perf(Scope): limit propagation of $broadcast to scopes that have registered the event

Update $on and $destroy to maintain a count of event keys registered for each scope and its children.
$broadcast will not descend past a node that has a count of 0/undefined for the $broadcasted event key.

Resolves angular#5341

Author: kseamon

src/ng/rootScope.js

@@ -133,6 +133,7 @@ function $RootScopeProvider(){
       this.$$asyncQueue = [];
       this.$$postDigestQueue = [];
       this.$$listeners = {};
+      this.$$listenerCount = {};
       this.$$isolateBindings = {};
     }
 
@@ -192,6 +193,7 @@ function $RootScopeProvider(){
         }
         child['this'] = child;
         child.$$listeners = {};
+        child.$$listenerCount = {};
         child.$parent = this;
         child.$$watchers = child.$$nextSibling = child.$$childHead = child.$$childTail = null;
         child.$$prevSibling = this.$$childTail;
@@ -696,6 +698,8 @@ function $RootScopeProvider(){
         this.$$destroyed = true;
         if (this === $rootScope) return;
 
+        forEach(this.$$listenerCount, decrementListenerCount, this);
+
         if (parent.$$childHead == this) parent.$$childHead = this.$$nextSibling;
         if (parent.$$childTail == this) parent.$$childTail = this.$$prevSibling;
         if (this.$$prevSibling) this.$$prevSibling.$$nextSibling = this.$$nextSibling;
@@ -885,8 +889,18 @@ function $RootScopeProvider(){
         }
         namedListeners.push(listener);
 
+        var current = this;
+        do {
+          if (!current.$$listenerCount[name]) {
+            current.$$listenerCount[name] = 0;
+          }
+          current.$$listenerCount[name]++;
+        } while ((current = current.$parent));
+
+        var self = this;
         return function() {
           namedListeners[indexOf(namedListeners, listener)] = null;
+          decrementListenerCount.call(self, 1, name);
         };
       },
 
@@ -998,8 +1012,7 @@ function $RootScopeProvider(){
             listeners, i, length;
 
         //down while you can, then up and next sibling or up and next sibling until back at root
-        do {
-          current = next;
+        while ((current = next)) {
           event.currentScope = current;
           listeners = current.$$listeners[name] || [];
           for (i=0, length = listeners.length; i<length; i++) {
@@ -1021,12 +1034,14 @@ function $RootScopeProvider(){
           // Insanity Warning: scope depth-first traversal
           // yes, this code is a bit crazy, but it works and we have tests to prove it!
           // this piece should be kept in sync with the traversal in $digest
-          if (!(next = (current.$$childHead || (current !== target && current.$$nextSibling)))) {
+          // (though it differs due to having the extra check for $$listenerCount)
+          if (!(next = ((current.$$listenerCount[name] && current.$$childHead) ||
+              (current !== target && current.$$nextSibling)))) {
             while(current !== target && !(next = current.$$nextSibling)) {
               current = current.$parent;
             }
           }
-        } while ((current = next));
+        }
 
         return event;
       }
@@ -1055,6 +1070,17 @@ function $RootScopeProvider(){
       return fn;
     }
 
+    function decrementListenerCount(count, name) {
+      var current = this;
+      do {
+        current.$$listenerCount[name] -= count;
+
+        if (current.$$listenerCount[name] === 0) {
+          delete current.$$listenerCount[name];
+        }
+      } while ((current = current.$parent));
+    }
+
     /**
      * function used as an initial value for watchers.
      * because it's unique we can easily tell it apart from other values

test/ng/rootScopeSpec.js

@@ -730,6 +730,27 @@ describe('Scope', function() {
       first.$apply();
       expect(log).toBe('1232323');
     }));
+
+    it('should decrement anscestor $$listenerCount entries', inject(function($rootScope) {
+      var EVENT = 'fooEvent',
+          spy = jasmine.createSpy('listener'),
+          firstSecond = first.$new();
+
+      firstSecond.$on(EVENT, spy);
+      firstSecond.$on(EVENT, spy);
+      middle.$on(EVENT, spy);
+
+      expect($rootScope.$$listenerCount[EVENT]).toBe(3);
+      expect(first.$$listenerCount[EVENT]).toBe(2);
+
+      firstSecond.$destroy();
+
+      expect($rootScope.$$listenerCount[EVENT]).toBe(1);
+      expect(first.$$listenerCount[EVENT]).toBeUndefined();
+
+      $rootScope.$broadcast(EVENT);
+      expect(spy.callCount).toBe(1);
+    }));
   });
 
 
@@ -1107,13 +1128,34 @@ describe('Scope', function() {
         child.$emit('abc');
         child.$broadcast('abc');
         expect(log).toEqual('XX');
+        expect($rootScope.$$listenerCount['abc']).toBe(1);
 
         log = '';
         listenerRemove();
         child.$emit('abc');
         child.$broadcast('abc');
         expect(log).toEqual('');
+        expect($rootScope.$$listenerCount['abc']).toBeUndefined();
       }));
+
+
+      it('should increment ancestor $$listenerCount entries', inject(function($rootScope) {
+        var child1 = $rootScope.$new(),
+            child2 = child1.$new(),
+            spy = jasmine.createSpy();
+
+        $rootScope.$on('event1', spy);
+        expect($rootScope.$$listenerCount).toEqual({event1: 1});
+
+        child1.$on('event1', spy);
+        expect($rootScope.$$listenerCount).toEqual({event1: 2});
+        expect(child1.$$listenerCount).toEqual({event1: 1});
+
+        child2.$on('event2', spy);
+        expect($rootScope.$$listenerCount).toEqual({event1: 2, event2: 1});
+        expect(child1.$$listenerCount).toEqual({event1: 1, event2: 1});
+        expect(child2.$$listenerCount).toEqual({event2: 1});
+      }))
     });
 
 
@@ -1358,6 +1400,23 @@ describe('Scope', function() {
           $rootScope.$broadcast('fooEvent');
           expect(log).toBe('');
         }));
+        
+        
+        it('should not descend past scopes with a $$listerCount of 0 or undefined', 
+            inject(function($rootScope) {
+          var EVENT = 'fooEvent',
+              spy = jasmine.createSpy('listener');
+
+          // Precondition: There should be no listeners for fooEvent.
+          expect($rootScope.$$listenerCount[EVENT]).toBeUndefined();
+
+          // Add a spy listener to a child scope.
+          $rootScope.$$childHead.$$listeners[EVENT] = [spy];
+
+          // $rootScope's count for 'fooEvent' is undefined, so spy should not be called.
+          $rootScope.$broadcast(EVENT);
+          expect(spy).not.toHaveBeenCalled();
+        }));
 
 
         it('should return event object', function() {