Resource-shadow NPM

ResourceShadow

keep a synchronized copy of a remote http resource. Modify freely and changes will be synced in the background automatically.

modify the local copy without blocking I/O (don't make users wait for a server response)
localStorage is used to keep realtime sync with other frames/tabs/windows in same domain
saving/loading to remote http server is done in the background
works in offline mode (only localStorage), switch to online mode (http) at any time
custom 3-way merge is supported (used when both local and server versions were modified)
retries (with a delay) on timeout or other network errors

Usage

  // will load it from localStorage if exists
  var preferences = resourceShadow.create({ localKey: 'preferences'});

  // set initial value
  preferences.apply({ color = 'white' });

  // make changes offline using .apply (no async callbacks!)
  preferences.apply(function(data){
    data.color = 'red';
  });

  // change was immediately persisted to localStorage

  // go online
  preferences.goOnline('http://myhost/user/' + currentUser.id + '/preferences').load();

  // value from server is loading

  // you can make changes without waiting
  // just beware that might trigger a 3-way merge (by default local wins)
  preferences.apply(function(data){
    data.color = 'blue';
  });

  // you can wait using events
  preferences.once('loaded', function(data){
    data.color = 'blue';
  });

  // you can load server-side changes at any time
  setTimeout(function(){
    preferences.load();
  }, 3000);


  // custom 3-way merge
  preferences.options.threeWayMerge = function(originalJson, serverJson, localJson) {
    // using a naive object properties merge (check lodash .merge)
    return _.merge({},
      JSON.parse(originalJson),
      JSON.parse(serverJson),
      JSON.parse(localJson));
  });

  // using load and "rebase"
  var preferences = resourceShadow.create({ localKey: 'preferences2'});
  preferences.apply({ someInitialLocal: 'values' });

  // will load server value, and apply local value on top of it (using 3 way merge)
  // the result will be as if local version were created *after* (or on top) of server version
  preferences.loadAndRebase();

  // reset to initial state (and go offline)
  preferences.reset();

  // listen for changes
  preferences.on('change', function(data, info){
    if (info.source === 'localStorageEvent') {
      console.log('a change arrived from another tab/frame/window');
    } else if (info.source === 'apply') {
      console.log('a change was just applied locally');
    } else if (info.source === 'server') {
      console.log('a change was loaded from server');
    } else if (info.source === 'localStorage') {
      console.log('a change was found at localStorage');
    }
  });

  preferences.on('saved', function(){
    console.log('changes were saved to the server');
  });