promise of the context, that is an object that maps fragment id to fragment url, to be able to override urls of the fragments on the page, defaults to `Promise.resolve({})`
-* `fetchTemplate(request, parseTemplate)` - Function that should fetch the template, call `parseTemplate` and return a promise of the result. Useful to implement your own way to retrieve and cache the templates, e.g. from s3.
-Default implementation [`lib/fetch-template.js`](https://github.com/zalando/tailor/blob/master/lib/fetch-template.js) fetches the template from  the file system
+* `fetchTemplate(request, parseTemplate, eventEmitter)` - Function that should fetch the template, call `parseTemplate` and return a promise of the result. Useful to implement your own way to retrieve and cache the templates, e.g. from s3.
+Default implementation [`lib/fetch-template.js`](https://github.com/zalando/tailor/blob/master/lib/fetch-template.js) fetches the template from  the file system.
+`eventEmitter` allow you to emit your own events that you can catch like native [Tailor events](https://github.com/zalando/tailor/blob/master/docs/Events.md).
 * `templatesPath` - To specify the path where the templates are stored locally, Defaults to `/templates/`
 * `fragmentTag` - Name of the fragment tag, defaults to `fragment`
 * `handledTags` - An array of custom tags, check [`tests/handle-tag`](https://github.com/zalando/tailor/blob/master/tests/handle-tag.js) for more info

docs/Events.md

@@ -20,5 +20,19 @@ Events may be used for logging and monitoring. Check [perf/benchmark.js](https:/
 * Error: `fragment:error(request, fragment.attributes, error)` in case of socket error, timeout, 50x
 * Fallback: `fragment:fallback(request, fragment.attributes, error)` in case of timeout/error from the fragment if the *fallback-src* is specified
 
+## Custom events
+
+It is possible to emit custom event during template fetching. You need to override `fetchTemplate` Tailor option. `fetchTemplate` is Tailor event emitter that you can use to emit your own events like this : 
+
+```js
+const fetchTemplate = (request, parseTemplate, eventEmitter) => {
+    // [Your logic]
+
+    eventEmitter.emit('myEvent', {foo: 'bar' });
+
+    parseTemplate();
+}
+```
+
 
 **Note:**  `fragment:response`, `fragment:fallback` and `fragment:error` are mutually exclusive. `fragment:end` happens only in case of successful response.

lib/request-handler.js

@@ -64,7 +64,7 @@ module.exports = function processRequest(options, request, response) {
         this.emit('context:error', request, err);
         return {};
     });
-    const templatePromise = fetchTemplate(request, parseTemplate);
+    const templatePromise = fetchTemplate(request, parseTemplate, this);
     const responseHeaders = {
         // Disable cache in browsers and proxies
         'Cache-Control': 'no-cache, no-store, must-revalidate',

tests/tailor.events.js

@@ -16,8 +16,9 @@ describe('Tailor events', () => {
         tailor = new Tailor({
             fetchContext: mockContext,
             pipeDefinition: () => Buffer.from(''),
-            fetchTemplate: (request, parseTemplate) => {
+            fetchTemplate: (request, parseTemplate, tailor) => {
                 const template = mockTemplate(request);
+                tailor.emit('customEvent', 'bar');
                 if (template) {
                     return parseTemplate(template);
                 } else {
@@ -159,4 +160,22 @@ describe('Tailor events', () => {
             response.on('end', done);
         });
     });
+
+    it('emits a custom event on fetching Template', done => {
+        const onCustomEvent = sinon.spy();
+        nock('https://fragment')
+            .get('/')
+            .reply(200, 'hello');
+        mockTemplate.returns('<fragment src="https://fragment">');
+        tailor.on('customEvent', onCustomEvent);
+        http.get('http://localhost:8080/template', response => {
+            response.resume();
+            response.on('end', () => {
+                const foo = onCustomEvent.args[0][0];
+                assert.equal(foo, 'bar');
+                assert.equal(onCustomEvent.callCount, 1);
+                done();
+            });
+        });
+    });
 });