fix($animate): ensure the DOM operation isn't run twice

Depending on the animations placed on ngClass, the DOM operation may
run twice causing a race condition between addClass and removeClass.
Depending on what classes are removed and added via $compile this may
cause all CSS classes to be removed accidentally from the element
being animated.

Closes angular#4949

src/ngAnimate/animate.js

@@ -563,7 +563,7 @@ angular.module('ngAnimate', ['ng'])
         //the animation if any matching animations are not found at all.
         //NOTE: IE8 + IE9 should close properly (run closeAnimation()) in case a NO animation is not found.
         if (animationsDisabled(element, parentElement) || matches.length === 0) {
-          domOperation();
+          fireDOMOperation();
           closeAnimation();
           return;
         }
@@ -596,7 +596,7 @@ angular.module('ngAnimate', ['ng'])
         //this would mean that an animation was not allowed so let the existing
         //animation do it's thing and close this one early
         if(animations.length === 0) {
-          domOperation();
+          fireDOMOperation();
           fireDoneCallbackAsync();
           return;
         }
@@ -616,7 +616,7 @@ angular.module('ngAnimate', ['ng'])
         //is so that the CSS classes present on the element can be properly examined.
         if((animationEvent == 'addClass'    && element.hasClass(className)) ||
            (animationEvent == 'removeClass' && !element.hasClass(className))) {
-          domOperation();
+          fireDOMOperation();
           fireDoneCallbackAsync();
           return;
         }
@@ -627,6 +627,7 @@ angular.module('ngAnimate', ['ng'])
 
         element.data(NG_ANIMATE_STATE, {
           running:true,
+          className:className,
           structural:!isClassBased,
           animations:animations,
           done:onBeforeAnimationsComplete
@@ -637,7 +638,7 @@ angular.module('ngAnimate', ['ng'])
         invokeRegisteredAnimationFns(animations, 'before', onBeforeAnimationsComplete);
 
         function onBeforeAnimationsComplete(cancelled) {
-          domOperation();
+          fireDOMOperation();
           if(cancelled === true) {
             closeAnimation();
             return;
@@ -695,6 +696,15 @@ angular.module('ngAnimate', ['ng'])
           doneCallback && $timeout(doneCallback, 0, false);
         }
 
+        //it is less complicated to use a flag than managing and cancelling
+        //timeouts containing multiple callbacks.
+        function fireDOMOperation() {
+          if(!fireDOMOperation.hasBeenRun) {
+            fireDOMOperation.hasBeenRun = true;
+            domOperation();
+          }
+        }
+
         function closeAnimation() {
           if(!closeAnimation.hasBeenRun) {
             closeAnimation.hasBeenRun = true;

test/ngAnimate/animateSpec.js

@@ -2599,4 +2599,38 @@ describe("ngAnimate", function() {
     });
   });
 
+  it('should only perform the DOM operation once',
+    inject(function($sniffer, $compile, $rootScope, $rootElement, $animate, $timeout) {
+
+    if (!$sniffer.transitions) return;
+
+    ss.addRule('.base-class', '-webkit-transition:1s linear all;' +
+                                      'transition:1s linear all;');
+
+    $animate.enabled(true);
+
+    var element = $compile('<div class="base-class one two"></div>')($rootScope);
+    $rootElement.append(element);
+    jqLite($document[0].body).append($rootElement);
+
+    $animate.removeClass(element, 'base-class one two');
+
+    //still true since we're before the reflow
+    expect(element.hasClass('base-class')).toBe(true);
+
+    //this will cancel the remove animation
+    $animate.addClass(element, 'base-class one two');
+
+    //the cancellation was a success and the class was added right away
+    //since there was no successive animation for the after animation
+    expect(element.hasClass('base-class')).toBe(true);
+
+    //the reflow...
+    $timeout.flush();
+
+    //the reflow DOM operation was commenced but it ran before so it
+    //shouldn't run agaun
+    expect(element.hasClass('base-class')).toBe(true);
+  }));
+
 });