Added 'onJsonFetch' callback option

[Stanko]

I added onJsonFetch callback option, which will be called every time JSON file is fetched.

This will allow users to cache json files on file system and create gatsby-like system, where everything, including json data is static.

Relates to issue #221. I left a question there some time ago, and finally got some time to implement it.

Let me know what do you think.

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• onJsonFetch

  Called every time JSON file is fetched, with route and json passed to it.

  It needs to be a string, path to the file which contains a callback function. Function needs to be file's default export.

  Example usage in package json:

  "reactSnap": {
  "onJsonFetch": "./callbacks/on-json-fetch.js"
  }

  Example of the script:

  function onJsonFetch(route, json) {
  console.log('onJsonFetch callback', route, json);
  }
  
  module.exports = onJsonFetch;

[Stanko]

Just to mention that my team implemented this on the project we are working on, and it is working like a charm. We are caching all responses to ./data folder, and then using it in production.

We can differentiate between app running in headless and user's browser by checking user agent:

const isSnap = navigator.userAgent.toLowerCase().search('reactsnap') >= 0;

Then we switch to using cached data instead of the real API:

// In production, if app is not being crawled by react-snap
// we want to use locally cached API data instead of the real API
const useLocalData = !isSnap && process.env.NODE_ENV === 'production';

const API_URL = useLocalData ? '/data' : process.env.API_URL;