Move path option into tracing.start (#192)

This consolidates all the tracing options in the tracing.start()

docs/api.md

@@ -73,8 +73,8 @@
     + [mouse.move(x, y)](#mousemovex-y)
     + [mouse.up([options])](#mouseupoptions)
   * [class: Tracing](#class-tracing)
-    + [tracing.start([options])](#tracingstartoptions)
-    + [tracing.stop(path)](#tracingstoppath)
+    + [tracing.start(options)](#tracingstartoptions)
+    + [tracing.stop()](#tracingstop)
   * [class: Dialog](#class-dialog)
     + [dialog.accept([promptText])](#dialogacceptprompttext)
     + [dialog.dismiss()](#dialogdismiss)
@@ -839,18 +839,18 @@ Dispatches a `mouseup` event.
 You can use [`tracing.start`](#tracingstartoptions) and [`tracing.stop`](#tracingstoppath) to create a trace file which can be opened in Chrome DevTools or [timeline viewer](https://chromedevtools.github.io/timeline-viewer/).
 
 ```js
-await page.tracing.start();
+await page.tracing.start({path: 'trace.json'});
 await page.navigate('https://www.google.com');
-await page.tracing.stop('trace.json');
+await page.tracing.stop();
 ```
 
-#### tracing.start([options])
+#### tracing.start(options)
 - `options` <[Object]>
+  - `path` <[string]> A path to write the trace file to. **required**
   - `screenshots` <[boolean]> captures screenshots in the trace.
 - returns: <[Promise]>
 
-#### tracing.stop(path)
-- `path` <[string]> A path to write the trace file to.
+#### tracing.stop()
 - returns: <[Promise]>
 
 ### class: Dialog

lib/Tracing.js

@@ -23,14 +23,16 @@ class Tracing {
   constructor(client) {
     this._client = client;
     this._recording = false;
+    this._path = '';
   }
 
   /**
-   * @param {Object=} options
+   * @param {!Object} options
    * @return {!Promise}
    */
-  async start(options = {}) {
+  async start(options) {
     console.assert(!this._recording, 'Cannot start recording trace while already recording trace.');
+    console.assert(options.path, 'Must specify a path to write trace file to.');
 
     const categoriesArray = [
       '-*', 'devtools.timeline', 'v8.execute', 'disabled-by-default-devtools.timeline',
@@ -42,6 +44,7 @@ class Tracing {
     if (options.screenshots)
       categoriesArray.push('disabled-by-default-devtools.screenshot');
 
+    this._path = options.path;
     this._recording = true;
     await this._client.send('Tracing.start', {
       transferMode: 'ReturnAsStream',
@@ -50,14 +53,13 @@ class Tracing {
   }
 
   /**
-   * @param {string} path
    * @return {!Promise}
    */
-  async stop(path) {
+  async stop() {
     let fulfill;
     let contentPromise = new Promise(x => fulfill = x);
     this._client.once('Tracing.tracingComplete', event => {
-      this._readStream(event.stream, path).then(fulfill);
+      this._readStream(event.stream, this._path).then(fulfill);
     });
     await this._client.send('Tracing.end');
     this._recording = false;

test/test.js

@@ -1503,23 +1503,23 @@ describe('Page', function() {
       fs.unlinkSync(outputFile);
     });
     it('should output a trace', SX(async function() {
-      await page.tracing.start({screenshots: true});
+      await page.tracing.start({screenshots: true, path: outputFile});
       await page.navigate(PREFIX + '/grid.html');
-      await page.tracing.stop(outputFile);
+      await page.tracing.stop();
       expect(fs.existsSync(outputFile)).toBe(true);
     }));
     it('should throw if tracing on two pages', SX(async function() {
-      await page.tracing.start();
+      await page.tracing.start({path: outputFile});
       let newPage = await browser.newPage();
       let error = null;
       try {
-        await newPage.tracing.start();
+        await newPage.tracing.start({path: outputFile});
       } catch (e) {
         error = e;
       }
       await newPage.close();
       expect(error).toBeTruthy();
-      await page.tracing.stop(outputFile);
+      await page.tracing.stop();
     }));
   });
 });