getLoadingOrNotReady() {
    const layerStatesArray = this.getLayerGroup().getLayerStatesArray();
    for (let i = 0, ii = layerStatesArray.length; i < ii; ++i) {
      const state = layerStatesArray[i];
      if (!state.visible) {
        continue;
      }
      const renderer = state.layer.getRenderer();
      if (renderer && !renderer.ready) {
        return true;
      }
      const source = state.layer.getSource();
      if (source && source.loading) {
        return true;
      }
    }
    return false;
  }
@return {boolean} Layers have sources that are still loading.