add request loaded event

[spelkey-ucd]

Summary

• auto-registers additional event in store for each lru, with -request-loaded suffix.
• adds method to service that allows user to emit this event.

Example

class FooService extends BaseService {

  constructor() {
    super();
    this.store = FooStore;
    this.basePath = `${appConfig.apiRoot}/foo`;
  }

  async list(){
    let ido = {action: 'list'};
    let id = payload.getKey(ido);

    await this.checkRequesting(
      id, this.store.data.list,
      () => this.request({
        url : this.basePath,
        checkCached : () => this.store.data.list.get(id),
        onUpdate : resp => this.store.set(
          payload.generate(ido, resp),
          this.store.data.list
        )
      })
    );

    // new method 
    this.emitIfLoaded(id, this.store.data.list);

    return this.store.data.list.get(id);
  }


}

Element then can listen via _onFooListRequestLoaded method

Utility

This is mostly useful when doing code-splitting. If data are retrieved by an element in chunk A, and then chunk B is loaded that has an element that needs that data, it's _onFooListUpdate event won't fire since the data has already been set in the cache.

Although, maybe a better pattern would be to call the callback in the element constructor. e.g.

this._onFooListUpdate(this.FooModel.store.data.list.get(payloadUtils.getKey({action: 'list'}))