perf($scope): Add a property $$watchersCount to scope

Add a property $$watchersCount to scope that keeps the number of
watchers in the scope plus all the child scopes. Use this property
when traversing scopes looking for watches

Author: lgalfaso

src/Angular.js

@@ -651,7 +651,7 @@ function arrayRemove(array, value) {
   var index = indexOf(array, value);
   if (index >=0)
     array.splice(index, 1);
-  return value;
+  return index >=0;
 }
 
 function isLeafNode (node) {

src/ng/rootScope.js

@@ -134,6 +134,7 @@ function $RootScopeProvider(){
       this.$$postDigestQueue = [];
       this.$$listeners = {};
       this.$$listenerCount = {};
+      this.$$watchersCount = 0;
       this.$$isolateBindings = {};
     }
 
@@ -196,6 +197,7 @@ function $RootScopeProvider(){
         child.$$listenerCount = {};
         child.$parent = this;
         child.$$watchers = child.$$nextSibling = child.$$childHead = child.$$childTail = null;
+        child.$$watchersCount = 0;
         child.$$prevSibling = this.$$childTail;
         if (this.$$childHead) {
           this.$$childTail.$$nextSibling = child;
@@ -340,7 +342,9 @@ function $RootScopeProvider(){
           var originalFn = watcher.fn;
           watcher.fn = function(newVal, oldVal, scope) {
             originalFn.call(this, newVal, oldVal, scope);
-            arrayRemove(array, watcher);
+            if (arrayRemove(array, watcher)) {
+              incrementWatchersCount(scope, -1);
+            }
           };
         }
 
@@ -350,10 +354,13 @@ function $RootScopeProvider(){
         // we use unshift since we use a while loop in $digest for speed.
         // the while loop reads in reverse order.
         array.unshift(watcher);
+        incrementWatchersCount(this, 1);
 
         return function() {
-          arrayRemove(array, watcher);
-          lastDirtyWatch = null;
+          if (arrayRemove(array, watcher)) {
+            incrementWatchersCount(scope, -1);
+            lastDirtyWatch = null;
+          }
         };
       },
 
@@ -622,7 +629,8 @@ function $RootScopeProvider(){
             // Insanity Warning: scope depth-first traversal
             // yes, this code is a bit crazy, but it works and we have tests to prove it!
             // this piece should be kept in sync with the traversal in $broadcast
-            if (!(next = (current.$$childHead ||
+            // (though it differs due to having the extra check for $$watchersCount)
+            if (!(next = ((current.$$watchersCount && current.$$childHead) ||
                 (current !== target && current.$$nextSibling)))) {
               while(current !== target && !(next = current.$$nextSibling)) {
                 current = current.$parent;
@@ -699,6 +707,7 @@ function $RootScopeProvider(){
         this.$$destroyed = true;
         if (this === $rootScope) return;
 
+        incrementWatchersCount(this, -this.$$watchersCount);
         forEach(this.$$listenerCount, bind(null, decrementListenerCount, this));
 
         if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
@@ -1071,6 +1080,12 @@ function $RootScopeProvider(){
       return fn;
     }
 
+    function incrementWatchersCount(current, count) {
+      do {
+        current.$$watchersCount += count;
+      } while ((current = current.$parent));
+    }
+
     function decrementListenerCount(current, count, name) {
       do {
         current.$$listenerCount[name] -= count;

test/ng/rootScopeSpec.js

@@ -108,11 +108,68 @@ describe('Scope', function() {
     it('should not keep constant expressions on watch queue', inject(function($rootScope) {
       $rootScope.$watch('1 + 1', function() {});
       expect($rootScope.$$watchers.length).toEqual(1);
+      expect($rootScope.$$watchersCount).toEqual(1);
       $rootScope.$digest();
 
       expect($rootScope.$$watchers.length).toEqual(0);
+      expect($rootScope.$$watchersCount).toEqual(0);
     }));
 
+    it('should decrement the watcherCount when destroying a child scope', inject(function($rootScope) {
+      var child1 = $rootScope.$new(),
+        child2 = $rootScope.$new(),
+        grandChild1 = child1.$new(),
+        grandChild2 = child2.$new();
+
+      child1.$watch('a', function() {});
+      child2.$watch('a', function() {});
+      grandChild1.$watch('a', function() {});
+      grandChild2.$watch('a', function() {});
+
+      expect($rootScope.$$watchersCount).toBe(4);
+      expect(child1.$$watchersCount).toBe(2);
+      expect(child2.$$watchersCount).toBe(2);
+      expect(grandChild1.$$watchersCount).toBe(1);
+      expect(grandChild2.$$watchersCount).toBe(1);
+
+      grandChild2.$destroy();
+      expect(child2.$$watchersCount).toBe(1);
+      expect($rootScope.$$watchersCount).toBe(3); 
+      child1.$destroy();
+      expect($rootScope.$$watchersCount).toBe(1); 
+    }));
+
+    it('should decrement the watcherCount when calling the remove function', inject(function($rootScope) {
+      var child1 = $rootScope.$new(),
+        child2 = $rootScope.$new(),
+        grandChild1 = child1.$new(),
+        grandChild2 = child2.$new(),
+        remove1,
+        remove2;
+
+      remove1 = child1.$watch('a', function() {});
+      child2.$watch('a', function() {});
+      grandChild1.$watch('a', function() {});
+      remove2 = grandChild2.$watch('a', function() {});
+
+      remove2();
+      expect(grandChild2.$$watchersCount).toBe(0);
+      expect(child2.$$watchersCount).toBe(1);
+      expect($rootScope.$$watchersCount).toBe(3);
+      remove1();
+      expect(grandChild1.$$watchersCount).toBe(1);
+      expect(child1.$$watchersCount).toBe(1);
+      expect($rootScope.$$watchersCount).toBe(2);
+
+      // Execute everything a second time to be sure that calling the remove funciton
+      // several times, it only decrements the counter once
+      remove2();
+      expect(child2.$$watchersCount).toBe(1);
+      expect($rootScope.$$watchersCount).toBe(2);
+      remove1();
+      expect(child1.$$watchersCount).toBe(1);
+      expect($rootScope.$$watchersCount).toBe(2);
+    }));
 
     it('should delegate exceptions', function() {
       module(function($exceptionHandlerProvider) {