/* Augments the WebExtension storage API to allow listening for changes on a 
 * single variable of a single storage area (i.e. local or sync).
 *
 * Usage example:
 *   const storage = listenableStorage(browser.storage)
 *   storage.local.onChanged('someVar', newValue => { .... })
 */

export default storage => {
  const mkOnChange = storageName => {
    return (key, cb) => {
      const listener = (changes, areaName) => {
        if (changes[key]) {
          const newValue = changes[key].newValue
          cb(newValue)
        }
      }
      storage.onChanged.addListener(listener)
    }
  }

  return storage && ({
    ...storage,
    local: storage.local && { ...storage.local, onChanged: mkOnChange('local') },
    sync: storage.sync && { ...storage.sync, onChanged: mkOnChange('sync') },
  })
}