perf($interpolate): provide a simplified result for constant expressions

Closes: #10414

Author: jbedard

Diff for: src/ng/interpolate.js

@@ -128,6 +128,15 @@ function $InterpolateProvider() {
       return value;
     }
 
+    //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) {
+        unwatch();
+        return constantInterp(scope);
+      }, listener, objectEquality);
+    }
+
     /**
      * @ngdoc service
      * @name $interpolate
@@ -223,6 +232,18 @@ 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 = valueFn(unescapedText);
+          constantInterp.expressions = [];
+          constantInterp.$$watchDelegate = constantWatchDelegate;
+        }
+        return constantInterp;
+      }
+
       allOrNothing = !!allOrNothing;
       var startIndex,
           endIndex,

Diff for: test/ng/interpolateSpec.js

@@ -157,6 +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');
     }));