solr/server/solr-webapp/webapp/js/lib/jquery.sammy.js
function(callback) {
      if (!_isFunction(callback)) {
        // if a string is passed to then, assume we want to call
        // a helper on the event context in its context
        if (typeof callback === 'string' && callback in this.event_context) {
          var helper = this.event_context[callback];
          callback = function(content) {
            return helper.apply(this.event_context, [content]);
          };
        } else {
          return this;
        }
      }
      var context = this;
      if (this.waiting) {
        this.callbacks.push(callback);
      } else {
        this.wait();
        window.setTimeout(function() {
          var returned = callback.apply(context, [context.content, context.previous_content]);
          if (returned !== false) {
            context.next(returned);
          }
        }, 13);
      }
      return this;
    }
function(callback) {
      if (!_isFunction(callback)) {
        // if a string is passed to then, assume we want to call
        // a helper on the event context in its context
        if (typeof callback === 'string' && callback in this.event_context) {
          var helper = this.event_context[callback];
          callback = function(content) {
            return helper.apply(this.event_context, [content]);
          };
        } else {
          return this;
        }
      }
      var context = this;
      if (this.waiting) {
        this.callbacks.push(callback);
      } else {
        this.wait();
        window.setTimeout(function() {
          var returned = callback.apply(context, [context.content, context.previous_content]);
          if (returned !== false) {
            context.next(returned);
          }
        }, 13);
      }
      return this;
    }
The "core" of the `RenderContext` object, adds the `callback` to the queue. If the context is `waiting` (meaning an async operation is happening) then the callback will be executed in order, once the other operations are complete. If there is no currently executing operation, the `callback` is executed immediately.  The value returned from the callback is stored in `content` for the subsiquent operation. If you return `false`, the queue will pause, and the next callback in the queue will not be executed until `next()` is called. This allows for the guarunteed order of execution while working with async operations.  If then() is passed a string instead of a function, the string is looked up as a helper method on the event context.  ### Example  this.get('#/', function() { // initialize the RenderContext // Even though `load()` executes async, the next `then()` // wont execute until the load finishes this.load('myfile.txt') .then(function(content) { // the first argument to then is the content of the // prev operation $('#main').html(content); }); });