Method onSerialize

packages/core/src/transfer_state.ts:124–126

* Register a callback to provide the value for a key when `toJson` is called.

(key: StateKey<T>, callback: () => T)

   * Register a callback to provide the value for a key when `toJson` is called.
   */
  onSerialize<T>(key: StateKey<T>, callback: () => T): void {
    this.onSerializeCallbacks[key] = callback;
  }

  /**
   * Serialize the current state of the store to JSON.