spokesman.js

Super flexible Node.js Service Emitter, you ask and you decide how spokesman will answer.

Install

$ npm install spokesman.js --save

How to create custom ServiceEmitter

First of all you need to extend emitter class.

// MyEmitter.js
const ServiceEmitter = require('spokesman.js').ServiceEmitter;

let singleton = Symbol();
let singletonEnforcer = Symbol();

class MyEmitter extends ServiceEmitter {

  constructor (enforcer) {

    // This disables creating instance outside.
    if (enforcer !== singletonEnforcer) {
      throw 'Cannot construct singleton';
    }

    super({ delay: 2000 }); // Call `ServiceEmitter` constructor.
  }

  /**
   * Get unique instance of `MyEmitter`.
   */
  static getInstance () {
    if (!this[singleton]) {
      this[singleton] = new MyEmitter(singletonEnforcer);
    }

    return this[singleton];
  }

  /**
   * @override
   */
  onProviderData (data, cb) {
    // Emit data received from provider to custom emitters.
    //
    // Note: if you call `super({ autoRequest: false })`, then this method will
    // not get fired unless you call first `this.provider.requestData(opts)` in
    // the override of `interval()`.

    // Validate data here.

    return cb(null, data);
  }
}

module.exports = MyEmitter;

Note: MyEmitter is a singleton, we recommend use this pattern to keep a clean implementation.

How to create custom ServiceProvider

Once we have our basic MyEmitter the next step is to create a custom provider.

// MyProvider.js
const ServiceProvider = require('spokesman.js').ServiceProvider;

class MyProvider extends ServiceProvider {

  constructor () {
    super(); // Call `ServiceProvider` constructor.
  }

  /**
   * @override
   */
  turnON () {

    if (this.isTurnedON()) { return }; // Return if already ON.

    super.turnON(); // Do not forget to call `turnON()` method of super class.

    // Listen for emitter data requests.
    this.on('wantsData', (opts) => this.setData('Hello world!'));
  }
}

module.exports = MyProvider;

Note: The default behavior of ServiceEmitter is to remove all provider listeners when no one listens 'data' event of ServiceEmitter.

MyEmitter + MyProvider usage

Now that our custom emitter and provider are ready we will see how they work together.

const MyEmitter = require('./MyEmitter');
const MyProvider = require('./MyProvider');

// First register `MyProvider` in `MyEmitter`.
MyEmitter.getInstance().registerProvider(new MyProvider());

// Then listen for 'data' event.
MyEmitter.getInstance().on('data', (data) => {
  console.log(data); // output: 'Hello world!' each 2 seconds.
});

You can create your own Emitters and Providers to adapt to your necessities.

ServiceEmitter Properties

.provider : ServiceProvider

Instance of ServiceProvider registered.

ServiceEmitter Methods

constructor (opts)

Constructor receives an Object with options:

• delay (Number) - Delay in milliseconds for intervals, default set to 1000.
• autoRequest (Boolean) - Whether to request data to provider automatically or not, default set to true.
• runImmediately (Boolean) - Determines if interval should be called immediately.

.dataHasListeners () : Boolean

Checks if any listeners listen 'data' event.

.isListeningTo (event, listener) : Boolean

Checks if listener provided is listening event name.

• event (String) - Event name to check.
• listener (Function) - Listener to check

.registerProvider (provider)

Public method for ServiceProvider registration.

• provider (ServiceProvider) - One of the available providers.

.isProviderRegistered () : Boolean

Verifies if a provider is already registered.

.getCurrentData ([pick]) : Object

Get current provider data with optional pick filter.

• pick (Array) - Array of properties to return in Object.

.interval ()

Abstract interval function to be implemented by custom emitter.

Note: This function will be called in each cycle of main interval.

.onNewListener (event, listener)

Abstract onNewListener function to be implemented by custom emitter.

• event (String) - Event name.
• listener (Function) - Listener Function.

Note: This function will be called on new event listener.

.onRemoveListener (event, listener)

Abstract onRemoveListener function to be implemented by custom emitter.

• event (String) - Event name.
• listener (Function) - Listener Function.

Note: This function will be called on remotion of event listener.

.onProviderData (data, [callback])

Abstract onProviderData function to be implemented by custom emitter.

• data (Object) - Data to emit.
• callback (Function) - Function to be called for data validation.

Callback arguments:

• error (Object) - Reason of validation failure.
• validatedData (Object) - Validated data to be applied to data in ServiceEmitter.

This callback does three things:

1. Emits any error provided as first argument on 'error' event.
2. If no errors were provided then set validated data taken from second argument as ServiceEmitter current data.
3. Emits validated data on 'data' event as long as error is a falsy value.

Notes:

• This function will be called when provider emits new data.
• If you set autoRequest as false, then this method will not get fired unless you call first this.provider.requestData(opts) in the override of interval().
• The callback is optional, and if you decide to omit it then you should emit data received on 'data' event to let know listeners when there is new data.

.onProviderError (err)

Abstract onProviderError function to be implemented by custom emitter.

• err (Object) - Error to emit.

Notes:

• This function will be called when provider emits an error.
• By default emits error received on 'error' event.

.removeListener (event, listener)

• event (String) - Event name of listener to remove.
• listener (Function) - Listener of event name to remove.

.removeAllListeners ()

Removes all listeners of data and error events.

ServiceEmitter Events

.on('data', callback)

Listens for main emitter event.

Callback arguments:

• data (Object) - Data received from custom emitter.

.on('error', callback)

Listens for error emitter event.

Callback arguments:

• error (Object) - Error received from custom emitter.

ServiceProvider Methods

.turnON ()

Turns ON provider, usually you would like to override default behavior for this action in providers.

Note: Even if you override this, you should always call super class turnON method to update ServiceProvider status.

.turnOFF ()

Turns OFF provider, usually you would like to override default behavior for this action in providers.

Note: Even if you override this, you should always call super class turnOFF method to update ServiceProvider status.

.isTurnedON () : Boolean

Whether provider is turned 'ON' or not.

.requestData ([opts])

Notifies to custom provider that custom emitter wants new data.

• opts (Object) - Optional Object to be passed along 'wantsData' event.

Note: 'wantsData' event should be listened by custom provider in order to be able to process request and return response to custom emitter.s

.setData (data)

Emits 'data' event which is listened by ServiceEmitter with data received from custom provider to make it available for consumers of your custom emitter.

• data (Object) - Data to transmit to ServiceEmitter.

Note: You should NOT override this method on your providers.

.setError (err)

Emits 'error' event which is listened by ServiceEmitter with error received from custom provider to make it available for consumers of your custom emitter.

• err (Object) - Possible error to transmit to ServiceEmitter.

ServiceProvider Events

.on('wantsData', callback)

Listens for wantsData event, this event should be listen by custom provider to know when custom emitters are requesting new data.

Callback arguments:

• opts (Object) - Optional Object passed from custom emitter.

Testing

$ npm test

License

MIT