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utils-event-emitter implements event-driven programming for the SDK. It connects the JavaScript scope (book event handlers, state machines) to Svelte component scope without passing props through deep component trees.

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createEventEmitter()

Factory function typed by a union of all possible emitter events. Create it once per game in src/game/eventEmitter.ts. 
import { createEventEmitter } from 'utils-event-emitter';

export function createEventEmitter<TEmitterEvent extends EmitterEventBase>()

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Constraint

TEmitterEvent must extend EmitterEventBase: 
export type EmitterEventBase = {
  type: string;
};
In practice every game defines a discriminated union type: 
export type EmitterEvent =
  | { type: 'boardSpin'; board: RawBoard }
  | { type: 'boardSettle'; board: RawBoard }
  | { type: 'totalWinUpdate'; amount: number }
  | { type: 'freeSpinCounterShow' }
  | { type: 'freeSpinCounterHide' }
  | { type: 'freeSpinCounterUpdate'; current?: number; total?: number };

export const { eventEmitter } = createEventEmitter<EmitterEvent>();

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Return value

{
  eventEmitter: {
    subscribeOnMount: (map: Partial<EmitterEventHandlerMap>) => void;
    broadcast:       (emitterEvent: TEmitterEvent) => void;
    broadcastAsync:  (emitterEvent: TEmitterEvent) => Promise<any[]>;
  }
}

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Methods

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subscribeOnMount()

Registers a map of handlers inside a Svelte component’s onMount lifecycle. Automatically unsubscribes when the component is destroyed. 
const subscribeOnMount = (
  emitterEventHandlerMap: Partial<EmitterEventHandlerMap>,
) => void
The EmitterEventHandlerMap type is keyed by every type in the TEmitterEvent union, and each value receives the narrowed event type for that key: 
type EmitterEventHandlerMap = {
  [T in EmitterEventType]: (emitterEvent: Extract<TEmitterEvent, { type: T }>) => any
};
Example — sync handler: 
<!-- FreeSpinCounter.svelte -->
<script lang="ts">
  const context = getContext();

  context.eventEmitter.subscribeOnMount({
    freeSpinCounterShow: () => (show = true),
    freeSpinCounterHide: () => (show = false),
    freeSpinCounterUpdate: (emitterEvent) => {
      if (emitterEvent.current !== undefined) current = emitterEvent.current;
      if (emitterEvent.total  !== undefined) total  = emitterEvent.total;
    },
  });
</script>

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broadcast()

Synchronously dispatches an event to all current subscribers. Use this for fire-and-forget updates (show/hide a component, update a counter). 
const broadcast = (emitterEvent: TEmitterEvent) => void

eventEmitter.broadcast({ type: 'freeSpinCounterShow' });
eventEmitter.broadcast({ type: 'freeSpinCounterUpdate', current: 3, total: 10 });
Internally it iterates the subscription Set and calls each handler synchronously: 
const broadcast = (emitterEvent: TEmitterEvent) => {
  subscriptions.forEach((emitterEventHandler) => {
    emitterEventHandler(emitterEvent);
  });
};

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broadcastAsync()

Dispatches an event to all subscribers and returns Promise.all of every handler’s return value. Use await eventEmitter.broadcastAsync(...) when you need to wait for all animations or async operations triggered by the event to finish. 
const broadcastAsync = (emitterEvent: TEmitterEvent) => Promise<any[]>

// bookEventHandlerMap.ts
await eventEmitter.broadcastAsync({
  type: 'freeSpinIntroUpdate',
  totalFreeSpins: bookEvent.totalFs,
});

<!-- FreeSpinIntro.svelte -->
<script lang="ts">
  context.eventEmitter.subscribeOnMount({
    freeSpinIntroUpdate: async (emitterEvent) => {
      freeSpinsFromEvent = emitterEvent.totalFreeSpins;
      // The broadcastAsync caller waits until this resolves
      await waitForResolve((resolve) => (oncomplete = resolve));
    },
  });
</script>

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ContextEventEmitter

utils-event-emitter also exports context helpers so eventEmitter can be read by any descendant component without prop drilling: 
import {
  setContextEventEmitter,
  getContextEventEmitter,
} from 'utils-event-emitter';

// context.ts (context key: '@@eventEmitter')
export function setContextEventEmitter<TEmitterEvent extends EmitterEventBase>(
  value: ContextEventEmitter<TEmitterEvent>,
): void

export function getContextEventEmitter<TEmitterEvent extends EmitterEventBase>(): 
  ContextEventEmitter<TEmitterEvent>
setContextEventEmitter must be called at the entry level of the app (same place as the other setContext* calls).

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Typical setup pattern

// apps/lines/src/game/eventEmitter.ts
import { createEventEmitter } from 'utils-event-emitter';
import type { EmitterEventUi } from 'components-ui-pixi';
import type { EmitterEventGame } from './typesEmitterEvent';

export type EmitterEvent = EmitterEventUi | EmitterEventGame;
export const { eventEmitter } = createEventEmitter<EmitterEvent>();

// apps/lines/src/game/context.ts
import { setContextEventEmitter } from 'utils-event-emitter';
import { eventEmitter } from './eventEmitter';

export const setContext = () => {
  setContextEventEmitter<EmitterEvent>({ eventEmitter });
  // ... other contexts
};

<!-- Any child component -->
<script lang="ts">
  import { getContextEventEmitter } from 'utils-event-emitter';
  const { eventEmitter } = getContextEventEmitter();

  eventEmitter.subscribeOnMount({
    myEvent: (e) => { /* handle */ },
  });
</script>

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