import uniqueId from 'lodash/utility/uniqueId';
import { isFSA } from 'flux-standard-action';

function mergeMeta(action, meta) {
  return {...action.meta, ...meta};
}

export default function webWorkerMiddleware(scriptURL) {
  let dispatch = null;
  const worker = new Worker(scriptURL);

  worker.onmessage = (e) => {
    if (dispatch && isFSA(e.data)) {
      dispatch(e.data);
    }
  };

  return methods => {
    dispatch = methods.dispatch;

    return next => action => {
      if (!isFSA(action) || !action.meta || !action.meta.background) {
        return next(action);
      }

      const backgroundTaskId = uniqueId('bt');

      worker.postMessage({ 
        ...action, 
        meta: mergeMeta(action, { 
          backgroundTaskId, 
          background: false 
        })
      });

      next({ 
        ...action, 
        meta: mergeMeta(action, { 
          backgroundTaskId, 
          sequence: 'begin' 
        }) 
      });

      return backgroundTaskId;
    }
  }
}