Implementing Direct URL in loklak Media Wall

Direct URL is a web address which redirects the user to the preset customized media wall so that the media wall can directly be used to be displayed on the screen. Loklak media wall provides direct URL which has information related to customizations set by the user included in the web address. These customizations, as the query parameters are detected when the page is initialized and actions are dispatched to make changes in the state properties, and hence, the UI properties and the expected behaviour of media wall.

In this blog, I would be explaining how I implemented direct URL in loklak media wall and how customizations are detected to build on initialization of component, a customized media wall.

Flow Chart

Working

Media Wall Direct URL effect

This effect detects when the WALL_GENERATE_DIRECT_URL action is dispatched and creates a direct URL string from all the customization state properties and dispatches a side action WallShortenDirectUrlAction() and stores direct URL string as a state property. For this, we need to get individual wall customization state properties and create an object for it and supply it as a parameter to the generateDirectUrl() function. Direct URL string is returned from the function and now, the action is dispatched to store this string as a state property.

@Effect()
generateDirectUrl$: Observable<Action>
= this.actions$
.ofType(mediaWallDirectUrlAction.ActionTypes.WALL_GENERATE_DIRECT_URL)
.withLatestFrom(this.store$)
.map(([action, state]) => {
return {
query: state.mediaWallQuery.query,
.
.
.
wallBackground: state.mediaWallCustom.wallBackground
};
})
.map(queryObject => {
const configSet = {
queryString: queryObject.query.displayString,
.
.
.
wallBackgroundColor: queryObject.wallBackground.backgroundColor
}
const shortenedUrl = generateDirectUrl(configSet);
return new mediaWallDirectUrlAction.WallShortenDirectUrlAction(shortenedUrl);
});

Generate Direct URL function

This function generates Direct URL string from all the current customization options value. Now,  keys of the object are separated out and for each element of the object, it checks if there is some current value for the elements and it then first parses the value of the element into URI format and then, adds it to the direct URL string. In such a way, we are creating a direct URL string with these customizations provided as the query parameters.

export function generateDirectUrl(customization: any): string {
const shortenedUrl = ;const activeFilterArray: string[] = new Array<string>();
let qs = ;
Object.keys(customization).forEach(config => {
if (customization[config] !== undefined && customization[config] !== null) {
if (config !== ‘blockedUser’ && config !== ‘hiddenFeedId’) {
qs += `${config}=${encodeURIComponent(customization[config])}&`;
}
else {
if (customization[config].length > 0) {
qs += `${config}= ${encodeURIComponent(customization[config].join(‘,’))}&`;
}
}
}
});
qs += `ref=share`;
return qs;
}

Creating a customized media wall

Whenever the user searches for the URL link on the web, a customized media wall must be created on initialization. The media wall component detects and subscribes to the URL query parameters using the queryParams API of the ActivatedRoute. Now, the values are parsed to a required format of payload and the respective actions are dispatched according to the value of the parameters. Now, when all the actions are dispatched, state properties changes accordingly. This creates a unidirectional flow of the state properties from the URL parameters to the template. Now, the state properties that are supplied to the template are detected and a customized media wall is created.

private queryFromURL(): void {
this.__subscriptions__.push(
this.route.queryParams
.subscribe((params: Params) => {
const config = {
queryString: params[‘queryString’] || ,
imageFilter: params[‘imageFilter’] || ,
profanityCheck: params[‘profanityCheck’] || ,
removeDuplicate: params[‘removeDuplicate’] || ,
wallHeaderBackgroundColor: params[‘wallHeaderBackgroundColor’] || ,
wallCardBackgroundColor: params[‘wallCardBackgroundColor’] || ,
wallBackgroundColor: params[‘wallBackgroundColor’] ||
}
this.setConfig(config);
})
);
}public setConfig(configSet: any) {
if (configSet[‘displayHeader’]) {
const isTrueSet = (configSet[‘displayHeader’] === ‘true’);
this.store.dispatch(new mediaWallDesignAction.WallDisplayHeaderAction(isTrueSet));
}
.
.
if (configSet[‘queryString’] || configSet[‘imageFilter’] || configSet[‘location’]) {
if (configSet[‘location’] === ‘null’) {
configSet[‘location’] = null;
}
const isTrueSet = (configSet[‘imageFilter’] === ‘true’);
const query = {
displayString: configSet[‘queryString’],
queryString: ,
routerString: configSet[‘queryString’],
filter: {
video: false,
image: isTrueSet
},
location: configSet[‘location’],
timeBound: {
since: null,
until: null
},
from: false
}
this.store.dispatch(new mediaWallAction.WallQueryChangeAction(query));
}
}

Now, the state properties are rendered accordingly and a customized media wall is created. This saves a lot of effort by the user to change the customization options whenever uses the loklak media wall.

Reference

Generating Ticket PDFs in Open Event API Server

In the ordering system of Open Event API Server, there is a requirement to send email notifications to the attendees. These attendees receive the URL of the pdf of the generated ticket. On creating the order, first the pdfs are generated and stored in the preferred storage location and then these are sent to the users through the email.

Generating PDF is a simple process, using xhtml2pdf we can generate PDFs from the html. The generated pdf is then passed to storage helpers to store it in the desired location and pdf-url is updated in the attendees record.

Sample PDF

PDF Template

The templates are written in HTML which is then converted using the module xhtml2pdf.
To store the templates a new directory was created at  app/templates where all HTML files are stored. Now, The template directory needs to be updated at flask initializing app so that template engine can pick the templates from there. So in app/__init__.py we updated flask initialization with

template_dir = os.path.dirname(__file__) + "/templates"

app = Flask(__name__, static_folder=static_dir, template_folder=template_dir)

This allows the template engine to pick the templates files from this template directory.

Generating PDFs

Generating PDF is done by rendering the html template first. This html content is then parsed into the pdf

file = open(dest, "wb")

pisa.CreatePDF(cStringIO.StringIO(pdf_data.encode('utf-8')), file)

file.close()

The generated pdf is stored in the temporary location and then passed to storage helper to upload it.

uploaded_file = UploadedFile(dest, filename)

upload_path = UPLOAD_PATHS['pdf']['ticket_attendee'].format(identifier=get_file_name())

new_file = upload(uploaded_file, upload_path)

This generated pdf path is returned here

Rendering HTML and storing PDF

for holder in order.ticket_holders:

  if holder.id != current_user.id:

      pdf = create_save_pdf(render_template('/pdf/ticket_attendee.html', order=order, holder=holder))

  else:

      pdf = create_save_pdf(render_template('/pdf/ticket_purchaser.html', order=order))

  holder.pdf_url = pdf

  save_to_db(holder)

The html is rendered using flask template engine and passed to create_save_pdf and link is updated on the attendee record.

Sending PDF on email

These pdfs are sent as a link to the email after creating the order. Thus a ticket is sent to each attendee and a summarized order details with attendees to the purchased.

send_email(

  to=holder.email,

  action=TICKET_PURCHASED_ATTENDEE,

  subject=MAILS[TICKET_PURCHASED_ATTENDEE]['subject'].format(

      event_name=order.event.name,

      invoice_id=order.invoice_number

  ),

  html= MAILS[TICKET_PURCHASED_ATTENDEE]['message'].format(

      pdf_url=holder.pdf_url,

      event_name=order.event.name

  )

)

References

  1. Readme – xhtml2pdf
    https://github.com/xhtml2pdf/xhtml2pdf/blob/master/README.rst
  2. Using xhtml2pdf and create pdfs
    https://micropyramid.com/blog/generating-pdf-files-in-python-using-xhtml2pdf/

 

Creating an Infobox for Mobile View Using Angular in Susper

In Susper, the Information and Analytics boxes disappeared for widths smaller than 1100px, since they were too big to fit in the existing page layout.
In Laptop view:

In Mobile view:

Hence we decided to design a new Info-cum-Analytics box for mobile devices, where the Analytics are displayed only if the ‘Show Analytics’ button is clicked and they are hidden on clicking the ‘Hide Analytics’ button.

The following is the html code for the Infobox:

<div class=“combo-box”>
<appinfobox></app-infobox>
<button class=“btn” id=“toggle-button” (click)=“BoxToggle()” type=“button” datatoggle=“collapse” datatarget=“#statbox” ariaexpanded=“false” ariacontrols=“collapseExample”>
{{boxMessage}} Analytics
</button>
<appstatsbox class=“collapse” id=“statbox”></app-statsbox>
</div>

We can make the following observations:

  • The combo box has both the Info and the Stats box, in addition to a button, which toggles the display status of the Analytics box.
  • To display the appropriate message on the button, we have a special function called BoxToggle() which sets the value of boxMessage()
  • The data-toggle attribute of the button has been set to collapse and the data-target is statbox
  • The statsbox has the class collapse, so that it collapses on clicking on the Toggle button.

The typescript code is as follows:

BoxToggle() {
if (this.boxMessage === ‘Show’) {
this.boxMessage = ‘Hide’;
} else {
this.boxMessage = ‘Show’;
}
}

This code thus effectively toggles between the messages Show and Hide, if the button is clicked when the message says Show, it changes to Hide and vice-versa

The CSS code:

@media screen and (min-width:768px) {
div.combo-box {
display: none;
}
}

The css code ensures that this combo-box is displayed only for widths less than 768px (only mobile and tablet screens).

To view the entire code you can check results.component.html and results.component.css in the Susper repository.

References

  1. W3 Schools for Collapsible buttons and their implementation: https://www.w3schools.com/bootstrap/bootstrap_collapse.asp
  2. Official Bootstrap documentation for collapsible buttons: https://v4-alpha.getbootstrap.com/components/collapse/

How to Parse HTML Tags and Anchor Clickable Links in SUSI Android App

Html tags are used to define how contents of a webpage should be formatted and displayed. Sometimes the SUSI answer type response contains some html tags but showing these html tags without parsing would distort the normal text flow in SUSI Android.

For the query ‘Ask me something’ SUSI’s reply is

“data”: [
     {
      “question”: “Which soccer team won the Copa Am&eacute;rica 2015 Championship ? “,                                          
     }]

In SUSI Android this message looks like

As you can see that showing html tags without parsing distort the normal text flow. So we need to parse html tags properly. We use Html class for this purpose. Html class is present in android.text package and you can import it in the class where you want to use it.

import android.text.Html

fromHtml method of Html class is used to parse html tags. But for API level less than 24 and equal to or greater than 24 we use different parameters in fromHtml method.

For API level less than 24 we used

Html.fromHtml(model.getContent())

But for API level equal to or greater than 24 we have to use

Html.fromHtml(model.getContent(), Html.FROM_HTML_MODE_COMPACT)

Here the second parameter is legacy flags which decides how text inside a tag will be shown after parsing.

In case of Html.fromHtml(String string) legacy flag is by default FROM_HTML_MODE_LEGACY. It indicates that separate block-level elements with blank lines.

So after parsing html tags using fromHtml

But return type of fromHtml method is Spanned so if you need String then you have to convert it into string using toString() method.

Anchor action type in susi response contains link and text.

       “link”: “https://www.openstreetmap.org/#map=13/1.2896698812440377/103.85006683126556”,
       “text”: “Link to Openstreetmap: Singapore”

Here the text is the text we show in textview and link is used to show web content in the browser when user click on text. So first link and text are attached together like

“<a href=\”” +susiResponse.getAnswers().get(0).getActions().get(i).getAnchorLink() + “\”>”
+ susiResponse.getAnswers().get(0).getActions().get(1).getAnchorText() + “</a>”

Here text between the tag is description of link and after parsing we show this text in textview. It can be parsed using fromHtml method of Html class and textview is made clickable by

chatTextView.setMovementMethod(LinkMovementMethod.getInstance());

Resources

Implementing Advanced Search Feature In Susper

Susper has been provided ‘Advanced Search’ feature which provides the user a great experience to search for desired results. Advanced search has been implemented in such a way it shows top authors, top providers, and distribution regarding protocols. Users can choose any of these options to get best results.

We receive data of each facet name from Yacy using yacy search endpoint. More about yacy search endpoint can be found here:  http://yacy.searchlab.eu/solr/select?query=india&fl=last_modified&start=0&rows=15&facet=true&facet.mincount=1&facet.field=host_s&facet.field=url_protocol_s&facet.field=author_sxt&facet.field=collection_sxt&wt=yjson

For implementing this feature, we created Actions and Reducers using concepts of Redux. The implemented actions can be found here: https://github.com/fossasia/susper.com/blob/master/src/app/actions/search.ts

Actions have been implemented because these actually represent some kind of event. For e.g. like the beginning of an API call here.

We also have created an interface for search action which can be found here under reducers as filename index.ts: https://github.com/fossasia/susper.com/blob/master/src/app/reducers/index.ts

Reducers are a pure type of function that takes the previous state and an action and returns the next state. We have used Redux to implement actions and reducers for the advanced search.

For advanced search, the reducer file can be found here: https://github.com/fossasia/susper.com/blob/master/src/app/reducers/search.ts

The main logic has been implemented under advancedsearch.component.ts:

export class AdvancedsearchComponent implements OnInit {
  querylook = {}; // array of urls
  navigation$: Observable<any>;
  selectedelements: Array<any> = []; // selected urls by user
changeurl
(modifier, element) {
// based on query urls are fetched
// if an url is selected by user, it is decoded
  this.querylook[‘query’] = this.querylook[‘query’] + ‘+’ + decodeURIComponent(modifier);
  this.selectedelements.push(element);
// according to selected urls
// results are loaded from yacy
  this.route.navigate([‘/search’], {queryParams: this.querylook});
}

// same method is implemented for removing an url
removeurl(modifier) {
  this.querylook[‘query’] = this.querylook[‘query’].replace(‘+’ + decodeURIComponent(modifier), );

  this.route.navigate([‘/search’], {queryParams: this.querylook});
}

 

The changeurl() function replaces the query with a query and selected URL and searches for the results only from the URL provider. The removeurl() function removes URL from the query and works as a normal search, searching for the results from all providers.

The source code for the implementation of advanced search feature can be found here: https://github.com/fossasia/susper.com/tree/master/src/app/advancedsearch

Resources

Writing Browser Specific CSS for Susper in Angular

In Susper, we were facing a unique problem for Information box and Analytics box alignment.
At a width of around 1290 px, the boxes fit perfectly in Firefox as shown:

However, they were slipping to the next line in Google Chrome browsers for the same dimension(1290 px)

The solution to this issue was to write browser-specific CSS.
The two most commonly used browser specific tags are

  1. @-moz-document url-prefix() { }: This tag is used to target the Mozilla Firefox browser in particular. Anything written within the curly braces will not apply to any other browser.
  2. @media screen and (-webkit-min-device-pixel-ratio:0) { }: This tag is used to target all browsers that support webkit such as Chrome, Safari etc.

For our problem, we need to use @media screen and (-webkit-min-device-pixel-ratio:0) { }
This was how the code was written for both the components (Information box and Analytics box). Please refer to infobox.component.css and statsbox.component.css for the entire code.

@media screen and (-webkit-min-device-pixel-ratio:0) {
@media screen and (max-width: 1300px) {
.card {
width: 366px;
}
}
}
@media screen and (max-width: 1280px) {
.card {
width: 366px;
}

As a result of this snippet of code, we see the following effects:

  • In Chrome, the boxes change to a smaller width at 1300px itself, thus preventing it from slipping to the next line
  • In Firefox, the boxes change to a smaller width only at 1280px, and not at 1300px, thus achieving the exact design we envisioned.

This is how the display finally looks in Chrome:

References:

  1. Stack overflow on specific CSS tags for Chrome: https://stackoverflow.com/questions/9328832/how-to-apply-specific-css-rules-to-chrome-only
  2. Stack overflow on specific CSS tags for Firefox: https://stackoverflow.com/questions/952861/targeting-only-firefox-with-css

Making a Sticky Top Navigation bar for Susper using Angular

A lot of websites, require a top navigation bar that sticks to the top, irrespective of the screen dimension size. This blog deals with how the top navigation bar was made sticky in Susper.

  1. Using the correct Bootstrap classes. Notice the code enveloping the navigation bar.

<nav class=“top-nav navbar navbar-static-top navbar-default”>

class=“container-fluid”>

class=“navbar-header” id=“navcontainer”>

</div>
</div>
</nav>

Points to note:

  • Using navbar and navbar-default creates a standard gray navigation bar.
  • Using navbar-static-top makes the navbar stick only to the top of the page and disappear on scrolling down.
  • Using container-fluid creates a container for the contents of the navbar with wide margins
  1.  Now we also need to write some personalized CSS code. Notice the classes navcontainer and  top-nav. This is the CSS code for these classes:

.top-nav{
margin-bottom: 0;
}
#navcontainer {
height: 65px;
width: 100vw;
}#navcontainer ul {
margin: 0;
padding: 0;
list-style-type: none;
}

Points to note:

  • Margin and padding can be set according to how the navbar should look. Click here to know the difference between margins and padding.
  • The height has been customized to 65px in Susper, with a width of 100vw(entire viewpost width).
  1. Lastly, if your navigation bar is inside the body tag, remember that by default, body has a top margin of 57 px. As a result you may see an extra white space on top of your navigation bar. To remove this:
  • Move the navigation bar code out of the body tag. If you can’t then,
  • Place your navigation bar in a container ( resultContainer on the Susper result page) and write this in your CSS file.

.resultContainer{
margin-top: -57px;
}

References:

Making Customized and Mobile Responsive Drop-down Menus in Susper using Angular

In  Susper, the drop-down menu is customized with colorful search icons and we wanted to maintain the same menu for mobile screens too, however the drop-down menu disappeared for all screens with width less than 767px. This blog can be used to learn how to create css classes for such drop-down menus without using any bootstrap.
This is how the issue was solved.

  1. Replacing standard bootstrap classes : The drop-down menu blocks had a source code as follows:

class=“dropdown-menu”>

class=“row”>

class=“col-sm-4”>

class=“block”>

</div>
</div>

Using col-sm-4 will do the following

  • For widths greater than 767px: Divide each row into four equally sized columns.
  • For widths smaller than 767px: Stack all the columns on top of each other.

Since the drop-down menu’s design was to remain intact, I made the following changes:

  • Replace row with menu-row
  • Replace col-sm-4 with menu-item

Now I wrote personalized css for these classes.

.menu-row{
width: 267px;
gridtemplatecolumns: 1fr 1fr 1fr;
background-color: white;
}
.menu-item{
display: inlineblock;
width: 86px;
}
  • Width: It is used to set the width of the div class, each row now has a width of 267px, with each column in it having a width of 86px.
  • Grid-template-columns: It is used to layout the structure of the template, here 1fr 1fr 1fr represents that there will be three columns in a row.
  • Display: The display is set to inline block to overwrite the default property of the div element to start in a new line.
  1. Custom css for small screens : In standard bootstrap, for screen sizes less than 767px, dropdown class has properties like transparent background, no border etc. that need to be over written. So we add a new id for the div tag as shown:

<div id=“small-drop” class=“dropdown-menu”>

/** Now we add css for it, as shown: **/
@media screen and (max-width: 767px) {
#small-drop{
position: absolute;
background-color: white;
border: 1px solid #cccccc;
right: -38px;
left: auto;
}

  • Position : absolute is used to make sure all our values are absolute and not relative to the higher div hierarchically
  • Border: The values for the border represent the following respectively: Thickness, Style and Color.
  • Auto: Here the value auto for left signifies that there is no fixed value for the left margin, it can take the default value

References:

  1. For working of grids in Bootstrap: https://www.w3schools.com/bootstrap/bootstrap_grid_examples.asp
  2. A useful article for difference between id and class: https://css-tricks.com/the-difference-between-id-and-class

 

Designing a Remote Laboratory with PSLab using Python Flask Framework

In the introductory post about remote laboratories, a general set of tools to create a framework and handle its various aspects was also introduced. In this blog post, we will explore the implementation of several aspects of the backend app designed with python-flask, and the frontend based on EmberJS. A clear separation of the frontend and backend facilitates minimal disruption of either sections due to the other.

Implementing API methods in Python-Flask

In the Flask web server, page requests are handled via ‘routes’ , which are essentially URLs linked to a python function. Routes are also capable of handling payloads such as POST data, and various return types are also supported.

We shall use an example to demonstrate how a Sign-Up request sent from the sign-up form in the remote lab frontend for PSLab is handled.

@app.route('/signUp',methods=['POST'])
def signUp():
	"""Sign Up for Virtual Lab

	POST: Submit sign-up parameters. The following must be present:
	 inputName : The name of your account. does not need to be unique
	 inputEmail : e-mail ID used for login . must be unique.
	 inputPassword: password .
	Returns HTTP 404 when data does not exist.
	"""
	# read the posted values from the UI
	_name = request.form['inputName']
	_email = request.form['inputEmail']
	_password = request.form['inputPassword']

	# validate the received values
	if _name and _email and _password:
		_hashed_password = generate_password_hash(_password)
		newUser = User(_email, _name,_hashed_password)
		try:
			db.session.add(newUser)
			db.session.commit()
			return json.dumps({'status':True,'message':'User %s created successfully. e-mail:%s !'%(_name,_email)})
		except Exception as exc:
			reason = str(exc)
			return json.dumps({'status':False,'message':str(reason)})

 

In this example, the first line indicates that all URL requests made to <domain:port>/signUp will be handled by the function signUp . During development, we host the server on localhost, and use the default PORT number 8000, so sign-up forms must be submitted to 127.0.0.1:8000/signUp .

For deployment on a globally accessible server, a machine with a static IP, and a DNS record must be used. An example for such a deployment would be the heroku subdomain where pslab-remote is automatically deployed ; https://pslab-stage.herokuapp.com/signUp

A closer look at the above example will tell you that POST data can be accessed via the request.form dictionary, and that the sign-up routine requires inputName,inputEmail, and inputPassword. A password hash is generated before writing the parameters to the database.

Testing API methods using the Postman chrome extension

The route described in the above example requires form data to be submitted along with the URL, and we will use a rather handy developer tool called Postman to help us do this. In the frontend apps , AJAX methods are usually employed to do such tasks as well as handle the response from the server.

 

The above screenshot shows Postman being used to submit form data to /signUp on our API server running at localhost:8000 . The fields inputName, inputDescription, and inputPassword are also posted along with it.

In the bottom section, one can see that the server returned a positive status variable, as well as a descriptive message.

Submitting the sign up form via an Ember controller.
  • Setting up a template
    We first need to set up a template that we shall call sign-up.hbs , and add the following form to it. This form contains the details essential for signing up , and its submit action is linked to an action called `signMeUp` . This action will be defined in the controller which we shall explore shortly

<form class="form-signin" {{action "signMeUp" on="submit"}} >
        <label for="inputName" class="sr-only">Your Name</label>
        {{input value=inputName type="text" name="inputName" id="inputName" class="form-control" placeholder="name" required=true autofocus=true}}
        <label for="inputEmail" class="sr-only">Email address</label>
        {{input value=inputEmail type="email" name="inputEmail" id="inputEmail" class="form-control" placeholder="Email address" required=true autofocus=true}}
        <label for="inputPassword" class="sr-only">Password</label>
        {{input value=inputPassword type="password" name="inputPassword" id="inputPassword" class="form-control" placeholder="Password" required=true autofocus=true}}
         
        <button class="btn btn-lg btn-primary btn-block" type="submit">Sign Up</button>
</form>

 

  • Defining the controller
    The controller contains the actions and variables that the template links to. In this case, we require an action called signMeUp. The success, failure, and error handlers are hidden for clarity.

import Ember from 'ember';
export default Ember.Controller.extend({
  actions:{
    signMeUp() {
        var request = Ember.$.post("/signUp",
 this.getProperties("inputName","inputEmail","inputPassword"),this,'json');
        request.then(this.success.bind(this), this.failure.bind(this),
this.error.bind(this));
    },
  },
});

The signMeUp action submits the contents of the form to the signUp route on the API server, and the results are handled by functions called success, failure, or error depending on the type of response from the backend server.

Resources:

 

Reducing Initial Load Time Of Susper

Susper used to take long time to load the initial page. So, there was a discussion on how to decrease the initial loading time of Susper.

Later on going through issues raised in official Angular repository regarding the time takento load angular applications, we found some of the solutions. Those include:

  • Migrating from development to production during build:
    • This shrinks vendor.js and main.js by minimising them and also removing all packages that are not required on production.
    • Enables inline html and extracting CSS.
    • Disables source maps.
  • Enable Ahead of Time (AoT) compilation.
  • Enable service workers.

After these changes we found the following changes in Susper:

File Name Before (content-length) After
vendor.js 709752 216764
main.js 56412 138361

LOADING TIMES GRAPHS:

After:

Vendor file:

Main.js

Before:

Vendor file:

Main.js

Also we could see that all files are now initiated by service worker:

More about Service Workers could be read at Mozilla and Service Workers in Angular.

Implementation:

While deploying our application, we have added –prod and –aot as extra attributes to ng build .This enables angular to use production mode and ahead of time compilation.

For service workers we have to install @angular/service-worker module. One can install it by using:

npm install @angular/service-worker --save
ng set apps.0.serviceWorker=true

The whole implementation of this is available at this pull:

https://github.com/fossasia/susper.com/pull/597/files

Resources: