Method subscribe

packages/query-core/src/subscribable.ts:8–17

(listener: TListener)

  }

  subscribe(listener: TListener): () => void {
    this.listeners.add(listener)

    this.onSubscribe()

    return () => {
      this.listeners.delete(listener)
      this.onUnsubscribe()
    }
  }

  hasListeners(): boolean {
    return this.listeners.size > 0