Tags: serviceworkers

8

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Making Resilient Web Design work offline

I’ve written before about taking an online book offline, documenting the process behind the web version of HTML5 For Web Designers. A book is quite a static thing so it’s safe to take a fairly aggressive offline-first approach. In fact, a static unchanging book is one of the few situations that AppCache works for. Of course a service worker is better, but until AppCache is removed from browsers (and until service worker is supported across the board), I’m using both. I wouldn’t recommend that for most sites though—for most sites, use a service worker to enhance it, and avoid AppCache like the plague.

For Resilient Web Design, I took a similar approach to HTML5 For Web Designers but I knew that there was a good chance that some of the content would be getting tweaked at least for a while. So while the approach is still cache-first, I decided to keep the cache fairly fresh.

Here’s my service worker. It starts with the usual stuff: when the service worker is installed, there’s a list of static assets to cache. In this case, that list is literally everything; all the HTML, CSS, JavaScript, and images for the whole site. Again, this is a pattern that works well for a book, but wouldn’t be right for other kinds of websites.

The real heavy lifting happens with the fetch event. This is where the logic sits for what the service worker should do everytime there’s a request for a resource. I’ve documented the logic with comments:

// Look in the cache first, fall back to the network
  // CACHE
  // Did we find the file in the cache?
      // If so, fetch a fresh copy from the network in the background
      // NETWORK
          // Stash the fresh copy in the cache
  // NETWORK
  // If the file wasn't in the cache, make a network request
      // Stash a fresh copy in the cache in the background
  // OFFLINE
  // If the request is for an image, show an offline placeholder
  // If the request is for a page, show an offline message

So my order of preference is:

  1. Try the cache first,
  2. Try the network second,
  3. Fallback to a placeholder as a last resort.

Leaving aside that third part, regardless of whether the response is served straight from the cache or from the network, the cache gets a top-up. If the response is being served from the cache, there’s an additional network request made to get a fresh copy of the resource that was just served. This means that the user might be seeing a slightly stale version of a file, but they’ll get the fresher version next time round.

Again, I think this acceptable for a book where the tweaks and changes should be fairly minor, but I definitely wouldn’t want to do it on a more dynamic site where the freshness matters more.

Here’s what it usually likes like when a file is served up from the cache:

caches.match(request)
  .then( responseFromCache => {
  // Did we find the file in the cache?
  if (responseFromCache) {
      return responseFromCache;
  }

I’ve introduced an extra step where the fresher version is fetched from the network. This is where the code can look a bit confusing: the network request is happening in the background after the cached file has already been returned, but the code appears before the return statement:

caches.match(request)
  .then( responseFromCache => {
  // Did we find the file in the cache?
  if (responseFromCache) {
      // If so, fetch a fresh copy from the network in the background
      event.waitUntil(
          // NETWORK
          fetch(request)
          .then( responseFromFetch => {
              // Stash the fresh copy in the cache
              caches.open(staticCacheName)
              .then( cache => {
                  cache.put(request, responseFromFetch);
              });
          })
      );
      return responseFromCache;
  }

It’s asynchronous, see? So even though all that network code appears before the return statement, it’s pretty much guaranteed to complete after the cache response has been returned. You can verify this by putting in some console.log statements:

caches.match(request)
.then( responseFromCache => {
  if (responseFromCache) {
      event.waitUntil(
          fetch(request)
          .then( responseFromFetch => {
              console.log('Got a response from the network.');
              caches.open(staticCacheName)
              .then( cache => {
                  cache.put(request, responseFromFetch);
              });
          })
      );
      console.log('Got a response from the cache.');
      return responseFromCache;
  }

Those log statements will appear in this order:

Got a response from the cache.
Got a response from the network.

That’s the opposite order in which they appear in the code. Everything inside the event.waitUntil part is asynchronous.

Here’s the catch: this kind of asynchronous waitUntil hasn’t landed in all the browsers yet. The code I’ve written will fail.

But never fear! Jake has written a polyfill. All I need to do is include that at the start of my serviceworker.js file and I’m good to go:

// Import Jake's polyfill for async waitUntil
importScripts('/js/async-waituntil.js');

I’m also using it when a file isn’t found in the cache, and is returned from the network instead. Here’s what the usual network code looks like:

fetch(request)
  .then( responseFromFetch => {
    return responseFromFetch;
  })

I want to also store that response in the cache, but I want to do it asynchronously—I don’t care how long it takes to put the file in the cache as long as the user gets the response straight away.

Technically, I’m not putting the response in the cache; I’m putting a copy of the response in the cache (it’s a stream, so I need to clone it if I want to do more than one thing with it).

fetch(request)
  .then( responseFromFetch => {
    // Stash a fresh copy in the cache in the background
    let responseCopy = responseFromFetch.clone();
    event.waitUntil(
      caches.open(staticCacheName)
      .then( cache => {
          cache.put(request, responseCopy);
      })
    );
    return responseFromFetch;
  })

That all seems to be working well in browsers that support service workers. For legacy browsers, like Mobile Safari, there’s the much blunter caveman logic of an AppCache manifest.

Here’s the JavaScript that decides whether a browser gets the service worker or the AppCache:

if ('serviceWorker' in navigator) {
  // If service workers are supported
  navigator.serviceWorker.register('/serviceworker.js');
} else if ('applicationCache' in window) {
  // Otherwise inject an iframe to use appcache
  var iframe = document.createElement('iframe');
  iframe.setAttribute('src', '/appcache.html');
  iframe.setAttribute('style', 'width: 0; height: 0; border: 0');
  document.querySelector('footer').appendChild(iframe);
}

Either way, people are making full use of the offline nature of the book and that makes me very happy indeed.

Less JavaScript

Every front-end developer at Clearleft went to FFConf last Friday: me, Mark, Graham, Charlotte, and Danielle. We weren’t about to pass up the opportunity to attend a world-class dev conference right here in our home base of Brighton.

The day was unsurprisingly excellent. All the speakers brought their A-game on a wide range of topics. Of course JavaScript was covered, but there was also plenty of mindfood on CSS, accessibility, progressive enhancement, dev tools, creative coding, and even emoji.

Normally FFConf would be a good opportunity to catch up with some Pauls from the Google devrel team, but because of an unfortunate scheduling clash this year, all the Pauls were at Chrome Dev Summit 2016 on the other side of the Atlantic.

I’ve been catching up on the videos from the event. There’s plenty of tech-related stuff: dev tools, web components, and plenty of talk about progressive web apps. But there was also a very, very heavy focus on performance. I don’t just mean performance at the shallow scale of file size and optimisation, but a genuine questioning of the impact of our developer workflows and tools.

In his talk on service workers (what else?), Jake makes the point that not everything needs to be a single page app, echoing Ada’s talk at FFConf.

He makes the point that if you really want fast rendering, nothing on the client side quite beats a server render.

They’ve written a lot of JavaScript to make this quite slow.

Unfortunately, all too often, I hear people say that a progressive web app must be a single page app. And I am not so sure. You might not need a single page app. A single page app can end up being a lot of work and slower. There’s a lot of cargo-culting around single page apps.

Alex followed up his barnstorming talk from the Polymer Summit with some more uncomfortable truths about how mobile phones work.

Cell networks are basically kryptonite to the protocols and assumptions that the web was built on.

And JavaScript frameworks aren’t helping. Quite the opposite.

But make no mistake: if you’re using one of today’s more popular JavaScript frameworks in the most naive way, you are failing by default. There is no sugarcoating this.

Today’s frameworks are mostly a sign of ignorance, or privilege, or both. The good news is that we can fix the ignorance.

The imitation game

Jason shared some thoughts on designing progressive web apps. One of the things he’s pondering is how much you should try make your web-based offering look and feel like a native app.

This was prompted by an article by Owen Campbell-Moore over on Ev’s blog called Designing Great UIs for Progressive Web Apps. He begins with this advice:

Start by forgetting everything you know about conventional web design, and instead imagine you’re actually designing a native app.

This makes me squirm. I mean, I’m all for borrowing good ideas from other media—native apps, TV, print—but I don’t think that inspiration should mean imitation. For me, that always results in an interface that sits in a kind of uncanny valley of being almost—but not quite—like the thing it’s imitating.

With that out of the way, most of the recommendations in Owen’s article are sensible ideas about animation, input, and feedback. But then there’s recommendation number eight: Provide an easy way to share content:

PWAs are often shown in a context where the current URL isn’t easily accessible, so it is important to ensure the user can easily share what they’re currently looking at. Implement a share button that allows users to copy the URL to the clipboard, or share it with popular social networks.

See, when a developer has to implement a feature that the browser should be providing, that seems like a bad code smell to me. This is a problem that Opera is solving (and Google says it is solving, while meanwhile penalising developers who expose the URL to end users).

Anyway, I think my squeamishness about all the advice to imitate native apps is because it feels like a cargo cult. There seems to be an inherent assumption that native is intrinsically “better” than the web, and that the only way that the web can “win” is to match native apps note for note. But that misses out on all the things that only the web can do—instant distribution, low-friction sharing, and the ability to link to any other resource on the web (and be linked to in turn). Turning our beautifully-networked nodes into standalone silos just because that’s the way that native apps have to work feels like the cure that kills the patient.

If anything, my advice for building a progressive web app would be the exact opposite of Owen’s: don’t forget everything you’ve learned about web design. In my opinion, the term “progressive web app” can be read in order of priority:

  1. Progressive—build in a layered way so that anyone can access your content, regardless of what device or browser they’re using, rewarding the more capable browsers with more features.
  2. Web—you’re building for the web. Don’t lose sight of that. URLs matter. Accessibility matters. Performance matters.
  3. App—sure, borrow what works from native apps if it makes sense for your situation.

Jason asks questions about how your progressive web app will behave when it’s added to the home screen. How much do you match the platform? How do you manage going chromeless? And the big one: what do users expect?

Will people expect an experience that maps to native conventions? Or will they be more accepting of deviation because they came to the app via the web and have already seen it before installing it?

These are good questions and I share Jason’s hunch:

My gut says that we can build great experiences without having to make it feel exactly like an iOS or Android app because people will have already experienced the Progressive Web App multiple times in the browser before they are asked to install it.

In all the messaging from Google about progressive web apps, there’s a real feeling that the ability to install to—and launch from—the home screen is a real game changer. I’m not so sure that we should be betting the farm on that feature (the offline possibilities opened up by service workers feel like more of a game-changer to me).

People have been gleefully passing around the statistic that the average number of native apps installed per month is zero. So how exactly will we measure the success of progressive web apps against native apps …when the average number of progressive web apps installed per month is zero?

I like Android’s add-to-home-screen algorithm (although it needs tweaking). It’s a really nice carrot to reward the best websites with. But let’s not carried away. I think that most people are not going to click that “add to home screen” prompt. Let’s face it, we’ve trained people to ignore prompts like that. When someone is trying to find some information or complete a task, a prompt that pops up saying “sign up to our newsletter” or “download our native app” or “add to home screen” is a distraction to be dismissed. The fact that only the third example is initiated by the operating system, rather than the website, is irrelevant to the person using the website.

Getting the “add to home screen” prompt for https://huffduffer.com/ on Android Chrome.

My hunch is that the majority of people will still interact with your progressive web app via a regular web browser view. If, then, only a minority of people are going to experience your site launched from the home screen in a native-like way, I don’t think it makes sense to prioritise that use case.

The great thing about progressive web apps is that they are first and foremost websites. Literally everyone who interacts with your progressive web app is first going to do so the old-fashioned way, by following a link or typing in a URL. They may later add it to their home screen, but that’s just a bonus. I think it’s important to build progressive web apps accordingly—don’t pretend that it’s just like building a native app just because some people will be visiting via the home screen.

I’m worried that developers are going to think that progressive web apps are something that need to built from scratch; that you have to start with a blank slate and build something new in a completely new way. Now, there are some good examples of these kind of one-off progressive web apps—The Guardian’s RioRun is nicely done. But I don’t think that the majority of progressive web apps should fall into that category. There’s nothing to stop you taking an existing website and transforming it step-by-step into a progressive web app:

  1. Switch over to HTTPS if you aren’t already.
  2. Use a service worker, even if it’s just to provide a custom offline page and cache some static assets.
  3. Make a manifest file to point to an icon and specify some colours.

See? Not exactly a paradigm shift in how you approach building for the web …but those deceptively straightforward steps will really turbo-boost your site.

I’m really excited about progressive web apps …but mostly for the “progressive” and “web” parts. Maybe I’ll start calling them progressive web sites. Or progressive web thangs.

Backdoor Service Workers

When I was moderating that panel at the Progressive Web App dev Summit, I brought up this point about twenty minutes in:

Alex, in your talk yesterday you were showing the AMP demo there with the Washington Post. You click through and there’s the Washington Post AMP thing, and it was able to install the Service Worker with that custom element. But I was looking at the URL bar …and that wasn’t the Washington Post. It was on the CDN from AMP. So I talked to Paul Backaus from the AMP team, and he explained that it’s an iframe, and using an iframe you can install a Service Worker from somewhere else.

Alex and Emily explained that, duh, that’s the way iframes work. It makes sense when you think about it—an iframe is pretty much the same as any other browser window. Still, it feels like it might violate the principle of least surprise.

Let’s say you followed my tongue-in-cheek advice to build a progressive web app store. Your homepage might have the latest 10 or 20 progressive web apps. You could also include 10 or 20 iframes so that those sites are “pre-installed” for the person viewing your page.

Enough theory. Here’s a practical example…

Suppose you’ve never visited the website for my book, html5forwebdesigners.com (if you have visited it, and you want to play along with this experiment, go to your browser settings and delete anything stored by that domain).

You happen to visit my website adactio.com. There’s a little blurb buried down on the home page that says “Read my book” with a link through to html5forwebdesigners.com. I’ve added this markup after the link:

<iframe src="https://html5forwebdesigners.com/iframe.html" style="width: 0; height: 0; border: 0">
</iframe>

That hidden iframe pulls in an empty page with a script element:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>HTML5 For Web Designers</title>
<script>
if ('serviceWorker' in navigator) {
  navigator.serviceWorker.register('/serviceworker.js');
}
</script>
</head>
</html>

That registers the Service Worker on my book’s site which then proceeds to install all the assets it needs to render the entire site offline.

There you have it. Without ever visiting the domain html5forwebdesigners.com, the site has been pre-loaded onto your device because you visited the domain adactio.com.

A few caveats:

  1. I had to relax the Content Security Policy for html5forwebdesigners.com to allow the iframe to be embedded on adactio.com:

    Header always set Access-Control-Allow-Origin: "https://adactio.com"
    
  2. If your browser’s settings has “Block third-party cookies and site data” selected in the preferences, the iframe-invoked Service Worker won’t install:

    Uncaught (in promise) DOMException: Failed to register a ServiceWorker: The user denied permission to use Service Worker.
    

The example I’ve put together here is relatively harmless. But it’s possible to imagine more extreme scenarios. Imagine there’s a publishing company that has 50 websites for 50 different publications. Each one of them could have an empty page waiting to be embedded via iframe from the other 49 sites. You only need to visit one page on one of those 50 sites to have 50 Service Workers spun up and caching assets in the background.

There’s the potential here for a tragedy of the commons. I hope we’ll be sensible about how we use this power.

Just don’t tell the advertising industry about this.

The Progressive Web App Dev Summit

I was in Amsterdam again at the start of last week for the Progressive Web App Dev Summit, organised by Google. Most of the talks were given by Google employees, but not all—this wasn’t just a European version of Google I/O. Representatives from Opera, Mozilla, Samsung, and Microsoft were also there, and there were quite a few case studies from independent companies. That was very gratifying to see.

Almost all the talks were related to progressive web apps. I say, “almost all” because there were occasional outliers. There was a talk on web components, which don’t have anything directly to do with progressive web apps (and I hope there won’t be any attempts to suggest otherwise), and another on rendering performance that had good advice for anyone building any kind of website. Most of the talks were about the building blocks of progressive web apps: HTTPS, Service Workers, push notifications, and all that jazz.

I was very pleased to see that there was a move away from the suggesting that single-page apps with the app-shell architecture model were the only way of building progressive web apps.

There were lots of great examples of progressively enhancing existing sites into progressive web apps. Jeff Posnick’s talk was a step-by-step walkthrough of doing exactly that. Reading through the agenda, I was really happy to see this message repeated again and again:

In this session we’ll take an online-only site and turn it into a fully network-resilient, offline-first installable progressive web app. We’ll also break out of the app shell and look at approaches that better-suit traditional server-driven sites.

Progressive Web Apps should work everywhere for every user. But what happens when the technology and API’s are not available for in your users browser? In this talk we will show you how you can think about and build sites that work everywhere.

Progressive Web Apps should load fast, work great offline, and progressively enhance to a better experience in modern browsers.

How do you put the “progressive” into your current web app?

You can (and should!) build for the latest and greatest browsers, but through a collection of fallbacks and progressive enhancements you can bring a lot tomorrow’s web to yesterday’s browsers.

I think this is a really smart move. It’s a lot easier to sell people on incremental changes than it is to convince them to rip everything out and start from scratch (another reason why I’m dubious about any association between web components and progressive web apps—but I’ll save that for another post).

The other angle that I really liked was the emphasis on emerging markets, not just wealthy westerners. Tal Oppenheimer’s talk Building for Billions was superb, and Alex kicked the whole thing off with some great facts and figures on mobile usage.

In my mind, these two threads are very much related. Progressive enhancement allows us to have our progressive web app cake and eat it too: we can make websites that can be accessed on devices with limited storage and slow networks, while at the same time ensuring those same sites take advantage of all the newest features in the latest and greatest browsers. I talked to a lot of Google devs about ways to measure the quality of a progressive web app, and I’m coming to the conclusion that a truly high-quality site is one that can still be accessed by a proxy browser like Opera Mini, while providing a turbo-charged experience in the latest version of Chrome. If you think that sounds naive or unrealistic, then I think you might want to dive deeper into all the technologies that make progressive web apps so powerful—responsive design, Service Workers, a manifest file, HTTPS, push notifications; all of those features can and should be used in a layered fashion.

Speaking of Opera, Andreas kind of stole the show, demoing the latest interface experiments in Opera Mobile.

That ambient badging that Alex was talking about? Opera is doing it. The importance of being able to access URLs that I’ve been ranting about? Opera is doing it.

Then we had the idea to somehow connect it to the “pull-to-refresh” spinner, as a secondary gesture to the left or right.

Nice! I’m looking forward to seeing what other browsers come up with it. It’s genuinely exciting to see all these different browser makers in complete agreement on which standards they want to support, while at the same time differentiating their products by competing on user experience. Microsoft recently announced that progressive web apps will be indexed in their app store just like native apps—a really interesting move.

The Progressive Web App Dev Summit wrapped up with a closing panel, that I had the honour of hosting. I thought it was very brave of Paul to ask me to host this, considering my strident criticism of Google’s missteps.

Initially there were going to be six people on the panel. Then it became eight. Then I blinked and it suddenly became twelve. Less of a panel, more of a jury. Half the panelists were from Google and the other half were from Opera, Microsoft, Mozilla, and Samsung. Some of those representatives were a bit too media-trained for my liking: Ali from Microsoft tried to just give a spiel, and Alex Komoroske from Google wouldn’t give me a straight answer about whether he wants Android Instant apps to succeed—Jake was a bit more honest. I should have channelled my inner Paxman a bit more.

Needless to say, nobody from Apple was at the event. No surprise there. They’ve already promised to come to the next event. There won’t be an Apple representative on stage, obviously—that would be asking too much, wouldn’t it? But at least it looks like they’re finally making an effort to engage with the wider developer community.

All in all, the Progressive Web App Dev Summit was good fun. I found the event quite inspiring, although the sausage festiness of the attendees was depressing. It would be good if the marketing for these events reached a wider audience—I met a lot of developers who only found out about it a week or two before the event.

I really hope that people will come away with the message that they can get started with progressive web apps right now without having to re-architect their whole site. Right now the barrier to entry is having your site running on HTTPS. Once you’ve got that up and running, it’s pretty much a no-brainer to add a manifest file and a basic Service Worker—to boost performance if nothing else. From there, you’re in a great position to incrementally add more and more features—an offline-first approach with your Service Worker, perhaps? Or maybe start dabbling in push notifications. The great thing about all of these technologies (with the glaring exception of web components in their current state) is that you don’t need to bet the farm on any of them. Try them out. Use them as enhancements. You’ve literally got nothing to lose …and your users have everything to gain.

The web on my phone

It’s funny how times have changed. Remember back in the 90s when Microsoft—quite rightly—lost an anti-trust case? They were accused of abusing their monopolistic position in the OS world to get an unfair advantage in the browser world. By bundling a copy of Internet Explorer with every copy of Windows, they were able to crush the competition from Netscape.

Mind you, it was still possible to install a Netscape browser on a Windows machine. Could you imagine if Microsoft had tried to make that impossible? There would’ve been hell to pay! They wouldn’t have had a legal leg to stand on.

Yet here we are two decades later and that’s exactly what an Operating System vendor is doing. The Operating System is iOS. It’s impossible to install a non-Apple browser onto an Apple mobile computer. For some reason, the fact that it’s a mobile device (iPhone, iPad) makes it different from a desktop-bound device running OS X. Very odd considering they’re all computers.

“But”, I hear you say, “What about Chrome for iOS? Firefox for iOS? Opera for iOS?”

Chrome for iOS is not Chrome. Firefox for iOS is not Firefox. Opera for iOS is not Opera. They are all using WebKit. They’re effectively the same as Mobile Safari, just with different skins.

But there won’t be any anti-trust case here.

I think it’s a real shame. Partly, I think it’s a shame because as a developer, I see an Operating System being let down by its browser. But mostly, I think it’s a shame because I use an iPhone and I’m being let down by its browser.

It’s kind of ironic, because when the iPhone first launched, it was all about the web apps. Remember, there was no App Store for the first year of the iPhone’s life. If you wanted to build an app, you had to use web technologies. Apple were ahead of their time. Alas, the web technologies weren’t quite up to the task back in 2007. These days, though, there are web technologies landing in browsers that are truly game-changing.

In case you hadn’t noticed, I’m very excited about Service Workers. It’s doubly exciting to see the efforts the Chrome on Android team are making to make the web a first-class citizen. As Remy put it:

If I add this app to my home screen, it will work when I open it.

I’d like to be able to use Chrome, Firefox, or Opera on my iPhone—real Chrome, real Firefox, or real Opera; not a skinned version of Safari. Right now the only way for me to switch browsers is to switch phones. Switching phones is a pain in the ass, but I’m genuinely considering it.

Whereas I’m all talk, Henrik has taken action. Like me, he doesn’t actually care about the Operating System. He cares about the browser:

Android itself bores me, honestly. There’s nothing all that terribly new or exciting here.’

save one very important detail…

IT’S CURRENTLY THE BEST MOBILE WEB APP PLATFORM

That’s true for now. The pole position for which browser is “best” is bound to change over time. The point is that locking me into one particular browser on my phone doesn’t sit right with me. It’s not very …webby.

I’m sure that Apple are not quaking in their boots at the thought of myself or Henrik switching phones. We are minuscule canaries in a very niche mine.

But what should give Apple pause for thought is the user experience they can offer for using the web. If they gain a reputation for providing a sub-par web experience compared to the competition, then maybe they’ll have to make the web a first-class citizen.

If I want to work towards that, switching phones probably won’t help. But what might help is following Alex’s advice in his answer to the question “What do we do about Safari?”:

What we do about Safari is we make websites amazing …and then they can’t not implement.

I’ll be doing that here on adactio, over on The Session (and Huffduffer when I get around to overhauling it), making progressively enhanced, accessible, offline-first, performant websites.

I’ll also be doing it at Clearleft. If you work at an organisation that wants a progressively-enhanced, accessible, offline-first, performant website, we should talk.

Service Worker notes

Here’s a disconnected hodge-podge of things related to Service Workers I’ve noticed recently…

Service Workers have landed in Firefox. When it was first unveiled in a nightly build a few people spotted some weird character issues on sites like mine that are using Service Workers, but that should all be fixed in the next release.

A while back I voted up Service Workers on Microsoft’s roadmap for Edge. I got an email today to say that the roadmap priority is high:

We intend to begin development soon.

We’re getting there.

Here’s a little gotcha that had me tearing my hair out until I tracked down the culprit: don’t use Header append Vary User-Agent in your site’s Apache config file. I don’t know why it snuck in there in the first place, but once I removed it, it fixed a weird issue that Aaron T. Grogg pointed out to me whereby my offline page would get cached, but not my CSS.

I really like this proposal for:

<link rel="serviceworker" href="/serviceworker.js">

It makes sense to me that I should be able to point to the Service Worker of a page in the same way that I point to a style sheet. It makes sense as a rel value too: “the linked resource has the relationship of ‘serviceworker’ to the current document.”

Also, I’m just generally a fan of declarative solutions. This feels like another good example of functionality that starts life in an imperative language (JavaScript) and then becomes declarative over time (see also: :hover, the required attribute, etc.).

Lyza wrote a fantastic article on Smashing Magazine that goes into all the details of her Service Worker. I must admit to feeling extremely gratified when she wrote:

First, I’m hugely indebted to Jeremy Keith for the implementation of service workers on his own website, which served as the starting point for my own code.

Going through her code, she made this remark:

Note: I use certain ECMAScript6 (or ES2015) features in the sample code for service workers because browsers that support service workers also support these features.

That’s a really good point. I haven’t messed around much with ES6 HipsterScript stuff partly because I haven’t yet set up a transpiler, so it was nice to know that my Service Worker is a “safe space” to try some stuff out in the browser. I refactored my JavaScript to use const, let, and =>.

Jake is looking for feedback on a specific part of Service Worker functionality around URLs. If I can wrap my head around what’s being described, I’ll chime in.

Finally, I had a nice little Service Worker moment earlier today. I was doing some updates on my web server that required a reboot. When I checked in Chrome to see how long adactio.com was down, I was surprised to see that the downtime appeared to be …zero. “That’s odd” I thought, “How can my site still be reachable if the server is …oh!” That’s when I realised I was seeing a cached version of my homepage. My Service Worker was doing it’s thing.

I had been thinking of Service Workers as a tool to help in situations where the user agent goes offline. But of course it’s an equally useful tool for when the server goes offline. This was a nice reminder of that.

Ice cold in Copenhagen

I went to Copenhagen last week for the Coldfront conference. It was lovely to be back in Denmark’s capital. I used to go over there ever year when the Reboot conference was running, but that wrapped up a few year’s back so it’s been quite a while since I had the opportunity to savour Copenhagen’s architecture, culture, coffee, food, and beer.

Coldfront was fun. Kenneth has modelled the format of the event on Remy’s Full Frontal conference—one day of a single track of front-end dev talks in a comfy cinema.

Going to a focused conference like this is a great way of getting a short sharp shock of what’s hot—like a State of the Union address for the web. At Coldfront there were some very clear themes around building for resilience, and specifically routing around the damage of inconsistent connectivity. There was a very clear message—from Paul, Alex, and Patrick (blog imminent)—that the network is not always on our side. Making our sites work offline should be much more of a priority than it currently is.

On a related note, the technology that was mentioned the most was Service Workers …and Jake wasn’t even there! Heck, even I mentioned it in glowing terms in my own little presentation. I was admiring the way it has been designed specifically to be used in a progressive enhancement kind of way.

So if I were Mr. McGuire in The Graduate, my line to a web developer equivalent of Dustin Hoffman would be “I want to say one word to you, just one word. Are you listening? …Service Workers.”