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Introduction

podium is an event emitter with support for tags, filters, channels, event update cloning, arguments spreading, and other features useful when building large scale applications. While node's native EventEmitter is strictly focused on maximum performance, it lacks many features that do not belong in the core implementation. podium is not restricted by node's performance requirement as it is designed for application layer needs where its overhead is largely insignificant as implementing these features will have similar cost on top of the native emitter.

Example

const Podium = require('@hapi/podium');

const emitter = new Podium()

const context = { count: 0 }

emitter.registerEvent({
    name: 'event',
    channels: ['ch1', 'ch2']
})

const handler1 = function () {

    ++this.count
    console.log(this.count)
};

const handler2 = function () {

    this.count = this.count + 2
    console.log(this.count)
}

emitter.on({
    name: 'event',
    channels: ['ch1']
}, handler1, context);

emitter.on({
    name: 'event',
    channels: ['ch2']
}, handler2, context)

emitter.emit({
    name: 'event',
    channel: 'ch1'
})

emitter.emit({
    name: 'event',
    channel: 'ch2'
})

emitter.hasListeners('event') // true

emitter.removeAllListeners('event') // Removes all listeners subscribed to 'event'

The above example uses podium's channel event parameter to restrict the event update to only the specified channel.

First you register the event by calling the registerEvent() method. Here, you name the event 'event' and give it channels ['ch1', 'ch2'].

Next you specify your listener handlers. These will be called when an event is updated. Here you make use of podium's listener context, data that you can bind to your listener handlers. In this case, handler1 will add 1 to count, which is specified as { count: 0 }, while handler2 will add 2.

Next you call the on() method to subscribe a handler to an event. You use the same event name, but two different channels. 'ch1' will use handler1 and 'ch2' will use handler2.

Lastly, you use emit() to emit and event update to the subscribers.

Using different parameters

Along with channels, podium allows you to specify other event parameters. Below are more examples:

channels

const Podium = require('@hapi/podium');
const podiumObject = new Podium();

podiumObject.registerEvent([
    {
        name: 'event1',
        channels: ['ch1', 'ch2', 'ch3', 'ch4'],
    },
    {
        name: 'event2',
        channels: ['ch1', 'ch2']
    }
]);
const listener1 = (data) => {

    console.log('listener1 called', data);
};
const listener2 = (data) => {

    console.log('listener2 called', data);
};

podiumObject.on({
    name: 'event1',
    channels: ['ch1']
}, listener1);

podiumObject.on({
    name: 'event1',
    channels: ['ch3', 'ch4']
}, listener2);


podiumObject.on({ name: 'event1', channels: 'ch2' }, (data) => { // autonomous function

    console.log('auto', data);
});

var arr = [0, 1, 2, 3, 4, 4, 5];

podiumObject.emit({
    name: 'event1',
    channel: 'ch3'
});

clone

const Podium = require('@hapi/podium');
const podiumObject = new Podium();

podiumObject.registerEvent([
    {
        name: 'event1',
        channels: ['ch1', 'ch2'],
        clone: true
    },
    {
        name: 'event2',
        channels: ['ch1', 'ch2']
    }
]);

const listener1 = (data) => {

    data[0] = 55;
    console.log('listener1 called', data);
};
const listener2 = (data) => {

    data[0] = 100;
    console.log('listener2 called', data);
};

podiumObject.on({
    name: 'event1',
    channels: ['ch1']
}, listener1);

podiumObject.on({
    name: 'event2',
    channels: ['ch1']
}, listener2);

var arr = [0, 1, 2, 3, 4, 4, 5];

console.log('initially: ', arr);

podiumObject.emit({
    name: 'event1',
    channel: 'ch1'
});

console.log('after event1, ch1: ', arr);

podiumObject.emit({
    name: 'event2',
    channel: 'ch1'
});

console.log('after event2, ch1: ', arr);

spread

const Podium = require('@hapi/podium');
const podiumObject = new Podium();

podiumObject.registerEvent([
    {
        name: 'event1',
        channels: ['ch1', 'ch2'],
        spread: true
    },
    {
        name: 'event2',
        channels: ['ch1', 'ch2']
    }
]);

const listener1 = (data1, data2, data3, data4) => {

    console.log('listener1 called', data1, data2, data3, data4);
};

const listener2 = (data) => {

    data[0] = 100;
    console.log('listener2 called', data);
};

podiumObject.on({
    name: 'event1',
    channels: ['ch1']
}, listener1);

podiumObject.on({
    name: 'event2',
    channels: ['ch1']
}, listener2);

var arr = [0, 1, 2, 3, 4, 4, 5];

console.log('initially: ', arr);

podiumObject.emit({
    name: 'event1',
    channel: 'ch1'
});

console.log('after event1, ch1: ', arr);

podiumObject.emit({
    name: 'event2',
    channel: 'ch1'
});
console.log('after event2, ch1: ', arr);

shared

const Podium = require('@hapi/podium');
const podiumObject = new Podium();

podiumObject.registerEvent([
    {
        name: 'event1',
        channels: ['ch1', 'ch2'],
    }
]);
podiumObject.registerEvent([
    {
        name: 'event1',
        channels: ['ch1', 'ch2'],
        shared: true
    }
]);
const listener2 = (data) => {

    console.log('listener2 called', data);
};

podiumObject.on({
    name: 'event1',
    channels: ['ch1']
}, listener2);

var arr = [0, 1, 2, 3, 4, 4, 5];

podiumObject.emit({
    name: 'event1',
    channel: 'ch1'
});

tag-filter

const Podium = require('@hapi/podium');
const emitter = new Podium('test');

const updates = [];
emitter.on('test', (data) => updates.push({ id: 1, data }));
emitter.on({ name: 'test', filter: ['a', 'b'] }, (data) => updates.push({ id: 2, data }));
emitter.on({ name: 'test', filter: 'b' }, (data) => updates.push({ id: 3, data }));
emitter.on({ name: 'test', filter: ['c'] }, (data) => updates.push({ id: 4, data }));
emitter.on({ name: 'test', filter: { tags: ['a', 'b'], all: true } }, (data) => updates.push({ id: 5, data }));

emitter.emit({ name: 'test', tags: 'a' }, 1);
emitter.emit({ name: 'test', tags: ['b'] }, 2);
emitter.emit({ name: 'test', tags: ['d'] }, 3);
emitter.emit({ name: 'test', tags: ['a'] }, 4);
emitter.emit({ name: 'test', tags: ['a', 'b'] }, 5);

emitter.emit('test', 6, () => {

    console.log(updates);
});

count

const Podium = require('@hapi/podium');
const podiumObject = new Podium();

podiumObject.registerEvent('event1');

const listener1 = function(data) {

    console.log('listener1 called', data);
};

podiumObject.on({
    name: 'event1',
    count: 2
}, listener1);

podiumObject.emit('event1', 'emit 1');
podiumObject.emit('event1', 'emit 2');
podiumObject.emit('event1', 'emit 3'); // this wont call listener1

new Podium(events)

Creates a new podium emitter where:

Returns a Podium object.

podium.registerEvent(events)

Register the specified events and their optional configuration. Events must be registered before they can be emitted or subscribed to. This is done to detect event name misspelling and invalid event activities. The events argument can be:

  • an event string.
  • an event options object with the following optional keys (unless noted otherwise):
    • name - the event name string (required).
    • channels - a string or array of strings specifying the event channels available. Defaults to no channel restrictions (event updates can specify a channel or not).
    • clone - if true, the data object passed to podium.emit() is cloned before it is passed to the listeners (unless an override specified by each listener). Defaults to false (data is passed as-is).
    • spread - if true, the data object passed to podium.emit() must be an array and the listener method is called with each array element passed as a separate argument (unless an override specified by each listener). This should only be used when the emitted data structure is known and predictable. Defaults to false (data is emitted as a single argument regardless of its type).
    • tags - if true and the criteria object passed to podium.emit() includes tags, the tags are mapped to an object (where each tag string is the key and the value is true) which is appended to the arguments list at the end. A configuration override can be set by each listener. Defaults to false.
    • shared - if true, the same event name can be registered multiple times where the second registration is ignored. Note that if the registration config is changed between registrations, only the first configuration is used. Defaults to false (a duplicate registration will throw an error). For detailed examples of event parameters see here
  • an array containing any of the above.

podium.emit(criteria, data)

Emits an event update to all the subscribed listeners where:

  • criteria - the event update criteria which must be one of:
    • the event name string.
    • an object with the following optional keys (unless noted otherwise):
      • name - the event name string (required).
      • channel - the channel name string.
      • tags - a tag string or array of tag strings.
  • data - the value emitted to the subscribers.

podium.on(criteria, listener, context)

Subscribe a handler to an event where:

  • criteria - the subscription criteria which must be one of the following:
    • event name string.
    • a criteria object with the following optional keys (unless noted otherwise):
      • name - the event name string (required).
      • channels - a string or array of strings specifying the event channels to subscribe to. If the event registration specified a list of allowed channels, the channels array must match the allowed channels. If channels are specified, event updates without any channel designation will not be included in the subscription. Defaults to no channels filter.
      • clone - if true, the data object passed to podium.emit() is cloned before it is passed to the listener method. Defaults to the event registration option (which defaults to false).
      • count - a positive integer indicating the number of times the listener can be called after which the subscription is automatically removed. A count of 1 is the same as calling podium.once(). Defaults to no limit.
      • filter - the event tags (if present) to subscribe to which can be one of the following:
        • a tag string.
        • an array of tag strings.
        • an object with the following:
          • tags - a tag string or array of tag strings.
          • all - if true, all tags must be present for the event update to match the subscription. Defaults to false (at least one matching tag).
      • spread - if true, and the data object passed to podium.emit() is an array, the listener method is called with each array element passed as a separate argument. This should only be used when the emitted data structure is known and predictable. Defaults to the event registration option (which defaults to false).
      • tags - if true and the criteria object passed to podium.emit() includes tags, the tags are mapped to an object (where each tag string is the key and the value is true) which is appended to the arguments list at the end. Defaults to the event registration option (which defaults to false).
  • listener - the handler method set to receive event updates. The function signature depends on the spread, and tags options.
  • context - an object that binds to the listener handler.

podium.addListener(criteria, listener, context)

Same as podium.on().

podium.once(criteria, listener, context)

Same as calling podium.on() with the count option set to 1.

podium.removeListener(name, listener)

Removes all listeners subscribed to a given event name matching the provided listener method where:

  • name - the event name string.
  • listener - the function reference provided when subscribed.

Returns a reference to the current emitter.

podium.removeAllListeners(name)

Removes all listeners subscribed to a given event name where:

  • name - the event name string.

Returns a reference to the current emitter.

podium.hasListeners(name)

Returns whether an event has any listeners subscribed where:

  • name - the event name string.

Returns true if the event name has any listeners, otherwise false.