perf($interpolate): move $sce/stringify from a parse interceptor to the watcher

Author: jbedard

src/ng/interpolate.js

@@ -202,7 +202,7 @@ function $InterpolateProvider() {
           }
           exp = text.substring(startIndex + startSymbolLength, endIndex);
           expressions.push(exp);
-          parseFns.push($parse(exp, parseStringifyInterceptor));
+          parseFns.push($parse(exp));
           index = endIndex + endSymbolLength;
           expressionPositions.push(concat.length);
           concat.push('');
@@ -232,7 +232,7 @@ function $InterpolateProvider() {
         var compute = function(values) {
           for (var i = 0, ii = expressions.length; i < ii; i++) {
             if (allOrNothing && isUndefined(values[i])) return;
-            concat[expressionPositions[i]] = values[i];
+            concat[expressionPositions[i]] = parseStringifyValue(values[i]);
           }
           return concat.join('');
         };
@@ -282,10 +282,10 @@ function $InterpolateProvider() {
           exp: text, //just for compatibility with regular watchers created via $watch
           expressions: expressions,
           $$watchDelegate: function(scope, listener, objectEquality) {
-            var lastValue;
+            var lastValue = NaN;
             return scope.$watchGroup(parseFns, function interpolateFnWatcher(values, oldValues) {
               var currValue = compute(values);
-              if (isFunction(listener)) {
+              if (isFunction(listener) && currValue !== lastValue) {
                 listener.call(this, currValue, values !== oldValues ? lastValue : currValue, scope);
               }
               lastValue = currValue;
@@ -299,7 +299,7 @@ function $InterpolateProvider() {
           replace(escapedEndRegexp, endSymbol);
       }
 
-      function parseStringifyInterceptor(value) {
+      function parseStringifyValue(value) {
         try {
           return stringify(getValue(value));
         } catch (err) {

test/ng/interpolateSpec.js

@@ -248,6 +248,32 @@ describe('$interpolate', function() {
     }));
   });
 
+  describe("watching", function() {
+    it("should invoke only on change of output", inject(function($interpolate, $rootScope) {
+      var watcherInvoked = 0;
+      $rootScope.a = $rootScope.b = $rootScope.c = 0;
+      $rootScope.$watch($interpolate("{{a + b}} {{c}}"), function() { watcherInvoked++; });
+      $rootScope.$apply();
+      expect(watcherInvoked).toBe(1);
+
+      $rootScope.a = 1;
+      $rootScope.$apply();
+      expect(watcherInvoked).toBe(2);
+
+      $rootScope.a = 0;
+      $rootScope.b = 1;
+      $rootScope.$apply();
+      expect(watcherInvoked).toBe(2);
+
+      $rootScope.c = '0';
+      $rootScope.$apply();
+      expect(watcherInvoked).toBe(2);
+
+      $rootScope.c = {toJSON: function() { return 0; }};
+      $rootScope.$apply();
+      expect(watcherInvoked).toBe(2);
+    }));
+  });
 
   describe('isTrustedContext', function() {
     it('should NOT interpolate a multi-part expression when isTrustedContext is true', inject(function($interpolate) {