PWA offer great native alike offline capabilities. But what about tracking? In this tutorial I will show you how simple it is to implement offline tracking with Google Analytics in your PWA.

As you may already understood from my previous post, I felt in love with PWA. For the ones "late to the party" of PWA, let's see the definition of what they are:

Progressive Web Apps are user experiences that have the reach of the web, and are:

• Reliable - Load instantly and never show the downasaur, even in uncertain network conditions.
• Fast - Respond quickly to user interactions with silky smooth animations and no janky scrolling.
• Engaging - Feel like a natural app on the device, with an immersive user experience.
• This new level of quality allows Progressive Web Apps to earn a place on the user's home screen.

Basically PWA brings native mobile alike experience to the web. One of the features that PWAs take from mobile apps is offline capabilities. In fact it is possible to develop a web application that works completely offline with the help of Service Workers. But what about tracking stuff when the web app is offline? The standard Google Analytics Web SDK doesn't support offline tracking out of the box. So is it possible to avoid to loose important tracking events when your user is using the PWA offline? Yes it is. Let's find out how we can do it .

Implementation

Make Google Analytics works offline is as simple as importing a script and call a function! This is possible thanks to Workbox. What is it? Let's the definition reported on its official website:

Welcome, Workbox is a set of libraries and Node modules that make it easy to cache assets and take full advantage of features used to build Progressive Web Apps.

Basically it is a JavaScript library that help developer to build full PWA. It has been developed by the some of the Google Chrome Core Member team. One if its main feature is the support for Google Analytics offline tracking . This is exactly what we need! . Let's see the implementation.
In my service worker file I imported the workbox javascript script with the importScript directive . Then I setup workbox by initializing it (and I also turn the debug features). Then I just need to initialize the workbox Google Analytics plugin and that's it. This is the final implementation.

importScripts('https://storage.googleapis.com/workbox-cdn/releases/4.3.1/workbox-sw.js');

//...other service worker code....

workbox.setConfig({ debug: false })
workbox.googleAnalytics.initialize();

//...other service worker code....

How it works

How does workbox make your GA tracking working when your PWA is offline? The workbox.googleAnalytics plugin setup a new fetch handler that intercepts all the requests made with Google Analytics. These requests are basically network calls done with the Google Analytics Measurement protocol . When your PWA goes offline this handler stores the failed call into an ad-hoc IndexedDB schema called workbox-background-sync. When the network came back the the requests saved are flushed and pushed to the Google Analytics servers as measurement protocol requests. Below you can find a video that shows you the entire flow described above.

One important thing: for all my request you can see two entries in workbox-background-sync schema: one for the page the user is visiting and one for /blog, the homepage of my blog. This is a consequence of the fact that for each page on my website I use the prefetch tag to cache the homepage of the blog on every page on my website (because by watching my tracking in google analytics, I saw that the majority of the visitors of my website goes to the homepage of the blog as second page in the step of their navigation).

Conclusion

Google Analytics offline tracking is another feature that makes PWAs more similar to native mobile apps. As I already said in other posts, the gap between web and native development is closing more and more each day. You can already start to think about giving almost the same user experience to your user on all platforms (web and native). I will say a prayer for this end of the year: please Apple don't stay behind your competitor and remember what you said in 2008 (when web technologies where not as advanced as of today) .