code: function(callback) {
      if (typeof callback === 'function') {
        this.afterSync(callback);
      }
  
      if (this.pendingSync) {
        clearTimeout(this.pendingSync.pop());
        this.pendingSync.forEach(this.cancelQueue);
        this.pendingSync = null;
      }
  
      if (this.syncing) {
        this.rerun = true;
        return false;
      }
  
      this.runSync();
      return true;
    }
docs: Runs the observer sync cycle which checks all the observers to see if they've changed.