mega-nice-change-event v1.0.3

Mega Nice Change Event

A mega nice change event to describe changes in a database entity.

Install

npm install mega-nice-change-event

Overview

Describe a change using an entity that changed

The journey begins when an entity changed.

import { ChangeDescription } from 'mega-nice-change-event'

let task = {
  id: 5,
  parentId: 33,
  userId: 12
  title: 'Clean up room'
}

let change = ChangeDescription.fullDescription(task, 'update', ['title'])

change == {
  entity: 'Task',

  idProps: {
    id: 5,
    parentId: 33,
    userId: 12
  },

  changes: [{
    method: 'update',
    props: ['title']
  }]
}

The static method fullDescription will create a full description of the changed object.

• entity: The entity name which basically is the class name but in this context we want to think about the changed thing as a database entity.
• idProps: Contains any property which identifies the entity. It includes any property named id or ending with Id.
• changes: An array of changes which consists of a change method and changed props.

If you need different behaviour you can either take the output of this method as a starting point or you just write your own.

Describe a change that should be relevant

Now that you described a change there are change listeners who would like to decide if a change is relevant for them or not. You do that by describing a change which acts as a template for any incoming change description. Let the code talk.

// listen to any change of any tasks
let listener1 = {
  entity: 'Task'
}

// listen to any change of a task with id 5
let listener2 = {
  entity: 'Task',
  idProps: { id: 5 }
}

// listen to any update change of a task with id 5
let listener3 = {
  entity: 'Task',
  idProps: { id: 5 },
  changes: { method: 'update' }
}

// listen to any title update change of a task with id 5
let listener4 = {
  entity: 'Task',
  idProps: { id: 5 },
  changes: { method: 'update', props: ['title'] }
}

// listen to any title update change of any task
let listener4 = {
  entity: 'Task',
  changes: { method: 'update', props: ['title'] }
}

// listen to any parentId or userId update change of the task with id 5
let listener5 = {
  entity: 'Task',
  idProps: { id: 5 },
  changes: { method: 'update', props: ['parentId', 'userId'] }
}

// listen to any delete change of any task with a userId of 12
let listener5 = {
  entity: 'Task',
  idProps: { userId: 12 },
  changes: { method: 'delete' }
}

As you can see, you are able to determine fine grained changes that listeners should react to.

Use method isRelevantFor to find out if a change is relevant for a listener.

change.isRelevantFor(listener)

Using ChangeEvent

To be able to describe numerous changes at once you can use the ChangeEvent class.

let event = new ChangeEvent(change1, change2, change3)
event.isRelevantFor(listener)

Test if a change is relevant to a set of listener rules

You can describe a set of rules a listener should react to.

let rule1 = {
  entity: 'Task',
  idProps: { id: 5 }
}

let rule2 = {
  entity: 'Task',
  idProps: { id: 5 },
  changes: { method: 'update' props: ['parentId'] }
}

let listenerRuleSet = [ description1, description2 ]

In this case we have two competing rules for tasks with an id of 5. If the incoming change is an update change then rule2 used. In any other case rule1 is used. That means in the case of an update the listener is only interested if the changed property was the parentId.

Unspecific change descriptions

A change may also be more unspecific.

// relevant to any listener which reacts to Task changes disregarding any idProps and changes
let change = {
  entity: 'Task'
}

let listener = {
  entity: 'Task',
  idProps: { id: 5 },
  changes: { method: 'delete' }
}

change.isRelevantFor(listener) == true

// relevant to any listener which reacts to Task changes of a task with id 5 disregarding any change methods or changed properties
let change = {
  entity: 'Task',
  idProps: { id: 5 }
}

let listener = {
  entity: 'Task',
  idProps: { id: 5 },
  changes: { method: 'delete' }
}

change.isRelevantFor(listener) == true