gistfile1.js

// wedata utility for Greasemonkey
// usage
/*

// ==UserScript==
// @name           foo bar
// @namespace      http://baz.com
// @require        http://gist.github.com/raw/34615/ba15cad2dd736f32a6d7868ce6595ac1bd01c0a3
// ==/UserScript==

var DATABASE_URL = 'http://wedata.net/databases/XXX/items.json';
var database = new Wedata.Database();

database.get(DATABASE_URL, function(items) {
  items.forEach(function(item) {
    // do something
  });
});

// clear cache
GM_registerMenuCommand('XXX - clear cache', function() {
  Wedata.Cache.set(DATABASE_URL, null, 0);
});

*/

var Wedata = {};

Wedata.Database = function() {
  this.items = [];
  this.expires = 24 * 60 * 60 * 1000; // 1 day
};

Wedata.Database.prototype.get = function(url, callback) { 
  var self = this;
  var cacheInfo;

  if(cacheInfo = Wedata.Cache.get(url)) {
    self.items = cacheInfo;
    callback(self.items);
  } else {
    GM_xmlhttpRequest({
      method : "GET",
      url : url,
      onload : function(res) {
        self.items = eval('(' + res.responseText + ')');
        callback(self.items);
        
        Wedata.Cache.set(url, self.items, self.expires);
      }
    });
  }
};

Wedata.Cache = {};

Wedata.Cache.set = function(key, value, expire) {
  var expire = new Date().getTime() + expire;
  GM_setValue(key, uneval({ value: value, expire: expire }));
};

Wedata.Cache.get = function(key) {
  var cached = eval(GM_getValue(key));
  if(!cached) {
    return;
  }
  
  if(cached.expire > new Date().getTime()) {
    return cached.value;
  }

  return;
};