#Events

Events play an essential role in being able to develop reactive plugins that act when state changes occur in Core (block received, transaction forged, etc).

The event dispatcher implementation that ships with Core supports both synchronous and asynchronous execution of event listeners. As always, you should think carefully about when to use an event listener as they are a great way of decoupling certain aspects of your application but can end up hogging resources.

#Prerequisites

Before we start, we need to establish what a few recurring variables and imports in this document refer to when they are used.

import { app } from "@arkecosystem/core-kernel";

• The app import refers to the application instance which grants access to the container, configurations, system information and more.

#Registering Events & Listeners

#Register a listener with the dispatcher

app.events.listen("firstEvent", ({ name, data }) => console.log(name, data.message));

#Register many listeners with the dispatcher

app.events.listenMany([
    ["firstEvent", listenerFunction],
    ["secondEvent", listenerFunction]
]);

#Register a one-time listener with the dispatcher

app.events.listenOnce("firstEvent", () => console.log("Hello World"));

#Remove a listener from the dispatcher

app.events.forget("firstEvent", listenerFunction);

#Remove many listeners from the dispatcher

app.events.forgetMany(["firstEvent", "secondEvent"]);

app.events.forgetMany([
    ["firstEvent", listenerFunction],
    ["secondEvent", listenerFunction]
]);

#Remove all listeners from the dispatcher

app.events.flush();

#Get all of the listeners for a given event name

app.events.getListeners("firstEvent");

#Determine if a given event has listeners

app.events.hasListeners("firstEvent");

#Dispatching Events

#Fire an event and call the listeners in asynchronous order

await app.events.dispatch("firstEvent", "Hello World");

#Fire an event and call the listeners in sequential order

await app.events.dispatchSeq("firstEvent", "Hello World");

#Fire an event and call the listeners in synchronous order

app.events.dispatchSync("firstEvent", "Hello World");

#Fire many events and call the listeners in asynchronous order

await app.events.dispatchMany([
    ["firstEvent", "Hello World"],
    ["secondEvent", "Hello World"]
]);

#Fire many events and call the listeners in sequential order

await app.events.dispatchManySeq([
    ["firstEvent", "Hello World"],
    ["secondEvent", "Hello World"]
]);

#Fire many events and call the listeners in synchronous order

app.events.dispatchManySync([[
    ["firstEvent", "Hello World"],
    ["secondEvent", "Hello World"]
]]);