ionic-framework/js/angular/service/delegateService.js

function delegateService(methodNames) {

  if (methodNames.indexOf('$getByHandle') > -1) {
    throw new Error("Method '$getByHandle' is implicitly added to each delegate service. Do not list it as a method.");
  }

  function trueFn() { return true; }

  return ['$log', function($log) {
    var delegate = this;

    var instances = this._instances = [];
    this._registerInstance = function(instance, handle, filterFn) {
      instance.$$delegateHandle = handle;
      instance.$$filterFn = filterFn || trueFn;
      instances.push(instance);

      return function deregister() {
        var index = instances.indexOf(instance);
        if (index !== -1) {
          instances.splice(index, 1);
        }
      };
    };

    this.$getByHandle = function(handle) {
      if (!handle) {
        return delegate;
      }
      return new InstanceForHandle(handle);
    };

    /*
     * Creates a new object that will have all the methodNames given,
     * and call them on the given the controller instance matching given
     * handle.
     * The reason we don't just let $getByHandle return the controller instance
     * itself is that the controller instance might not exist yet.
     *
     * We want people to be able to do
     * `var instance = $ionicScrollDelegate.$getByHandle('foo')` on controller
     * instantiation, but on controller instantiation a child directive
     * may not have been compiled yet!
     *
     * So this is our way of solving this problem: we create an object
     * that will only try to fetch the controller with given handle
     * once the methods are actually called.
     */
    function InstanceForHandle(handle) {
      this.handle = handle;
    }

    methodNames.forEach(function(methodName) {
      InstanceForHandle.prototype[methodName] = function() {
        var handle = this.handle;
        var args = arguments;
        var matchingInstancesFound = 0;
        var finalResult;
        var result;

        //This logic is repeated below; we could factor some of it out to a function
        //but don't because it lets this method be more performant (one loop versus 2)
        instances.forEach(function(instance) {
          if (instance.$$delegateHandle === handle && instance.$$filterFn(instance)) {
            matchingInstancesFound++;
            result = instance[methodName].apply(instance, args);
            //Only return the value from the first call
            if (matchingInstancesFound === 1) {
              finalResult = result;
            }
          }
        });

        if (!matchingInstancesFound) {
          return $log.warn(
            'Delegate for handle "' + this.handle + '" could not find a ' +
            'corresponding element with delegate-handle="' + this.handle + '"! ' +
            methodName + '() was not called!\n' +
            'Possible cause: If you are calling ' + methodName + '() immediately, and ' +
            'your element with delegate-handle="' + this.handle + '" is a child of your ' +
            'controller, then your element may not be compiled yet. Put a $timeout ' +
            'around your call to ' + methodName + '() and try again.'
          );
        }

        return finalResult;
      };

      delegate[methodName] = function() {
        var args = arguments;
        var matchingInstancesFound = 0;
        var finalResult;
        var result;

        //This logic is repeated above
        instances.forEach(function(instance, index) {
          if (instance.$$filterFn(instance)) {
            matchingInstancesFound++;
            result = instance[methodName].apply(instance, args);
            //Only return the value from the first call
            if (matchingInstancesFound === 1) {
              finalResult = result;
            }
          }
        });

        return finalResult;
      };

    });
  }];
}