Building Websites to Subscribe to MQTT Data

This worksheet builds on the Publish Subscribe one. Make sure you have finished this before starting. In this worksheet you will learn how to use the TDD methodology to develop websites that subscribe to MQTT brokers. You can find the exercise files in the exercises/02_tdd/web_client/ directory.

1 Basic MQTT Client

In this first exercise you will learn the basics of how to create a website that can subscribe to data from an MQTT broker. It will focus on writing JavaScript code that runs inside the browser. Open the chat/ directory where you will see three files plus a directory.

The index.html is the main webpage the user will see. You should open this in the browser to run the web app.
1. It contains links to the hivemq mqtt library to allow the page to publish and subscribe.
2. It links to the module.js script which contains our MVC Model.
3. It links to the mqtt.js script that interacts with the html view and JavaScript model.
  1. This is the MVC Controller and uses the Messaging object imported from the hivemq library to handle publishing to and subscribing from an MQTT broker which it does by creating a client object on line 6. This contains a number of properties that we can use to store anonymous functions, these will be triggered by different events, the most important being onMessageArrived. This property is assigned a function that gets triggered every time a new MQTT message arrives. This makes use of the functions in our model.
  2. At the bottom of the script we create an event listener that will get triggered once the html (the DOM) has been downloaded. This adds an event handler to the textbox that triggers each time a key on the keyboard is released. If the key is the enter key it retrieves the text in the textbox and passes it to the publish() function.
  3. The publish() function uses the Messaging object in the hivemq library to create a new Message object which is sent to the MQTT broker.
The test/ directory contains the unit tests for our MVC Model.
1. The testrunner.html runs the tests in the browser using the Jasmine 2 testing framework, you should open this file in your web browser.
2. The module.test.js script contains the unit tests for the module.js script.

As you can see you have a working website that conforms to the MVC design pattern plus a complete suite of unit tests that cover the model (business logic).

1.1 Immediately-Invoked Function Expressions

We are primarily concerned with writing tests for our model (module.js) Open the file now and make sure you understand how it works.

All variables in modern JavaScript are function-scoped, this means they are only accessible in the function they were declared in. Any variables defined outside a function have global scope. In our model we have an array to hold the messages which should not be directly accessed. To solve this we place the array declaration in an anonymous function which is stored in the messages constant.

We obviously want any properties and methods we define to be able to see this array and so these need to be defined in the same function. Unfortunately any functions defined inside the main function will also not be accessible externally however remember we can return data from a function by using the return statement. In this example we return an object containing all the functions and properties we want to be able to call!

To be able to access any of this we need to execute the function. To run a function we add a pair of parentheses at the end () and so by adding this to the end of the function expression it is immediately executed, hence the term Immediately Executed Function Expression or IIFE for short. Since all the data in a model should be private it is expected that any model you create will need to be defined as an IIFE.

Take time to go through the code carefully so you fully understand how IIFEs work. If you decide to search online for additional resources (and you should) be aware that IIFEs used to be called Function Closures or Closures for short.

1.2 TDD First Iteration

Try leaving the text field blank then clicking on the button, what happens? Clearly we need to make changes to the model. The most obvious solution is to modify the extractData() function to throw an error if the message is a blank string. Although we are using the Jasmine testing tools, these work in a very similar way to the Jest tools. Open the test/module.test.js script and study it carefully noting that the tests have been placed in nested groups.

1.2.1 Write a Failing Test

The first step is to define the functionality we want in the form of a test. Clearly we need to throw an error rather than simply add a blank item to the list. Since we are testing the add() function we need to add an extra test to this section of the suite. Locate the extractData tests which are grouped in the method group. Add the following test:

it('empty string parameter should throw an error', () => {
  try {
    messages.extractData('')
    expect(1).toBe(0) // this line should not be run!
  } catch(err) {
    expect(err.message).toBe('empty string parameter')
  }
})

Notice that if the extractData() function does not throw an error it runs the next line which obviously fails. This means that we can flag if the error is not thrown. Run the unit test suite, the new test should fail, this is the first step.

1.2.2 Write Code to Pass the Test

Your next step is to modify the extractData() function until all the tests pass. There is a solution below but see if you can do it without looking!

extractData: payloadString => {
    if(payloadString.length === 0) {
        throw new Error('empty string parameter')
    }
    return payloadString
}

1.2.3 Refactor the Code

The third and final step is to tidy up the code ensuring all the tests still pass. In this example there is only one line in the if statement block so we can reduce it to a single line as shown.

extractData: payloadString => {
    if(payloadString.length === 0) throw new Error('empty string parameter')
    return payloadString
}

Run the test suite again to make sure all the tests pass. You have now completed your first TDD iteration.

1.3 TDD Second Iteration

Try adding a message that only includes spaces, what happens? Use the TDD methodology to fix this bug (hint: try using the String.trim() command).

1.4 Test Your Understanding

The current webpage has very limited functionality, only displaying the last message with no additional information. Use a TDD methodology to make the following improvements:

change the topic, replacing the /cu segment with the chosen username.
store all the messages in the array rather than the last one by modifying the addMessage() function.
display the message thread in a table by modifying the buildHTML() function.
change the payload to an object containing the sender's username, message and the date/time and modify the extractData() function to return an object containing the three values. You will also need to modify the buildHTML() function to avoid breaking your code.
Modify the buildHTML() function to:
1. display the name of the sender in a second column in the table.
2. display the date/time in a third column in the table.

2 Maps

The third and final topic will show you how we can use the data to place markers on Google Maps.

LatLon

mosquitto_pub -h broker.mqttdashboard.com -t cov/map -m '{"lat": 52.4082, "lon": -1.5071, "label": "cathedral"}'

Location	Lat	Lon
Godiva Statue	52.4080	-1.5104
Frank Whittle Building	52.4053	-1.4998
Police Station	52.4045	-1.5084
Coventry College	52.4140	-1.5038
University Hospital	52.4213	-1.4416

need to create an array of markers to manage them.

The test/ directory contains a suite of tests to cover the code in the IIFE that you will find in the module.js script. Try running the tests, note that they all pass.

2.1 TDD First Iteration

need to handle incomplete data in the JSON string... missing label property.

2.1.1 Write a Failing Test

By now you should be confortable with the TDD process. Let's add a third test for the extractData function. The following test should work:

it('throws exception if label property missing', () => {
  try {
    const str = '{"lat": 52.4082, "lon": -1.5071}'
    maps.extractData(str)
    expect(0).toBe(1) // this line should not run!
  } catch(err) {
    expect(err.message).toBe('Error: missing label property')
  }
})

Add this new test and use the testrunner to run the test suite. The test should fail with the message Expected 0 to be 1 which shows that the error is not being thrown!

2.1.2 Write Code to Pass the Test

Now we need to modify the extractData() function so that this new test passes (and we dont break any of the other tests!). Try the following:

extractData: payloadString => {
  try {
    const data = JSON.parse(payloadString)
      if(data.label === undefined) {
        throw new Error('missing label property')
      }
    return data
  } catch(err) {
    if(err.message === 'Unexpected token h in JSON at position 0') {
      throw new Error('parameter is not a json string')
    }
  throw new Error(err) // we need to propagate all other errors
  }
}

Run the test suite again to check that all the tests now pass.

2.1.3 Refactor the Code

The third and final step in TDD is to refactor (tidy) the code, ensuring that the tests still pass. We can simplify the code by removing the brackets from single line if statements and clean up some of the conditionals.

try {
  const data = JSON.parse(payloadString)
  if(!data.label) throw new Error('missing label property')
  return data
} catch(err) {
  if(err.message === 'Unexpected token h in JSON at position 0') {
    throw new Error('parameter is not a json string')
  }
  throw new Error(err) // we need to propagate all other errors
}

Make sure all the tests still pass before marking this task as complete.

2.2 TDD Second (and third) Iteration

Try modifying the script to handle a missing lat or lon property (note this will take two TDD iterations).

2.3 Test Your Understanding

The sample code demonstrates a simple map with multiple pins. You should now modify this to track buses around Coventry:

Modify the extractData() function to accept and validate two additional pieces of data:
1. route which is a number representing the bus route
2. reg which should contain the vehicle registration number
Display both the route and reg number in the marker label.
Each reg should appear only once on the map:
1. If the vehicle appears for the first time add a new pin.
2. When the same vehicle updates its position you should replace the existing pin.
instead of using the generic marker you should instead use the following bus symbol as a custom marker. Hint: add another property called icon whem you create a new Marker.

TEACHING-MATERIALS/02 MQTT JavaScript.md