Method subscribe

packages/query-core/src/subscribable.ts:27–36

* Registers a listener to be called on every update this object notifies about. Returns a function * that removes the listener again — call it to stop listening. The base class never drops a listener * on its own, though some subclasses clear all of theirs in `destroy()`. * @param listener

(listener: TListener)

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

    this.onSubscribe()

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

  /**
   * Returns `true` while at least one listener is registered, `false` once they have all unsubscribed.