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 angular#4960

src/.jshintrc

@@ -100,6 +100,7 @@
     "assertNotHasOwnProperty": false,
     "getter": false,
     "getBlockElements": false,
+    "tokenDifference": false,
 
     /* AngularPublic.js */
     "version": false,
@@ -162,4 +163,4 @@
     "nullFormCtrl": false
 
   }
-}
+}

src/Angular.js

@@ -80,7 +80,8 @@
     -assertArgFn,
     -assertNotHasOwnProperty,
     -getter,
-    -getBlockElements
+    -getBlockElements,
+    -tokenDifference
 
 */
 
@@ -1338,3 +1339,24 @@ 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;
+}

src/ng/compile.js

@@ -677,8 +677,8 @@ 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,
@@ -736,22 +736,6 @@ 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;
-        }
       },
 
 

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 +32,34 @@ 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) {

test/ng/directive/ngClassSpec.js

@@ -411,4 +411,47 @@ describe('ngClass animations', function() {
       expect(enterComplete).toBe(true);
     });
   });
+
+  it("should not remove classes if they're going to be added back right after", function() {
+    module('mock.animate');
+
+    inject(function($rootScope, $compile, $animate) {
+      var className;
+
+      $rootScope.one = true;
+      $rootScope.two = true;
+      $rootScope.three = true;
+
+      var element = angular.element('<div ng-class="{one:one, two:two, three:three}"></div>');
+      $compile(element)($rootScope);
+      $rootScope.$digest();
+
+      //this fires twice due to the class observer firing
+      className = $animate.flushNext('addClass').params[1];
+      className = $animate.flushNext('addClass').params[1];
+      expect(className).toBe('one two three');
+
+      expect($animate.queue.length).toBe(0);
+
+      $rootScope.three = false;
+      $rootScope.$digest();
+
+      className = $animate.flushNext('removeClass').params[1];
+      expect(className).toBe('three');
+
+      expect($animate.queue.length).toBe(0);
+
+      $rootScope.two = false;
+      $rootScope.three = true;
+      $rootScope.$digest();
+
+      className = $animate.flushNext('removeClass').params[1];
+      expect(className).toBe('two');
+
+      className = $animate.flushNext('addClass').params[1];
+      expect(className).toBe('three');
+
+      expect($animate.queue.length).toBe(0);
+    });
+  });
 });