perf($rootScope): make postDigestQueue more efficient

Author: thorn0

src/ng/rootScope.js

@@ -848,7 +848,7 @@ function $RootScopeProvider() {
 
         clearPhase();
 
-        while (postDigestQueue.length) {
+        while (!postDigestQueue.isEmpty()) {
           try {
             postDigestQueue.shift()();
           } catch (e) {
@@ -1306,7 +1306,7 @@ function $RootScopeProvider() {
 
     //The internal queues. Expose them on the $rootScope for debugging/testing purposes.
     var asyncQueue = $rootScope.$$asyncQueue = [];
-    var postDigestQueue = $rootScope.$$postDigestQueue = [];
+    var postDigestQueue = $rootScope.$$postDigestQueue = new Queue();
     var applyAsyncQueue = $rootScope.$$applyAsyncQueue = [];
 
     return $rootScope;
@@ -1364,5 +1364,28 @@ function $RootScopeProvider() {
         });
       }
     }
+
+    // A simple queue implementation used for postDigestQueue,
+    // taken from http://code.stephenmorley.org/javascript/queues/
+    // See https://github.com/angular/angular.js/issues/14534
+    function Queue() {
+      var queue = [];
+      var offset = 0;
+      this.isEmpty = function() {
+        return queue.length === 0;
+      };
+      this.push = function(item) {
+        queue.push(item);
+      };
+      this.shift = function() {
+        if (queue.length === 0) return undefined;
+        var item = queue[offset];
+        if (++offset * 2 >= queue.length) {
+          queue = queue.slice(offset);
+          offset = 0;
+        }
+        return item;
+      };
+    }
   }];
 }

test/ng/rootScopeSpec.js

@@ -1320,6 +1320,29 @@ describe('Scope', function() {
       expect(externalWatchCount).toEqual(0);
     }));
 
+    it('should process $$postDigest callbacks as a queue (FIFO) when the scope is digested', inject(function($rootScope) {
+      var result = '';
+
+      $rootScope.$$postDigest(function() {
+        result += 'a';
+        $rootScope.$$postDigest(function() {
+          result += 'd';
+        });
+      });
+
+      $rootScope.$$postDigest(function() {
+        result += 'b';
+      });
+
+      $rootScope.$$postDigest(function() {
+        result += 'c';
+      });
+
+      expect(result).toBe('');
+      $rootScope.$digest();
+      expect(result).toBe('abcd');
+    }));
+
     it('should run a $$postDigest call on all child scopes when a parent scope is digested', inject(function($rootScope) {
       var parent = $rootScope.$new(),
           child = parent.$new(),