Receiving H Events

What Are H Events?

H events, so named as they are prefixed with the text H:, are events that are used to communicate with Javascript/HTML instruments. They can be triggered by any number of sources, including but not limited to: model behaviors, SimConnect add-ons, and even Javascript instruments themselves. H events are not required to be defined ahead of time, similar to L Vars, and can be named with any contiguous sequence of alphanumeric characters and underscores.

How To Receive H Events

Inside your main instrument class that extends BaseInstrument, you must define the method onInteractionEvent() in order to receive H events:

class MyInstrument extends BaseInstrument {
  get templateID(): string {
    return 'MyInstrument';
  }

  // Will be called by the VCockpit system when an H event is received.
  public onInteractionEvent(args: string[]): void {
    const hEventName = args[0];
    console.log(`Got a H Event named ${hEventName}!`);
  }
}

The args parameter of onInteractionEvent() will contain the name of the H event that was received as the first element of the array, and any arguments passed to the H event as additional elements in the array. If the H event does not have any arguments, then args will only contain the H event name.

The avionics framework also provides a convenient utility publisher class to publish H events to an EventBus:

import { EventBus, HEventPublisher } from '@microsoft/msfs-sdk';

class MyInstrument extends BaseInstrument {
  private readonly eventBus = new EventBus();
  private readonly hEventPublisher = new HEventPublisher(this.eventBus);

  get templateID(): string {
    return 'MyInstrument';
  }

  public connectedCallback(): void {
    super.connectedCallback();
    this.hEventPublisher.startPublish();
  }

  public onInteractionEvent(args: string[]): void {
    this.hEventPublisher.dispatchHEvent(args);
  }
}

HEventPublisher will publish the received H events to the event bus under the topics defined in HEvent. H events without any arguments will be published to the hEvent topic and those with at least one argument will be published to the hEvent_with_args topic.

Once HEventPublisher is set up, you can subscribe to H events on the event bus:

import { HEvent } from '@microsoft/msfs-sdk';

const subscriber = eventBus.getSubscriber<HEvent>();

subscriber.on('hEvent')
  .handle(eventName => {
    console.log(`Got a H Event named ${eventName}!`);
  });

subscriber.on('hEvent_with_args')
  .handle(data => {
    console.log(`Got a H Event named ${data.name} with args ${data.args}!`);
  });