examples/weather/angular.js
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);
      }
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#$watchCollection
@methodOf ng.$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, removing,
and moving items belonging to an object or array.


# 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, the internal watch operation is terminated.