perf(ngModel,form): avoid invoking $interpolate for empty/unset values

benchmarks/largetable-bp/main.html

@@ -19,6 +19,7 @@
       <div>interpolation + fnInvocation: <input type="radio" ng-model="benchmarkType" value="interpolationFn"></div>
       <div>ngBind + filter: <input type="radio" ng-model="benchmarkType" value="ngBindFilter"></div>
       <div>interpolation + filter: <input type="radio" ng-model="benchmarkType" value="interpolationFilter"></div>
+      <div>ngModel: <input type="radio" ng-model="benchmarkType" value="ngModel"></div>
 
       <ng-switch on="benchmarkType">
         <baseline-binding-table ng-switch-when="baselineBinding">
@@ -77,6 +78,13 @@ <h2>interpolation with filter</h2>
             <span ng-repeat="column in row">{{column.i | noop}}:{{column.j | noop}}|</span>
           </div>
         </div>
+        <div ng-switch-when="ngModel">
+          <h2>ngModels</h2>
+          <div ng-repeat="row in data">
+            <input type="text" ng-model="row.i" name="constName" />
+            <input type="text" ng-model="row.j" />
+          </div>
+        </div>
       </ng-switch>
     </div>
   </div>

src/ng/interpolate.js

@@ -88,6 +88,19 @@ function $InterpolateProvider() {
       return '\\\\\\' + ch;
     }
 
+    //TODO: this is the same as the constantWatchDelegate in parse.js
+    function constantWatchDelegate(scope, listener, objectEquality, constantInterp) {
+      var unwatch;
+      return unwatch = scope.$watch(function constantInterpolateWatch(scope) {
+        return constantInterp(scope);
+      }, function constantInterpolateListener(value, old, scope) {
+        if (isFunction(listener)) {
+          listener.apply(this, arguments);
+        }
+        unwatch();
+      }, objectEquality);
+    }
+
     /**
      * @ngdoc service
      * @name $interpolate
@@ -183,6 +196,19 @@ function $InterpolateProvider() {
      * - `context`: evaluation context for all expressions embedded in the interpolated text
      */
     function $interpolate(text, mustHaveExpression, trustedContext, allOrNothing) {
+      // Provide a quick exit and simplified result function for text with no interpolation
+      if (!text.length || text.indexOf(startSymbol) === -1) {
+        var constantInterp;
+        if (!mustHaveExpression) {
+          var unescapedText = unescapeText(text);
+          constantInterp = function constantInterpolationFn() { return unescapedText; };
+          constantInterp.exp = text;
+          constantInterp.expressions = [];
+          constantInterp.$$watchDelegate = constantWatchDelegate;
+        }
+        return constantInterp;
+      }
+
       allOrNothing = !!allOrNothing;
       var startIndex,
           endIndex,

test/ng/interpolateSpec.js

@@ -125,6 +125,28 @@ describe('$interpolate', function() {
 
       expect($rootScope.$countWatchers()).toBe(0);
     }));
+
+    it('should stop watching strings with no expressions after first execution',
+      inject(function($interpolate, $rootScope) {
+        var spy = jasmine.createSpy();
+        $rootScope.$watch($interpolate('foo'), spy);
+        $rootScope.$digest();
+        expect(($rootScope.$$watchers || []).length).toBe(0);
+        expect(spy).toHaveBeenCalledWith('foo', 'foo', $rootScope);
+        expect(spy.calls.length).toBe(1);
+      })
+    );
+
+    it('should stop watching strings with only constant expressions after first execution',
+      inject(function($interpolate, $rootScope) {
+        var spy = jasmine.createSpy();
+        $rootScope.$watch($interpolate('foo {{42}}'), spy);
+        $rootScope.$digest();
+        expect(($rootScope.$$watchers || []).length).toBe(0);
+        expect(spy).toHaveBeenCalledWith('foo 42', 'foo 42', $rootScope);
+        expect(spy.calls.length).toBe(1);
+      })
+    );
   });
 
   describe('interpolation escaping', function() {
@@ -135,6 +157,7 @@ describe('$interpolate', function() {
 
 
     it('should support escaping interpolation signs', inject(function($interpolate) {
+      expect($interpolate('\\{\\{')(obj)).toBe('{{');
       expect($interpolate('{{foo}} \\{\\{bar\\}\\}')(obj)).toBe('Hello {{bar}}');
       expect($interpolate('\\{\\{foo\\}\\} {{bar}}')(obj)).toBe('{{foo}} World');
     }));