Progressive Web Apps in Chrome

Progressive Web Apps (PWA) are HTML based applications that run as though they are desktop applications.  Google Chrome received support for PWAs on Chrome OS in May with the release of Chrome 67.  Linux and Windows received support in August with the release of Chrome 70.  Support for Mac OS X is yet to come.

Download code (415 KB)

siderealLarge

One of the first differences that stands out for PWAs is that they can run in their own application window and are indistinguishable from other applications running on a machine. That difference is largely visual. But the differences extend well beyond what is visible. Resources that are not usually available to an HTML page are available to a PWA such as access to Bluetooth, serial ports, UDP networking, and more.  Chrome PWAs can be installed and have their own icon in your programs menu and function offline.

There are requirements that must be satisfied before an HTML page can be installed as a PWA.  These are the conditions that must be met.

  • The page must be served over SSL/HTTPS.
  • The page must have a service worker with a fetch handler.
  • User engagement requirements must be met (interaction with the domain for at least 30 seconds).
  • A manifest must be present.
    • 192px and 512px icons must be included.
    • Application must have a short name and long name.
    • The display mode must be specified.
    • start_url must be specified.

 

If all of these requirements are met Chrome will trigger a beforeinstallprompt event for the web page. Once this event is triggered your application can present the user with an install prompt.  Depending on the Chrome version your application may be able to suppress this prompt and display it to the user later (allowing you to decide where in the interaction flow that the prompt shows up) or your app might not be allowed to suppress it.

I’ll make a minimilastic application that satisfies the requirements for being a PWA.  The application that I’ll make will calculate sidereal time. Sidereal time is a time tracking system used by astronomers and is always expressed in 24 hour format. The usual system of tracking time was formed around trying to map the time of the day to the position of the sun (solar time, though it is far less than perfect). Sidereal time is based on the position of the stars relative to the observer. I will not talk much about the algorithm behind this calculation much here. I talked about calculating sidereal time in an application I had made for the now defunct Windows Phone 7; while that OS is no more the description I gave on how sidereal time works is still applicable.

Using SVG I’ve made a simple 24 hour clock face. The clock face is really there for aesthetics. Chances are if you try to read the hands of the clock the hour hand will cause confusion since it’s position on a 24 hour clock will not meet expectations that have been formed from being able to read a 12 hour clock.  The digital readout is the part that will actually give the information of interest. Every second the time is updated and the hands animate to their new position. There’s also a gear icon for opening the settings interface.

sampleApp

Satisfying the SSL/HTTP Requirement

A lot of the necessary features are only available if your application is being served over SSL. If you don’t see HTTPS in the address bar then these features simply will not work. To satisfy this requirement for now I’m using Google Firebase and the temporary URL that it has assigned to me. I don’t plan on keeping this URL forever, but at the time of this post you can play with the application over at https://siderealtimepiece.firebaseapp.com.

Satisfying Manifest Resources Requirements

The manifest for my application is in the root directory of the application. It is a JSON formatted file with information on where the program icons can be found, the starting URL, and the name of the application as it should appear on the user’s machine.

{
    "short_name": "Sidereal",
    "name": "Sidereal Time Piece",
    "icons": [
      {
        "src": "./images/sidereal192.png",
        "type": "image/png",
        "sizes": "192x192"
      },
      {
        "src": "./images/sidereal512.png",
        "type": "image/png",
        "sizes": "512x512"
      }
    ],
    "start_url": "index.html?pwa=true",
    "background_color": "#000080",
    "display": "standalone",
    "scope": "./",
    "theme_color": "#FFFFFF"
  }
  

The Service Worker

To satisfy the service worker requirement there’s a JavaScript file in the root of this application’s files named sw.js. The service worker works in the background behind the page. For this application we only want the service worker to do two things; respond to an install event by caching the required files locally and serve up those files when needed. The list of the files that are to be cached are in an array named urlsToCache. When the service worker response to the install event it will pass this list of URLs to a call of the addAll method on the cache object. The cache object will then download the resources at these URLs and save them locally where we can use them offline.

var CACHE_NAME = 'siderealclock-cache';
var urlsToCache = [
  './',
  './styles/main.css',
  './scripts/app.js',
  './scripts/jquery-3.3.1.min.js',
  './images/sidereal192.png',
  './images/sidereal512.png',
  './images/siderealLarge.png',
  './404.html'
];

self.addEventListener('install', function(event) {
  // Perform install steps
  event.waitUntil(
    caches.open(CACHE_NAME)
      .then(function(cache) {
        console.log('Opened cache');
        return cache.addAll(urlsToCache);
      })
  );
});

For the fetch event I’m using code from a Google recommendation. This handler will serve the contents from the cache when there is a cache hit and also add new files to the cache when a request is made for a file that isn’t already there.

self.addEventListener('fetch', function(event) {
    event.respondWith(
      caches.match(event.request)
        .then(function(response) {
          // Cache hit - return response
          if (response) {
            return response;
          }
  
          // IMPORTANT: Clone the request. A request is a stream and
          // can only be consumed once. Since we are consuming this
          // once by cache and once by the browser for fetch, we need
          // to clone the response.
          var fetchRequest = event.request.clone();
  
          return fetch(fetchRequest).then(
            function(response) {
              // Check if we received a valid response
              if(!response || response.status !== 200 || response.type !== 'basic') {
                return response;
              }
  
              // IMPORTANT: Clone the response. A response is a stream
              // and because we want the browser to consume the response
              // as well as the cache consuming the response, we need
              // to clone it so we have two streams.
              var responseToCache = response.clone();
  
              caches.open(CACHE_NAME)
                .then(function(cache) {
                  cache.put(event.request, responseToCache);
                });
  
              return response;
            }
          );
        })
      );
  });
  

This file must be registered as the service worker for it to be able to do anything. In one of the JavaScript files loaded by the page I check the navigator object to ensure there is a serviceWorker member (if there isn’t then the browser in which the code is running currently doesn’t support service workers). If it is there then the service worker can be registered with navigator.serviceWorker.register(path_to_service_worker).

if('serviceWorker' in navigator) {
    navigator.serviceWorker
             .register('./sw.js')
             .then(function() { console.log("Service Worker Registered"); });
  }

Handling the Install Prompt

If your code is running on a Chrome implementation that supports it you can defer the presentation of the installation prompt. In my case I’ve decided to defer it and make a button available in the settings UI. The variable installPrompt will hold the reference to the event object that when activated will present the user with the Chrome install UI. When the event is raised the variable is populated with the event object and the install button within my settings UI is made visible.

var installPrompt;


function beforeInstall(e) { 
    console.log('beforeInstallPrompt()')
    e.preventDefault();
    installPrompt = e;
    $('.installUI').show();
}


window.addEventListener('beforeinstallprompt', beforeInstall);
    $('.installButton').on('click', function(){
        installPrompt.prompt();
        installPrompt.userChoice
          .then((choiceResult) => {
            $('.installUI').hide();
            installPrompt = null;
          });
      
    });

Testing the application on Chrome on Ubuntu Linux when I select my install button the Chrome install prompt shows.

Chrome Desktop Install Prompt
The Install Prompt that shows on Google Chrome on a desktop

Program Launchers on the Desktop

On the desktop once installed the icon for the PWA shows up in the computer’s program launcher.  It also shows up in the Chrome app list. When launched since this application was made to run in standalone mode the application runs in it’s own window with the OS appropriate buttons for going full screen, minimizing, and closing the window. My test application uses location services to acquire the longitude at which the sidereal time is being calculated. When run in a regular browser window I’m prompted each time I visit the page to give permission for location information. This gets a little annoying after a while. When the application is running in stand alone mode the application’s border shows an icon indicating that the location is being detected. Clicking on the icon gives the user the ability to change the location permissions for the application.

Samsung Internet Compatibility

Samsung Internet, the default browser for a long period on many Samsung phones, also supports PWAs. (Samsung Internet can also be installed on non-Samsung phones). Samsung Internet is a Chromium based browser and Samsung is one of the contributors to the Chromium project.  It may come as no surprise that no code changes are necessary for this application to work on  The UI it presents for installing PWAs is different than what Chrome presents. When Samsung Internet detects that a page can be installed as a PWA an icon is shown in the address bar that resembles a house with a plus in the center. Selecting it will add the icon to the home screen. The icon shows with a smaller image of the Samsung Internet icon indicating that it is a PWA.  The beforeinstallprompt event will never be triggered. Since the presentation of the custom install button was driven by this event it simply will not show.

SamsungPWACentered

Adding iOS Compatibility

If you saw the original iPhone announcement back in 2007 Steve Jobs had announced that making apps for the iPhone could be done with HTML; at the time there was no SDK available to developers and if they wanted to target the iPhone they were making a web app that had an icon on the home screen. From 2007 to 2018 Apple didn’t do much to advance the platform. It wasn’t until March 2018 that Apple made significant updates to their support to HTML based applications. Apple added support for web manifest, and services workers, web assembly, and other features.

There’s not 100% parity between iOS and Android for available features in PWA. On iOS storage is limited to 50MB per app. On Android the application can request more storage. Android BWAs also have access to Bluetooth features, speech recognition, background sync, and other features. For my sample application none of these mentioned differences matter. While the Android implementations have UI notifications that let the user know that the app can be installed on iOS there’s no visual notification. To install the application the user must select the share option and add the page to their home screen.

Safari ignores most of the attributes of the manifest. It also doesn’t save state if the user leaves the application. So the developers must make their own implementation to save state as the user jumps in and out of the application. If you want a custom icon to show in Safari for your application Apple has a document on specifying the icon using the link tag. An icon can be specified like the following.

    <link rel="apple-touch-icon"  href="./images/icons/apple-icon-57x57.png">

If you want to specify multiple icons and allow the phone to select the most appropriate one for the user’s resolution add a sizes attribute to the tag.

   <link rel="apple-touch-icon" sizes="57x57" href="./images/icons/apple-icon-57x57.png">
    <link rel="apple-touch-icon" sizes="60x60" href="./images/icons/apple-icon-60x60.png">
    <link rel="apple-touch-icon" sizes="72x72" href="./images/icons/apple-icon-72x72.png">

My clock icon for the program shows up in the iPhone favourites list as the following.

FavouriteIcon

Offline Functionality

This application doesn’t need the internet for any functionality. It’s only inputs are the current local time and the user’s longitude. With the lack of need for any network resources and the service worker caching the required files for the application it will work just fine offline after it has been installed. If you make an application that requires network access you will want to give some thought to what to do when there is no data connection. Even if the application can’t do anything without a connection it would be better to show a friendly message than to just let the application not work.

An Alternative to the App Store

PWAs longtime might turn out to be a good alternative to app stores for some types of applications. Whether or not it is a good fit for the needs that you have will depend on the functionality that your applications require and what is available on the devices that you need to target. Apple appears to be behind on this front at the moment. But I hope that the attention that they’ve put on the platform this year to be indicative of future efforts. I’m personally am interested in what could be done when PWAs and WebAssembly are combined together. These are topics to which I hope to give a good bit of attention over the following months.

Advertisements

Future of Microsoft Edge

Faster Updates, Cross Platform, 4 Channels

Edge

 

Microsoft had made an announcement some time ago stating that they were adopting Chromium as the foundation of the Edge browser. They are making more information available about their plans. A computer will be able to have up to 4 Edge. In addition to the general release version there will be a beta , development, and canary channel where canary is the most experimental channel.

Microsoft is going to make a web view control available. This is a feature I wish had been available a couple of years ago. I worked on a project in which it was necessary to insert a Chromium based web view and that required making modifications to the Chromium source. Compiling Chromium can take hours!

Microsoft Edge is the only browser that has achieved a 100% score in accessibility evaluation. Microsoft is planning to make contributions to Chromium which should help improve the scores of Chromium based browsers across the board.  One example of a change coming from Microsoft is a media query for detecting when a user has turned on high contrast in either their computer or browser. The web developer can choose to change the appearance of a page accordingly.

Windows Biometric sign in is also going to be available to web sites (see navigator.credentials.get). This allows for a more secure way of logging in that doesn’t require a password. Edge also has a hardware backed feature named “Play Ready” that allows for secure playback of premium media.

The user agent-string for Edge Chromium will be Edg (that’s not a type, there is no E on the end). Note that the non-chromium version of edge uses the user token Edge instead. But it is recommended that instead of using the user agent string for enabling or disabling functionality in a web page it is better to use feature detection; new features are being added to browsers all the time and relying on the user agent alone can lead to a page not using a feature that was actually available within a browser.  Edge gets supports for module import and much better much better speech synthesis.

Basic Hue Lighting Control: Part 1

screenshot
Screenshot of Chrome application for controlling Hue lighting.

Continuing from the post I made on SSDP discovery with Chrome, I’m making an application that will do more than just discovery. For this post I’m going to show the starting point of a Chrome application for controlling your home Hue lighting. I’ve divided this into two parts. In this first part I’m showing the process of pairing with the bridge. In the second part I’ll control the lights.

The features that this application will implement will include bridge discovery and pairing; the power state of the light; and the brightness level of the light. There’s many other features that could still be implemented.  Given the full range of capabilities that the Hue kits support (changing color, timers, response to motion sensors, etc.) this will not be an application that utilizes the full capability of the Hue lighting sets.

Chrome Only

This application is designed to only run in Chrome. If you want to adapt it to run outside of Chrome, you can do so by first disabling SSDP discovery. (Other HTML application platforms might not support UDP for discovery.)

The other discovery methods (querying Hue’s discovery web service or asking the user to enter the IP address) can still work. A non-chrome target will also need to allow CORS to be ignored and allow communication without SSL.

What is Hue Lighting?

Hue Lighting is an automated lighting solution made by Philips. Generally the lighting kits are sold in a package that contains three LED based light bulbs and a bridge. The bridge is a device that connects to your home network with an Ethernet jack and communicates with the light bulbs.

Philips also makes free applications for iOS and Android for controlling the lights. For any Hue light the light’s brightness and whether or not it is turned on can be controlled through the applications. Some lights also allow the color temperature to be changed (adjusting the tint between red, yellow and blue). Some lights support RGB (Red, Green, Blue) parameters so that their colors can be changed.  These settings can be individually adjusted or settings for a collection of the lights can be defined together as a “scene.” When a scene is activated the state of all of the lights that make up the scene are updated. Scenes can be activated through special light switches, through an app, through a schedule, or in response to a Hue motion sensor detecting motion.

Discovery: Review and New Methods

The central piece of hardware for the Hue lighting is the Hue Bridge. At the time of this writing there are two versions of the bridge. For the functionality that this application will utilize, the differences between the two bridges will not matter. The messaging and interaction to both versions of the bridge will be the same. My UI will properly represent the bridge that the system discovers. The first version of the Hue Bridge is round. The second version of the Hue Bridge is square. In either case we must first find the bridge’s IP address before we can begin interaction.

phillipsbridge
Phillips Hue Bridge Version 1 (left) and Version 2 (right)

The Hue bridge can be discovered in multiple ways. It can be discovered using SSDP. The basics of SSDP discovery were previously discussed here. Please refer back to it if you need more detail than what is found in this brief overview.  Devices that support SSDP discovery join a multicast group on the network that they are connected to. These devices generally wait for a request for discovery to be received. An SSDP request is sent as an HTTP over UDP message and every SSDP device that receives it responds with some basic information about itself and a URL to where more information on the device can be found. Examples of some devices that support SSDP are network attached storage; set top boxes like Android TVs and Rokus; printers; and home automation kits.

Two other methods of discovering a bridge include asking the user to enter an IP address and asking for a list of IP addresses of bridges on your network through the Hue discovery service.  If you have a Hue bridge connected to your network right now you can see it’s IP address by visiting https://discovery.meethue.com/ . If you are on a shared network then you may also see IP addresses of other bridges on your network. It is also possible that not all bridges on your network are reachable.  This method is much easier to implement than SSDP based discovery. But on a network for which there is no Internet connection (whether by design or from an outage) this method will not work. The SSDP method is only dependent on the local network.

function discoverBridge() { 
    discovredHueBridgeList = [];
    fetch(' https://discovery.meethue.com')
        .then(response => response.json())
    .then(function (hueBridgeList) {
        console.info(hueBridgeList);
        hueBridgeList.forEach((item)=> {
         // each item processed here has a bridge IP address
         // and serial number exposed through item.id and 
         // item.internalipaddress
       }
     );
}

Once I have a bridge IP address I attempt to query it for more information. If communication succeeds, then I show a representation of the bridge with an icon that matches the version of the bridge that the user has. The UI layout has two images ( one named hueBridgev1 and the other hueBridgev2) I show the appropriate image and hide the other.

Pairing

Now that the bridges have been discovered, it is up to the user to select one with which to pair. After the user selects a bridge, she is instructed to press the pairing button on the bridge. While this instruction is displayed the application is repeatedly attempting to request a new user ID name from the bridge. This should be viewed more as an access token. The Hue documentation uses the term “user name” but the actual value is what appears to be a random sequence of characters. To request a user name a JSON payload with one member named devicetype is posted to the bridge. The value assigned to devicetype matters little. It is recommended that it be a string that is unique to your application. The payload is posted to http://%5Byour bridge IP address]/api. A failure response will result. This is expected. The application must repeatedly make this request and prompt the user to press the link button on the bridge.  The request will fail until the pairing button on the bridge is passed.

function pairBridge(ipAddress) {
   console.info('attempting pairing with address ', ipAddress);
   var req = { devicetype: "hue.j2i.net#browser" };
   var reqStr = JSON.stringify(req);
   var tryCount = 0;
   return new Promise(function(resolve, reject)  {
      var tryInterval = setInterval(function () {
      console.log('attempt ', tryCount);
      ++tryCount;
      if (tryCount > 60) {
        clearInterval(tryInterval);
         reject();
         return;
      }
      fetch(`http://${ipAddress}/api`, {
         method: "POST",
         headers: {
            "Content-Type": "application/json"
         },
         body: reqStr
      })
      .then(function(response)  {
         console.log('text:',response);
         return response.json();
      })
      .then(function(data)  {
          console.log(data);
          if (data.length > 0) {
             var success = data[0].success;
             var error = data[0].error;
             if (success) {
                console.log('username:', success.username);
                var bridge = {
                   ipAddress: ipAddress,
                   username: success.username
                };
                clearInterval(tryInterval);
                 resolve(bridge);
                  return;
               }
               else if (error) {
                  if (error.type === 101) {
                     console.log('the user has not pressed the link button');
                  }
               }
            }
         });
      }, 2000);
   });
}

Once the button is pressed the bridge will respond to the first pairing request it receives with a user name that the application can use. This user name must be saved and used for calls to most of the functionality that is present in the bridge. I save the bridge’s serial number, IP address, and the name that must be used for the various API calls to an indexedDB object store. The access information for multiple paired bridges could be stored in the object store at once. But the application will only be able to communicate with one bridge at a time.

Continued in Part II