Event-Ag

Typed event aggregator implementation for Typescript.

Installation

Install via npm using npm i --save-dev event-ag

Usage

Creating an event

First create an event that extends PubSubBase as shown below; this event will be used as event arguments when publishing.

export default class NewUserEvent extends PubSubBase<NewUserEvent> {
  private userId: number;
  
  public get UserId: number {
    return this.userId;
  }
  
  public set PackId(value: number) {
    this.userId = value;
  }
}

Creating a Query

A query should be used when a return value is required, queries should not edit the state of the application i.e. be free of side effects.

To create a query, create a class that extends Query as shown below; this query class will represent return type of the query.

export default class UserQuery extends Query<UsernameQuery> {
  private user: IUser;

  public get User: IUser {
    return this.user;
  }

  public set User(value: IUser){
    this.user = value;
  }
}

Args can be passed when making a query, to do this, create an interface that extends the QueryArgs interface.

export interface UserQueryArgs implements QueryArgs<TestQuery> {
  Username: string;
}

The Event Aggregator

The EventAggregator is responsible for managing all of the available events, events are added to the EventAggregator the first time GetEvent is called with a new event. To get started with using the EventAggregator, create an instance of EventAggregator, then get the newly created PubSubBase event from the EventAggregator.

const eventAggregator = new EventAggregator();
const newUserEvent = eventAggregator.GetEvent(NewUserEvent);

Subscribing to events

Events can be subscribed to using the following method.

const eventAggregator = new EventAggregator();
const newUserEvent = eventAggregator.GetEvent(NewUserEvent);

newUserEvent.Subscribe((eventArgs: NewUserEvent) => console.log(`New user - UserId: ${eventArgs.UserId}`);

Publishing events

To publish an event, an instance of the event must be passed as event arguments.

const eventAggregator = new EventAggregator();
const newUserEvent = eventAggregator.GetEvent(NewUserEvent);

newUserEvent.Subscribe((eventArgs: NewUserEvent) => console.log(`New user - UserId: ${eventArgs.UserId}`);
newUserEvent.Publish({UserId: 0} as NewUserEvent);

Handling Queries

Unlike events, a query can only have one handler. A function can be registered as a handler of a query with the following code.

  const eventAggregator = new EventAggregator();
  eventAggregator.GetQuery(UserQuery).Handle((query: QueryArgs<UserQuery> | undefined) => {
    const userQueryArgs = query as UserQueryArgs;
    const user = // Code to find user with **userQuery.Username** goes here.
    return {
      User: user
    } as UserQuery;
  });

NOTE: parameter of handler must inlude Query<T> | undefined as query args may not always be passed.

Executing Queries

Executing a query is similar to publishing an event.

  const userQuery = eventAggregator.GetQuery(UserQuery);

  const user = userQuery.Execute({
    Username: 'test_username'
  } as UserQueryArgs);