fix(ngClass): ensure that ngClass only adds/removes the changed classes

ngClass works by removing all the former classes and then adding all the
new classes to the element during each watch change operation. This may
cause transition animations to never render. The ngClass directive will
now only add and remove the classes that change during each watch operation.

Closes #4960
Closes #4944

4 files changed, 44 insertions, 26 deletions

diff --git a/src/.jshintrc b/src/.jshintrc
index e29b09f3..2467d667 100644
--- a/src/.jshintrc
+++ b/src/.jshintrc
@@ -100,6 +100,7 @@
     "assertNotHasOwnProperty": false,
     "getter": false,
     "getBlockElements": false,
+    "tokenDifference": false,
 
     /* AngularPublic.js */
     "version": false,
@@ -162,4 +163,4 @@
     "nullFormCtrl": false
 
   }
-}
\ No newline at end of file
+}
diff --git a/src/Angular.js b/src/Angular.js
index 8409f971..b27f4b06 100644
--- a/src/Angular.js
+++ b/src/Angular.js
@@ -80,7 +80,8 @@
     -assertArgFn,
     -assertNotHasOwnProperty,
     -getter,
-    -getBlockElements
+    -getBlockElements,
+    -tokenDifference
 
 */
 
@@ -1350,3 +1351,24 @@ function getBlockElements(block) {
 
   return jqLite(elements);
 }
+
+/**
+ * Return the string difference between tokens of the original string compared to the old string
+ * @param {str1} string original string value
+ * @param {str2} string new string value
+ */
+function tokenDifference(str1, str2) {
+  var values = '',
+      tokens1 = str1.split(/\s+/),
+      tokens2 = str2.split(/\s+/);
+
+  outer:
+  for(var i=0;i<tokens1.length;i++) {
+    var token = tokens1[i];
+    for(var j=0;j<tokens2.length;j++) {
+      if(token == tokens2[j]) continue outer;
+    }
+    values += (values.length > 0 ? ' ' : '') + token;
+  }
+  return values;
+}
diff --git a/src/ng/compile.js b/src/ng/compile.js
index 4d83f379..ce3d0514 100644
--- a/src/ng/compile.js
+++ b/src/ng/compile.js
@@ -688,8 +688,8 @@ function $CompileProvider($provide) {
         if(key == 'class') {
           value = value || '';
           var current = this.$$element.attr('class') || '';
-          this.$removeClass(tokenDifference(current, value).join(' '));
-          this.$addClass(tokenDifference(value, current).join(' '));
+          this.$removeClass(tokenDifference(current, value));
+          this.$addClass(tokenDifference(value, current));
         } else {
           var booleanKey = getBooleanAttrName(this.$$element[0], key),
               normalizedVal,
@@ -747,22 +747,6 @@ function $CompileProvider($provide) {
             $exceptionHandler(e);
           }
         });
-
-        function tokenDifference(str1, str2) {
-          var values = [],
-              tokens1 = str1.split(/\s+/),
-              tokens2 = str2.split(/\s+/);
-
-          outer:
-          for(var i=0;i<tokens1.length;i++) {
-            var token = tokens1[i];
-            for(var j=0;j<tokens2.length;j++) {
-              if(token == tokens2[j]) continue outer;
-            }
-            values.push(token);
-          }
-          return values;
-        }
       },
 
 
diff --git a/src/ng/directive/ngClass.js b/src/ng/directive/ngClass.js
index a0d23a28..10ef7fd1 100644
--- a/src/ng/directive/ngClass.js
+++ b/src/ng/directive/ngClass.js
@@ -21,9 +21,9 @@ function classDirective(name, selector) {
             var mod = $index & 1;
             if (mod !== old$index & 1) {
               if (mod === selector) {
-                addClass(scope.$eval(attr[name]));
+                addClass(flattenClasses(scope.$eval(attr[name])));
               } else {
-                removeClass(scope.$eval(attr[name]));
+                removeClass(flattenClasses(scope.$eval(attr[name])));
               }
             }
           });
@@ -32,22 +32,33 @@ function classDirective(name, selector) {
 
         function ngClassWatchAction(newVal) {
           if (selector === true || scope.$index % 2 === selector) {
+            var newClasses = flattenClasses(newVal || '');
             if (oldVal && !equals(newVal,oldVal)) {
-              removeClass(oldVal);
+              var oldClasses = flattenClasses(oldVal);
+              var toRemove = tokenDifference(oldClasses, newClasses);
+              if(toRemove.length > 0) {
+                removeClass(toRemove);
+              }
+
+              var toAdd = tokenDifference(newClasses, oldClasses);
+              if(toAdd.length > 0) {
+                addClass(toAdd);
+              }
+            } else {
+              addClass(newClasses);
             }
-            addClass(newVal);
           }
           oldVal = copy(newVal);
         }
 
 
         function removeClass(classVal) {
-          attr.$removeClass(flattenClasses(classVal));
+          attr.$removeClass(classVal);
         }
 
 
         function addClass(classVal) {
-          attr.$addClass(flattenClasses(classVal));
+          attr.$addClass(classVal);
         }
 
         function flattenClasses(classVal) {