ux-capture

v4.0.0

Published

2 years ago

Browser instrumentation helper that makes it easier to capture UX speed metrics

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sergeychernyshev

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webperf perfmatters web performance ux speed user-timing

UX Capture

Browser instrumentation library that makes it easier to capture UX performance metrics.

Project Goals

There are multiple goals for this project, many dictated by the lack of real rendering instrumentation of paint events in the browser. These include:

Capture display and interactivity events for various UI elements (e.g. images, text, video, fonts, buttons, etc.)
Group together multiple display events for elements of a web page that represent the same design/product components.
Group together multiple components to reflect various phases of page load
Collect captured events and UX speed metrics for all users using RUM (Real User Measurement) tools.
Calibrate in-browser instrumentation by recording page load video using synthetic tools and deriving same UX speed metrics
Create uniform instrumentation for both page views and interactive views, to be usable with any back-end and front-end framework
Future compatibility with Element Timing API that aims at adding instrumentation directly into browser

JS library

The intent of this library is to help developers instrument technical events (marks) on their pages and group them into "zones" that represent "phases" of page load, with each phase representing distinct stages of user experience.

Usage

NOTE: this version of the library relies on UserTiming API to be available in the browser, but should not break if it doesn't. You can use a polyfill if you want to support older browsers.

Load the library by inlining the contents of ux-capture.min.js in a <script> tag in the HTML document <head>. Here's an example using server-side React:
```
const uxCaptureFilename = require.resolve('ux-capture/lib/ux-capture.min.js');
const uxCaptureJS = fs.readFileSync(uxCaptureFilename, 'utf8');
...
render() {
    <head>
        <title>My Page</title>
        <script dangerouslySetInnerHTML={{ __html: uxCaptureJS }} />
        ...
    </head>
    ...
}
```
NOTE: The script must be inlined. Do not use a script tag with a src attribute. Waiting for network requests might artifically skew event timings on the page and lead to race conditions.
NOTE: It is important to have this code available very early on the page since we need to instrument events that happen as early as HTML parsing, so ideally in the <head>.
Initialize UXCapture using UXCapture.create(), optionally with mark and measure event handlers, e.g.
```
    <script>
        UXCapture.create({
            onMark: name => console.log('marked', name),
            onMeasure: name => console.log('measured', name),
        });
    </script>
```
Custom event handlers are useful in cases where the monitoring solution you use (e.g., NewRelic) does not support the W3C UserTiming API natively. You can then provide a custom method of recording the results.
- onMark - provides a custom handler to be called every time a mark is recorded with the name of the mark as the only argument
- onMeasure - provides a custom handler to be called every time a measure is recorded with the name of the measure as the only argument
At the top of the view markup, define the expected zones and corresponding marks with UXCapture.startView(), e.g.
```
    <script>
        UXCapture.startView([
            {
                name: 'ux-destination-verified',
                marks: ['ux-1', 'ux-2']
            },{
                name: 'ux-primary-content-available',
                marks: ['ux-3', 'ux-4']
            }
            ...
        ]);
    </script>
```
NOTE: UXCapture.startView() will throw an error if called while previous view is active, so be careful to only call it once.
Each individual zone configuration object contains of zone's name that will be used as a name of corresponding W3C UserTiming API measure and marks array of individual event name strings that zone groups together, each individual name will be used when recording corresponding events as W3C UserTiming API mark.
You can optionally update a view that has already been started and add more zones by calling UXCapture.updateView().
Call UXCapture.mark in the HTML markup for each ‘mark’ name passed into UXCapture.startView()/updateView().
```
    <script>UXCapture.mark('ux-1')</script>
    <img onload="UXCapture.mark('ux-2')" … />
    ...
```
(SPA support) For 'interactive' view changes (usually associated with a route change), the client app must imperatively indicate when the current view is no longer valid using UXCapture.startTransition, and clear any marks that should not be considered valid for the subsequent view using UXCapture.clearMarks(name). To clear all marks, omit the name argument. For marks that are associated with elements that do not change between views, there is no need to clear the mark.
The call to UXCapture.startTransition does not need to be in the markup (and generally shouldn’t be).
```
    history.push(‘/foo’)
    window.UXCapture.startTransition();
```
Summary: A SPA view transition is comprised of the following calls:
1. UXCapture.startTransition() – required
2. UXCapture.clearMarks(name) – optional, but should be called for each existing mark that is no longer valid
Repeat from step 3.

Sample page

This repository contains a sample page that implements basic instrumentation for your reference: https://cdn.rawgit.com/sergeychernyshev/ux-capture/master/examples/index.html

Instrumentation

Individual Element Instrumentation

Each element that needs to be instrumented might require multiple snippets of code injected into a page to measure several events which in turn are aggregated into element-level measurements using element aggregation algorythm (can vary for different element instrumentation methods, but "latest of events" is generally a good default).

Number of snippets and exact code to be added to the page depends on the type of element being measured and must be determined experimentally or discovered in industry white papers or blogs as there is currently no direct method of instrumenting the display timings in modern browsers.

Below is the list of instrumentation methods with examples:

Image elements

Image tracking requires two measurements, one within the onload callback of the image itself and another within inline <script> tag directly after the image.

<img src="hero.jpg" onload="UXCapture.mark('ux-image-onload-logo')">
<script>UXCapture.mark('ux-image-inline-logo')</script>

Element aggregation algorythm: latest of the two (inline and onload) measurements.

References:

Steve Souders: Hero Image Custom Metrics, published on May 12, 2015

Text without custom font

Text that does not use a custom font can be instrumented by supplying one inline <script> tag directly after the text:

<h1>Headline</h1>
<script>UXCapture.mark("ux-text-headline");</script>

Element aggregation algorythm: no aggregation, just one event.

References:

Steve Souders: User Timing and Custom Metrics (example 5), published on November 12, 2015

Text with custom font

Many pages use custom fonts to display text and often experience Flash of Invisible Text or FOIT. It is important to take into account time to load custom fonts. You can do it using font loaders provided by using event tracking in Web Font Loader used by Typekit and Google.

You can inline the library in HTML and then use the following code to fire a mark when font loaded.

<script>
WebFont.load({
    custom: {
        families: ["Montserrat:n4"]
    },
    active: function() {
        UXCapture.mark("ux-font-montserrat-normal");
    }
});
</script>

NOTE: See Font Variation Description format used by Web Font Loader for specifying particular font variation to track.

Similarly to tracking text without custom font, inject a mark inline after text that uses custom font in question.

<h2>Title with font</h2>
<script>UXCapture.mark("ux-text-title-using-montserrat-normal");</script>

Element aggregation algorythm: latest of the two (font and text) measurements.

Event handler attachment

Some user activity requires custom JavaScript handler code to be attached to an event on the page, e.g. click of the button (e.g. it's only "available" when visible AND clickable). Instrumenting handler attachment is straightforward, just include the call right after handler attachment in JavaScript code.

var button_element = document.getElementById('mybutton');
button_element.addEventListener('click', myActionHandler);
UXCapture.mark('ux-handler-myaction');

Element aggregation algorythm: no aggregation, just one event.

Aggregating component metrics

Most design components on web page consist of multiple elements, e.g. tweet contains text, name of the person tweeting and their userpic.

On this level, it is very business-specific and needs attention of a product manager and/or graphics designer to identify. It is suggested to conduct business interviews to identify the elements and to group them into components accordingly.

Most common way / algorythm to aggregate performance metrics for individual tracked elements is to take timing of the latest element that showed up and treat it as completion timing for compoment overall, but business decision can be made to track earliest element (e.g. "as long as something is visible"), although these are less common and can create more variability in measurement.

Component-level aggregation might be an advanced detail and can be skipped early on with element timings aggregated directly into experience/perception phases, but can be useful for modularity and more detailed analysis across the system.

Aggregating experience/perception phase metrics

It is critical to group metrics into categories that are not specific to individual pages, but can be used universally across the property and just comprised of different components / elements on each page.

Well known example of such "category" is first meaningful paint which has different meaning on differeng parts of user experience, but represents a universal improvement over "first paint" technical metric — performance metrics that represent time spent executing various technical components of the application as opposed to metrics representing speed of the human-computer interface as it is perceived by the user'.

This can be taken further to represent user's intent in more detail. Each view can be broken down into several phases which all contribute to keeping user on task and giving them best experience in terms of percieved speed.

Here are 4 phases defining parts of experience that matter to business from different perspectives:

Destination verified (ux-destination-verified)
Primary content displayed (ux-primary-content-displayed)
Primary action available (ux-primary-action-available)
Secondary content displayed (ux-secondary-content-displayed)

Each phase's component or element metrics (marks) can be combined and reported as measures:

// assuming logo's onload event was last to fire among all element timers for this phase
performance.measure('ux-destination-verified', 0, 'ux-image-onload-logo');

the can be then collected using RUM beacon or using synthetic testing tool like WebPageTest or Chrome Developer Tools' Timeline tab.

This is done automatically by the library and no additional instrumentation is necessary.

Testing results

To confirm that your instrumentation was successful, open your page in Chrome Developer Tools Performance tab and hit reload to capture the timeline.

Verify that individual marks are captured as timestamps on the timeline (small vertical lines on Frames band), hovering over each should show the name used for the mark.

Also verify that measures are captured (long horizontal bars under User Timing band, starting at navigation and ending at the last mark comprizing the zone).

Note that you might need to zoom in on the timeline to see these marks and measures. Chrome DevTools Performance Timeline with marks and measures

You can also run W3C Performance Timeline API performance.getEntriesByType() method with "mark" and "measure" parameters to retrieve marks and measures respectively.

Chrome DevTools console showing captured performance marks and measures