fix(rootScope): $watchCollection returns in oldCollection a copy of the former array data

related to angular#1751
When watching arrays, $watchCollection returned the new data both in the newCollection
and the oldCollection arguments of the listener

Author: Gonzalo Ruiz de Villa

src/ng/rootScope.js

@@ -362,6 +362,7 @@ function $RootScopeProvider(){
       $watchCollection: function(obj, listener) {
         var self = this;
         var oldValue;
+        var oldArray;
         var newValue;
         var changeDetected = 0;
         var objGetter = $parse(obj);
@@ -371,6 +372,7 @@ function $RootScopeProvider(){
 
         function $watchCollectionWatch() {
           newValue = objGetter(self);
+          oldArray = null;
           var newLength, key;
 
           if (!isObject(newValue)) {
@@ -386,6 +388,8 @@ function $RootScopeProvider(){
               changeDetected++;
             }
 
+            oldArray = oldValue.slice(0);
+
             newLength = newValue.length;
 
             if (oldLength !== newLength) {
@@ -439,7 +443,7 @@ function $RootScopeProvider(){
         }
 
         function $watchCollectionAction() {
-          listener(newValue, oldValue, self);
+          listener(newValue, oldArray || oldValue, self);
         }
 
         return this.$watch($watchCollectionWatch, $watchCollectionAction);

test/ng/rootScopeSpec.js

@@ -510,6 +510,28 @@ describe('Scope', function() {
           $rootScope.$digest();
           expect(arrayLikelog).toEqual(['x', 'y']);
         });
+
+        it('should return a new array with old values', function(){
+          var watchArgs;
+          $rootScope.$watchCollection('obj', function (newValues, oldValues) {
+            watchArgs = {
+              newValues: newValues,
+              oldValues: oldValues
+            };
+          });
+
+          $rootScope.obj = ['a'];
+          $rootScope.$digest();
+
+          expect(watchArgs.newValues).toEqual($rootScope.obj);
+          expect(watchArgs.oldValues).toEqual([]);
+
+          $rootScope.obj.push('b');
+          $rootScope.$digest();
+
+          expect(watchArgs.newValues).toEqual(['a', 'b']);
+          expect(watchArgs.oldValues).toEqual(['a']);
+        })
       });