rootScope.js

'use strict';

/**
 * DESIGN NOTES
 *
 * The design decisions behind the scope are heavily favored for speed and memory consumption.
 *
 * The typical use of scope is to watch the expressions, which most of the time return the same
 * value as last time so we optimize the operation.
 *
 * Closures construction is expensive in terms of speed as well as memory:
 *   - No closures, instead use prototypical inheritance for API
 *   - Internal state needs to be stored on scope directly, which means that private state is
 *     exposed as $$____ properties
 *
 * Loop operations are optimized by using while(count--) { ... }
 *   - this means that in order to keep the same order of execution as addition we have to add
 *     items to the array at the beginning (shift) instead of at the end (push)
 *
 * Child scopes are created and removed often
 *   - Using an array would be slow since inserts in middle are expensive so we use linked list
 *
 * There are few watches then a lot of observers. This is why you don't want the observer to be
 * implemented in the same way as watch. Watch requires return of initialization function which
 * are expensive to construct.
 */


/**
 * @ngdoc object
 * @name ng.$rootScopeProvider
 * @description
 *
 * Provider for the $rootScope service.
 */

/**
 * @ngdoc function
 * @name ng.$rootScopeProvider#digestTtl
 * @methodOf ng.$rootScopeProvider
 * @description
 *
 * Sets the number of digest iterations the scope should attempt to execute before giving up and
 * assuming that the model is unstable.
 *
 * The current default is 10 iterations.
 *
 * @param {number} limit The number of digest iterations.
 */


/**
 * @ngdoc object
 * @name ng.$rootScope
 * @description
 *
 * Every application has a single root {@link ng.$rootScope.Scope scope}.
 * All other scopes are child scopes of the root scope. Scopes provide mechanism for watching the model and provide
 * event processing life-cycle. See {@link guide/scope developer guide on scopes}.
 */
function $RootScopeProvider(){
  var TTL = 10;

  this.digestTtl = function(value) {
    if (arguments.length) {
      TTL = value;
    }
    return TTL;
  };

  this.$get = ['$injector', '$exceptionHandler', '$parse',
      function( $injector,   $exceptionHandler,   $parse) {

    /**
     * @ngdoc function
     * @name ng.$rootScope.Scope
     *
     * @description
     * A root scope can be retrieved using the {@link ng.$rootScope $rootScope} key from the
     * {@link AUTO.$injector $injector}. Child scopes are created using the
     * {@link ng.$rootScope.Scope#$new $new()} method. (Most scopes are created automatically when
     * compiled HTML template is executed.)
     *
     * Here is a simple scope snippet to show how you can interact with the scope.
     * <pre>
     * <file src="./test/ng/rootScopeSpec.js" tag="docs1" />
     * </pre>
     *
     * # Inheritance
     * A scope can inherit from a parent scope, as in this example:
     * <pre>
         var parent = $rootScope;
         var child = parent.$new();

         parent.salutation = "Hello";
         child.name = "World";
         expect(child.salutation).toEqual('Hello');

         child.salutation = "Welcome";
         expect(child.salutation).toEqual('Welcome');
         expect(parent.salutation).toEqual('Hello');
     * </pre>
     *
     *
     * @param {Object.<string, function()>=} providers Map of service factory which need to be provided
     *     for the current scope. Defaults to {@link ng}.
     * @param {Object.<string, *>=} instanceCache Provides pre-instantiated services which should
     *     append/override services provided by `providers`. This is handy when unit-testing and having
     *     the need to override a default service.
     * @returns {Object} Newly created scope.
     *
     */
    function Scope() {
      this.$id = nextUid();
      this.$$phase = this.$parent = this.$$watchers =
                     this.$$nextSibling = this.$$prevSibling =
                     this.$$childHead = this.$$childTail = null;
      this['this'] = this.$root =  this;
      this.$$destroyed = false;
      this.$$asyncQueue = [];
      this.$$listeners = {};
      this.$$isolateBindings = {};
    }

    /**
     * @ngdoc property
     * @name ng.$rootScope.Scope#$id
     * @propertyOf ng.$rootScope.Scope
     * @returns {number} Unique scope ID (monotonically increasing alphanumeric sequence) useful for
     *   debugging.
     */


    Scope.prototype = {
      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$new
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Creates a new child {@link ng.$rootScope.Scope scope}.
       *
       * The parent scope will propagate the {@link ng.$rootScope.Scope#$digest $digest()} and
       * {@link ng.$rootScope.Scope#$digest $digest()} events. The scope can be removed from the scope
       * hierarchy using {@link ng.$rootScope.Scope#$destroy $destroy()}.
       *
       * {@link ng.$rootScope.Scope#$destroy $destroy()} must be called on a scope when it is desired for
       * the scope and its child scopes to be permanently detached from the parent and thus stop
       * participating in model change detection and listener notification by invoking.
       *
       * @param {boolean} isolate if true then the scope does not prototypically inherit from the
       *         parent scope. The scope is isolated, as it can not see parent scope properties.
       *         When creating widgets it is useful for the widget to not accidentally read parent
       *         state.
       *
       * @returns {Object} The newly created child scope.
       *
       */
      $new: function(isolate) {
        var Child,
            child;

        if (isolate) {
          child = new Scope();
          child.$root = this.$root;
        } else {
          Child = function() {}; // should be anonymous; This is so that when the minifier munges
            // the name it does not become random set of chars. These will then show up as class
            // name in the debugger.
          Child.prototype = this;
          child = new Child();
          child.$id = nextUid();
        }
        child['this'] = child;
        child.$$listeners = {};
        child.$parent = this;
        child.$$watchers = child.$$nextSibling = child.$$childHead = child.$$childTail = null;
        child.$$prevSibling = this.$$childTail;
        if (this.$$childHead) {
          this.$$childTail.$$nextSibling = child;
          this.$$childTail = child;
        } else {
          this.$$childHead = this.$$childTail = child;
        }
        return child;
      },

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$watch
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Registers a `listener` callback to be executed whenever the `watchExpression` changes.
       *
       * - The `watchExpression` is called on every call to {@link ng.$rootScope.Scope#$digest $digest()} and
       *   should return the value which will be watched. (Since {@link ng.$rootScope.Scope#$digest $digest()}
       *   reruns when it detects changes the `watchExpression` can execute multiple times per
       *   {@link ng.$rootScope.Scope#$digest $digest()} and should be idempotent.)
       * - The `listener` is called only when the value from the current `watchExpression` and the
       *   previous call to `watchExpression` are not equal (with the exception of the initial run,
       *   see below). The inequality is determined according to
       *   {@link angular.equals} function. To save the value of the object for later comparison, the
       *   {@link angular.copy} function is used. It also means that watching complex options will
       *   have adverse memory and performance implications.
       * - The watch `listener` may change the model, which may trigger other `listener`s to fire. This
       *   is achieved by rerunning the watchers until no changes are detected. The rerun iteration
       *   limit is 10 to prevent an infinite loop deadlock.
       *
       *
       * If you want to be notified whenever {@link ng.$rootScope.Scope#$digest $digest} is called,
       * you can register a `watchExpression` function with no `listener`. (Since `watchExpression`
       * can execute multiple times per {@link ng.$rootScope.Scope#$digest $digest} cycle when a change is
       * detected, be prepared for multiple calls to your listener.)
       *
       * After a watcher is registered with the scope, the `listener` fn is called asynchronously
       * (via {@link ng.$rootScope.Scope#$evalAsync $evalAsync}) to initialize the
       * watcher. In rare cases, this is undesirable because the listener is called when the result
       * of `watchExpression` didn't change. To detect this scenario within the `listener` fn, you
       * can compare the `newVal` and `oldVal`. If these two values are identical (`===`) then the
       * listener was called due to initialization.
       *
       *
       * # Example
       * <pre>
           // let's assume that scope was dependency injected as the $rootScope
           var scope = $rootScope;
           scope.name = 'misko';
           scope.counter = 0;

           expect(scope.counter).toEqual(0);
           scope.$watch('name', function(newValue, oldValue) { scope.counter = scope.counter + 1; });
           expect(scope.counter).toEqual(0);

           scope.$digest();
           // no variable change
           expect(scope.counter).toEqual(0);

           scope.name = 'adam';
           scope.$digest();
           expect(scope.counter).toEqual(1);
       * </pre>
       *
       *
       *
       * @param {(function()|string)} watchExpression Expression that is evaluated on each
       *    {@link ng.$rootScope.Scope#$digest $digest} cycle. A change in the return value triggers a
       *    call to the `listener`.
       *
       *    - `string`: Evaluated as {@link guide/expression expression}
       *    - `function(scope)`: called with current `scope` as a parameter.
       * @param {(function()|string)=} listener Callback called whenever the return value of
       *   the `watchExpression` changes.
       *
       *    - `string`: Evaluated as {@link guide/expression expression}
       *    - `function(newValue, oldValue, scope)`: called with current and previous values as parameters.
       *
       * @param {boolean=|function()} objectEquality Compare object for equality rather than for reference, or supply with comparator function.
       * @returns {function()} Returns a deregistration function for this listener.
       */
      $watch: function(watchExp, listener, objectEquality) {
        var equalityComparator;
        if(isFunction(objectEquality)){
            equalityComparator = objectEquality;
        }
        var scope = this,
            get = compileToFn(watchExp, 'watch'),
            array = scope.$$watchers,
            watcher = {
              fn: listener,
              last: initWatchVal,
              get: get,
              exp: watchExp,
              eq: equalityComparator || !!objectEquality
            };

        // in the case user pass string, we need to compile it, do we really need this ?
        if (!isFunction(listener)) {
          var listenFn = compileToFn(listener || noop, 'listener');
          watcher.fn = function(newVal, oldVal, scope) {listenFn(scope);};
        }

        if (typeof watchExp == 'string' && get.constant) {
          var originalFn = watcher.fn;
          watcher.fn = function(newVal, oldVal, scope) {
            originalFn.call(this, newVal, oldVal, scope);
            arrayRemove(array, watcher);
          };
        }

        if (!array) {
          array = scope.$$watchers = [];
        }
        // we use unshift since we use a while loop in $digest for speed.
        // the while loop reads in reverse order.
        array.unshift(watcher);

        return function() {
          arrayRemove(array, watcher);
        };
      },


      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$watchCollection
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Shallow watches the properties of an object and fires whenever any of the properties change
       * (for arrays this implies watching the array items, for object maps this implies watching the properties).
       * If a change is detected the `listener` callback is fired.
       *
       * - The `obj` collection is observed via standard $watch operation and is examined on every call to $digest() to
       *   see if any items have been added, removed, or moved.
       * - The `listener` is called whenever anything within the `obj` has changed. Examples include adding new items
       *   into the object or array, removing and moving items around.
       *
       *
       * # Example
       * <pre>
          $scope.names = ['igor', 'matias', 'misko', 'james'];
          $scope.dataCount = 4;

          $scope.$watchCollection('names', function(newNames, oldNames) {
            $scope.dataCount = newNames.length;
          });

          expect($scope.dataCount).toEqual(4);
          $scope.$digest();

          //still at 4 ... no changes
          expect($scope.dataCount).toEqual(4);

          $scope.names.pop();
          $scope.$digest();

          //now there's been a change
          expect($scope.dataCount).toEqual(3);
       * </pre>
       *
       *
       * @param {string|Function(scope)} obj Evaluated as {@link guide/expression expression}. The expression value
       *    should evaluate to an object or an array which is observed on each
       *    {@link ng.$rootScope.Scope#$digest $digest} cycle. Any shallow change within the collection will trigger
       *    a call to the `listener`.
       *
       * @param {function(newCollection, oldCollection, scope)} listener a callback function that is fired with both
       *    the `newCollection` and `oldCollection` as parameters.
       *    The `newCollection` object is the newly modified data obtained from the `obj` expression and the
       *    `oldCollection` object is a copy of the former collection data.
       *    The `scope` refers to the current scope.
       *
       * @returns {function()} Returns a de-registration function for this listener. When the de-registration function is executed
       * then the internal watch operation is terminated.
       */
      $watchCollection: function(obj, listener) {
        var self = this;
        var oldValue;
        var newValue;
        var changeDetected = 0;
        var objGetter = $parse(obj);
        var internalArray = [];
        var internalObject = {};
        var oldLength = 0;

        function $watchCollectionWatch() {
          newValue = objGetter(self);
          var newLength, key;

          if (!isObject(newValue)) {
            if (oldValue !== newValue) {
              oldValue = newValue;
              changeDetected++;
            }
          } else if (isArrayLike(newValue)) {
            if (oldValue !== internalArray) {
              // we are transitioning from something which was not an array into array.
              oldValue = internalArray;
              oldLength = oldValue.length = 0;
              changeDetected++;
            }

            newLength = newValue.length;

            if (oldLength !== newLength) {
              // if lengths do not match we need to trigger change notification
              changeDetected++;
              oldValue.length = oldLength = newLength;
            }
            // copy the items to oldValue and look for changes.
            for (var i = 0; i < newLength; i++) {
              if (oldValue[i] !== newValue[i]) {
                changeDetected++;
                oldValue[i] = newValue[i];
              }
            }
          } else {
            if (oldValue !== internalObject) {
              // we are transitioning from something which was not an object into object.
              oldValue = internalObject = {};
              oldLength = 0;
              changeDetected++;
            }
            // copy the items to oldValue and look for changes.
            newLength = 0;
            for (key in newValue) {
              if (newValue.hasOwnProperty(key)) {
                newLength++;
                if (oldValue.hasOwnProperty(key)) {
                  if (oldValue[key] !== newValue[key]) {
                    changeDetected++;
                    oldValue[key] = newValue[key];
                  }
                } else {
                  oldLength++;
                  oldValue[key] = newValue[key];
                  changeDetected++;
                }
              }
            }
            if (oldLength > newLength) {
              // we used to have more keys, need to find them and destroy them.
              changeDetected++;
              for(key in oldValue) {
                if (oldValue.hasOwnProperty(key) && !newValue.hasOwnProperty(key)) {
                  oldLength--;
                  delete oldValue[key];
                }
              }
            }
          }
          return changeDetected;
        }

        function $watchCollectionAction() {
          listener(newValue, oldValue, self);
        }

        return this.$watch($watchCollectionWatch, $watchCollectionAction);
      },

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$digest
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Processes all of the {@link ng.$rootScope.Scope#$watch watchers} of the current scope and its children.
       * Because a {@link ng.$rootScope.Scope#$watch watcher}'s listener can change the model, the
       * `$digest()` keeps calling the {@link ng.$rootScope.Scope#$watch watchers} until no more listeners are
       * firing. This means that it is possible to get into an infinite loop. This function will throw
       * `'Maximum iteration limit exceeded.'` if the number of iterations exceeds 10.
       *
       * Usually you don't call `$digest()` directly in
       * {@link ng.directive:ngController controllers} or in
       * {@link ng.$compileProvider#directive directives}.
       * Instead a call to {@link ng.$rootScope.Scope#$apply $apply()} (typically from within a
       * {@link ng.$compileProvider#directive directives}) will force a `$digest()`.
       *
       * If you want to be notified whenever `$digest()` is called,
       * you can register a `watchExpression` function  with {@link ng.$rootScope.Scope#$watch $watch()}
       * with no `listener`.
       *
       * You may have a need to call `$digest()` from within unit-tests, to simulate the scope
       * life-cycle.
       *
       * # Example
       * <pre>
           var scope = ...;
           scope.name = 'misko';
           scope.counter = 0;

           expect(scope.counter).toEqual(0);
           scope.$watch('name', function(newValue, oldValue) {
             scope.counter = scope.counter + 1;
           });
           expect(scope.counter).toEqual(0);

           scope.$digest();
           // no variable change
           expect(scope.counter).toEqual(0);

           scope.name = 'adam';
           scope.$digest();
           expect(scope.counter).toEqual(1);
       * </pre>
       *
       */
      $digest: function() {
        var watch, value, last, equalityFunc,
            watchers,
            asyncQueue = this.$$asyncQueue,
            length,
            dirty, ttl = TTL,
            next, current, target = this,
            watchLog = [],
            logIdx, logMsg;

        beginPhase('$digest');

        do { // "while dirty" loop
          dirty = false;
          current = target;

          while(asyncQueue.length) {
            try {
              current.$eval(asyncQueue.shift());
            } catch (e) {
              $exceptionHandler(e);
            }
          }

          do { // "traverse the scopes" loop
            if ((watchers = current.$$watchers)) {
              // process our watches
              length = watchers.length;
              while (length--) {
                try {
                  watch = watchers[length];
                  // If the watch provided with equality function, use that, else use default.
                  if(isFunction(watch.eq))
                      equalityFunc = watch.eq;
                  else
                      equalityFunc = equals;
                  // Most common watches are on primitives, in which case we can short
                  // circuit it with === operator, only when === fails do we use .equals
                  if ((value = watch.get(current)) !== (last = watch.last) &&
                      !(!!watch.eq
                          ? equalityFunc(value, last)
                          : (typeof value == 'number' && typeof last == 'number'
                             && isNaN(value) && isNaN(last)))) {
                    dirty = true;
                    watch.last = watch.eq ? copy(value) : value;
                    watch.fn(value, ((last === initWatchVal) ? value : last), current);
                    if (ttl < 5) {
                      logIdx = 4 - ttl;
                      if (!watchLog[logIdx]) watchLog[logIdx] = [];
                      logMsg = (isFunction(watch.exp))
                          ? 'fn: ' + (watch.exp.name || watch.exp.toString())
                          : watch.exp;
                      logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);
                      watchLog[logIdx].push(logMsg);
                    }
                  }
                } catch (e) {
                  $exceptionHandler(e);
                }
              }
            }

            // Insanity Warning: scope depth-first traversal
            // yes, this code is a bit crazy, but it works and we have tests to prove it!
            // this piece should be kept in sync with the traversal in $broadcast
            if (!(next = (current.$$childHead || (current !== target && current.$$nextSibling)))) {
              while(current !== target && !(next = current.$$nextSibling)) {
                current = current.$parent;
              }
            }
          } while ((current = next));

          if(dirty && !(ttl--)) {
            clearPhase();
            throw ngError(27,
                '{0} $digest() iterations reached. Aborting!\nWatchers fired in the last 5 iterations: {1}',
                TTL, toJson(watchLog));
          }
        } while (dirty || asyncQueue.length);

        clearPhase();
      },


      /**
       * @ngdoc event
       * @name ng.$rootScope.Scope#$destroy
       * @eventOf ng.$rootScope.Scope
       * @eventType broadcast on scope being destroyed
       *
       * @description
       * Broadcasted when a scope and its children are being destroyed.
       */

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$destroy
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Removes the current scope (and all of its children) from the parent scope. Removal implies
       * that calls to {@link ng.$rootScope.Scope#$digest $digest()} will no longer
       * propagate to the current scope and its children. Removal also implies that the current
       * scope is eligible for garbage collection.
       *
       * The `$destroy()` is usually used by directives such as
       * {@link ng.directive:ngRepeat ngRepeat} for managing the
       * unrolling of the loop.
       *
       * Just before a scope is destroyed a `$destroy` event is broadcasted on this scope.
       * Application code can register a `$destroy` event handler that will give it chance to
       * perform any necessary cleanup.
       */
      $destroy: function() {
        // we can't destroy the root scope or a scope that has been already destroyed
        if ($rootScope == this || this.$$destroyed) return;
        var parent = this.$parent;

        this.$broadcast('$destroy');
        this.$$destroyed = true;

        if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
        if (parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;
        if (this.$$prevSibling) this.$$prevSibling.$$nextSibling = this.$$nextSibling;
        if (this.$$nextSibling) this.$$nextSibling.$$prevSibling = this.$$prevSibling;

        // This is bogus code that works around Chrome's GC leak
        // see: https://github.com/angular/angular.js/issues/1313#issuecomment-10378451
        this.$parent = this.$$nextSibling = this.$$prevSibling = this.$$childHead =
            this.$$childTail = null;
      },

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$eval
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Executes the `expression` on the current scope returning the result. Any exceptions in the
       * expression are propagated (uncaught). This is useful when evaluating Angular expressions.
       *
       * # Example
       * <pre>
           var scope = ng.$rootScope.Scope();
           scope.a = 1;
           scope.b = 2;

           expect(scope.$eval('a+b')).toEqual(3);
           expect(scope.$eval(function(scope){ return scope.a + scope.b; })).toEqual(3);
       * </pre>
       *
       * @param {(string|function())=} expression An angular expression to be executed.
       *
       *    - `string`: execute using the rules as defined in  {@link guide/expression expression}.
       *    - `function(scope)`: execute the function with the current `scope` parameter.
       *
       * @returns {*} The result of evaluating the expression.
       */
      $eval: function(expr, locals) {
        return $parse(expr)(this, locals);
      },

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$evalAsync
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Executes the expression on the current scope at a later point in time.
       *
       * The `$evalAsync` makes no guarantees as to when the `expression` will be executed, only that:
       *
       *   - it will execute in the current script execution context (before any DOM rendering).
       *   - at least one {@link ng.$rootScope.Scope#$digest $digest cycle} will be performed after
       *     `expression` execution.
       *
       * Any exceptions from the execution of the expression are forwarded to the
       * {@link ng.$exceptionHandler $exceptionHandler} service.
       *
       * @param {(string|function())=} expression An angular expression to be executed.
       *
       *    - `string`: execute using the rules as defined in  {@link guide/expression expression}.
       *    - `function(scope)`: execute the function with the current `scope` parameter.
       *
       */
      $evalAsync: function(expr) {
        this.$$asyncQueue.push(expr);
      },

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$apply
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * `$apply()` is used to execute an expression in angular from outside of the angular framework.
       * (For example from browser DOM events, setTimeout, XHR or third party libraries).
       * Because we are calling into the angular framework we need to perform proper scope life-cycle
       * of {@link ng.$exceptionHandler exception handling},
       * {@link ng.$rootScope.Scope#$digest executing watches}.
       *
       * ## Life cycle
       *
       * # Pseudo-Code of `$apply()`
       * <pre>
           function $apply(expr) {
             try {
               return $eval(expr);
             } catch (e) {
               $exceptionHandler(e);
             } finally {
               $root.$digest();
             }
           }
       * </pre>
       *
       *
       * Scope's `$apply()` method transitions through the following stages:
       *
       * 1. The {@link guide/expression expression} is executed using the
       *    {@link ng.$rootScope.Scope#$eval $eval()} method.
       * 2. Any exceptions from the execution of the expression are forwarded to the
       *    {@link ng.$exceptionHandler $exceptionHandler} service.
       * 3. The {@link ng.$rootScope.Scope#$watch watch} listeners are fired immediately after the expression
       *    was executed using the {@link ng.$rootScope.Scope#$digest $digest()} method.
       *
       *
       * @param {(string|function())=} exp An angular expression to be executed.
       *
       *    - `string`: execute using the rules as defined in {@link guide/expression expression}.
       *    - `function(scope)`: execute the function with current `scope` parameter.
       *
       * @returns {*} The result of evaluating the expression.
       */
      $apply: function(expr) {
        try {
          beginPhase('$apply');
          return this.$eval(expr);
        } catch (e) {
          $exceptionHandler(e);
        } finally {
          clearPhase();
          try {
            $rootScope.$digest();
          } catch (e) {
            $exceptionHandler(e);
            throw e;
          }
        }
      },

      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$on
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Listens on events of a given type. See {@link ng.$rootScope.Scope#$emit $emit} for discussion of
       * event life cycle.
       *
       * The event listener function format is: `function(event, args...)`. The `event` object
       * passed into the listener has the following attributes:
       *
       *   - `targetScope` - `{Scope}`: the scope on which the event was `$emit`-ed or `$broadcast`-ed.
       *   - `currentScope` - `{Scope}`: the current scope which is handling the event.
       *   - `name` - `{string}`: Name of the event.
       *   - `stopPropagation` - `{function=}`: calling `stopPropagation` function will cancel further event
       *     propagation (available only for events that were `$emit`-ed).
       *   - `preventDefault` - `{function}`: calling `preventDefault` sets `defaultPrevented` flag to true.
       *   - `defaultPrevented` - `{boolean}`: true if `preventDefault` was called.
       *
       * @param {string} name Event name to listen on.
       * @param {function(event, args...)} listener Function to call when the event is emitted.
       * @returns {function()} Returns a deregistration function for this listener.
       */
      $on: function(name, listener) {
        var namedListeners = this.$$listeners[name];
        if (!namedListeners) {
          this.$$listeners[name] = namedListeners = [];
        }
        namedListeners.push(listener);

        return function() {
          namedListeners[indexOf(namedListeners, listener)] = null;
        };
      },


      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$emit
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Dispatches an event `name` upwards through the scope hierarchy notifying the
       * registered {@link ng.$rootScope.Scope#$on} listeners.
       *
       * The event life cycle starts at the scope on which `$emit` was called. All
       * {@link ng.$rootScope.Scope#$on listeners} listening for `name` event on this scope get notified.
       * Afterwards, the event traverses upwards toward the root scope and calls all registered
       * listeners along the way. The event will stop propagating if one of the listeners cancels it.
       *
       * Any exception emitted from the {@link ng.$rootScope.Scope#$on listeners} will be passed
       * onto the {@link ng.$exceptionHandler $exceptionHandler} service.
       *
       * @param {string} name Event name to emit.
       * @param {...*} args Optional set of arguments which will be passed onto the event listeners.
       * @return {Object} Event object, see {@link ng.$rootScope.Scope#$on}
       */
      $emit: function(name, args) {
        var empty = [],
            namedListeners,
            scope = this,
            stopPropagation = false,
            event = {
              name: name,
              targetScope: scope,
              stopPropagation: function() {stopPropagation = true;},
              preventDefault: function() {
                event.defaultPrevented = true;
              },
              defaultPrevented: false
            },
            listenerArgs = concat([event], arguments, 1),
            i, length;

        do {
          namedListeners = scope.$$listeners[name] || empty;
          event.currentScope = scope;
          for (i=0, length=namedListeners.length; i<length; i++) {

            // if listeners were deregistered, defragment the array
            if (!namedListeners[i]) {
              namedListeners.splice(i, 1);
              i--;
              length--;
              continue;
            }
            try {
              namedListeners[i].apply(null, listenerArgs);
              if (stopPropagation) return event;
            } catch (e) {
              $exceptionHandler(e);
            }
          }
          //traverse upwards
          scope = scope.$parent;
        } while (scope);

        return event;
      },


      /**
       * @ngdoc function
       * @name ng.$rootScope.Scope#$broadcast
       * @methodOf ng.$rootScope.Scope
       * @function
       *
       * @description
       * Dispatches an event `name` downwards to all child scopes (and their children) notifying the
       * registered {@link ng.$rootScope.Scope#$on} listeners.
       *
       * The event life cycle starts at the scope on which `$broadcast` was called. All
       * {@link ng.$rootScope.Scope#$on listeners} listening for `name` event on this scope get notified.
       * Afterwards, the event propagates to all direct and indirect scopes of the current scope and
       * calls all registered listeners along the way. The event cannot be canceled.
       *
       * Any exception emitted from the {@link ng.$rootScope.Scope#$on listeners} will be passed
       * onto the {@link ng.$exceptionHandler $exceptionHandler} service.
       *
       * @param {string} name Event name to broadcast.
       * @param {...*} args Optional set of arguments which will be passed onto the event listeners.
       * @return {Object} Event object, see {@link ng.$rootScope.Scope#$on}
       */
      $broadcast: function(name, args) {
        var target = this,
            current = target,
            next = target,
            event = {
              name: name,
              targetScope: target,
              preventDefault: function() {
                event.defaultPrevented = true;
              },
              defaultPrevented: false
            },
            listenerArgs = concat([event], arguments, 1),
            listeners, i, length;

        //down while you can, then up and next sibling or up and next sibling until back at root
        do {
          current = next;
          event.currentScope = current;
          listeners = current.$$listeners[name] || [];
          for (i=0, length = listeners.length; i<length; i++) {
            // if listeners were deregistered, defragment the array
            if (!listeners[i]) {
              listeners.splice(i, 1);
              i--;
              length--;
              continue;
            }

            try {
              listeners[i].apply(null, listenerArgs);
            } catch(e) {
              $exceptionHandler(e);
            }
          }

          // Insanity Warning: scope depth-first traversal
          // yes, this code is a bit crazy, but it works and we have tests to prove it!
          // this piece should be kept in sync with the traversal in $digest
          if (!(next = (current.$$childHead || (current !== target && current.$$nextSibling)))) {
            while(current !== target && !(next = current.$$nextSibling)) {
              current = current.$parent;
            }
          }
        } while ((current = next));

        return event;
      }
    };

    var $rootScope = new Scope();

    return $rootScope;


    function beginPhase(phase) {
      if ($rootScope.$$phase) {
        throw ngError(28, '{0} already in progress', $rootScope.$$phase);
      }

      $rootScope.$$phase = phase;
    }

    function clearPhase() {
      $rootScope.$$phase = null;
    }

    function compileToFn(exp, name) {
      var fn = $parse(exp);
      assertArgFn(fn, name);
      return fn;
    }

    /**
     * function used as an initial value for watchers.
     * because it's unique we can easily tell it apart from other values
     */
    function initWatchVal() {}
  }];
}