add a setNetworkActivityIndicatorVisible to every react-native fetch operations.

fetch-with-loading.js

const globalFetch = global.fetch;

global.fetch = (url: string, params: Object): Promise<*> => {
  return new Promise((resolve: any => void, reject: string => void) => {
    StatusBar.setNetworkActivityIndicatorVisible(true);

    globalFetch(url, params)
      .then(resolve)
      .then(reject)
      .finaly(() => {
        StatusBar.setNetworkActivityIndicatorVisible(false);
      });
  });
};