perf($rootScope): make queues more efficient

By using a pointer to the current start of the queue and only clearing
up the queue storage later, we save lots of time that was spent manipulating
arrays with `slice`

Closes #14545

Author: thorn0

src/ng/rootScope.js

@@ -769,15 +769,19 @@ function $RootScopeProvider() {
           dirty = false;
           current = target;
 
-          while (asyncQueue.length) {
+          // It's safe for asyncQueuePosition to be a local variable here because this loop can't
+          // be reentered recursively. Calling $digest from a function passed to $applyAsync would
+          // lead to a '$digest already in progress' error.
+          for (var asyncQueuePosition = 0; asyncQueuePosition < asyncQueue.length; asyncQueuePosition++) {
             try {
-              asyncTask = asyncQueue.shift();
+              asyncTask = asyncQueue[asyncQueuePosition];
               asyncTask.scope.$eval(asyncTask.expression, asyncTask.locals);
             } catch (e) {
               $exceptionHandler(e);
             }
             lastDirtyWatch = null;
           }
+          asyncQueue.length = 0;
 
           traverseScopesLoop:
           do { // "traverse the scopes" loop
@@ -848,13 +852,15 @@ function $RootScopeProvider() {
 
         clearPhase();
 
-        while (postDigestQueue.length) {
+        // postDigestQueuePosition isn't local here because this loop can be reentered recursively.
+        while (postDigestQueuePosition < postDigestQueue.length) {
           try {
-            postDigestQueue.shift()();
+            postDigestQueue[postDigestQueuePosition++]();
           } catch (e) {
             $exceptionHandler(e);
           }
         }
+        postDigestQueue.length = postDigestQueuePosition = 0;
       },
 
 
@@ -1309,6 +1315,8 @@ function $RootScopeProvider() {
     var postDigestQueue = $rootScope.$$postDigestQueue = [];
     var applyAsyncQueue = $rootScope.$$applyAsyncQueue = [];
 
+    var postDigestQueuePosition = 0;
+
     return $rootScope;
 
 

test/ng/rootScopeSpec.js

@@ -1320,46 +1320,6 @@ describe('Scope', function() {
       expect(externalWatchCount).toEqual(0);
     }));
 
-    it('should run a $$postDigest call on all child scopes when a parent scope is digested', inject(function($rootScope) {
-      var parent = $rootScope.$new(),
-          child = parent.$new(),
-          count = 0;
-
-      $rootScope.$$postDigest(function() {
-        count++;
-      });
-
-      parent.$$postDigest(function() {
-        count++;
-      });
-
-      child.$$postDigest(function() {
-        count++;
-      });
-
-      expect(count).toBe(0);
-      $rootScope.$digest();
-      expect(count).toBe(3);
-    }));
-
-    it('should run a $$postDigest call even if the child scope is isolated', inject(function($rootScope) {
-      var parent = $rootScope.$new(),
-          child = parent.$new(true),
-          signature = '';
-
-      parent.$$postDigest(function() {
-        signature += 'A';
-      });
-
-      child.$$postDigest(function() {
-        signature += 'B';
-      });
-
-      expect(signature).toBe('');
-      $rootScope.$digest();
-      expect(signature).toBe('AB');
-    }));
-
     it('should cause a $digest rerun', inject(function($rootScope) {
       $rootScope.log = '';
       $rootScope.value = 0;
@@ -1705,6 +1665,92 @@ describe('Scope', function() {
     }));
   });
 
+  describe('$$postDigest', function() {
+    it('should process callbacks as a queue (FIFO) when the scope is digested', inject(function($rootScope) {
+      var signature = '';
+
+      $rootScope.$$postDigest(function() {
+        signature += 'A';
+        $rootScope.$$postDigest(function() {
+          signature += 'D';
+        });
+      });
+
+      $rootScope.$$postDigest(function() {
+        signature += 'B';
+      });
+
+      $rootScope.$$postDigest(function() {
+        signature += 'C';
+      });
+
+      expect(signature).toBe('');
+      $rootScope.$digest();
+      expect(signature).toBe('ABCD');
+    }));
+
+    it('should support $apply calls nested in $$postDigest callbacks', inject(function($rootScope) {
+      var signature = '';
+
+      $rootScope.$$postDigest(function() {
+        signature += 'A';
+      });
+
+      $rootScope.$$postDigest(function() {
+        signature += 'B';
+        $rootScope.$apply();
+        signature += 'D';
+      });
+
+      $rootScope.$$postDigest(function() {
+        signature += 'C';
+      });
+
+      expect(signature).toBe('');
+      $rootScope.$digest();
+      expect(signature).toBe('ABCD');
+    }));
+
+    it('should run a $$postDigest call on all child scopes when a parent scope is digested', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+          child = parent.$new(),
+          count = 0;
+
+      $rootScope.$$postDigest(function() {
+        count++;
+      });
+
+      parent.$$postDigest(function() {
+        count++;
+      });
+
+      child.$$postDigest(function() {
+        count++;
+      });
+
+      expect(count).toBe(0);
+      $rootScope.$digest();
+      expect(count).toBe(3);
+    }));
+
+    it('should run a $$postDigest call even if the child scope is isolated', inject(function($rootScope) {
+      var parent = $rootScope.$new(),
+          child = parent.$new(true),
+          signature = '';
+
+      parent.$$postDigest(function() {
+        signature += 'A';
+      });
+
+      child.$$postDigest(function() {
+        signature += 'B';
+      });
+
+      expect(signature).toBe('');
+      $rootScope.$digest();
+      expect(signature).toBe('AB');
+    }));
+  });
 
   describe('events', function() {