feat(scope): early exit of digest loop

At any given time there are many watchers in the 
system. When watcher fires it can change the 
model. This change may now cause an earlier watch 
to be eligible for firing. For this reason the 
watches need to be re-checked several times, until 
no watchers fire. Currently the system loops until 
there is a clean run through the loop. This means 
that every watch needs to run at least twice. A 
better way is to remember the last fired watch, 
and then exit the loop early when the the last 
watch has been reached again.

lib/core/scope.dart

@@ -399,9 +399,11 @@ class Scope implements Map {
   }
 
   $digest() {
-    var innerAsyncQueue = _innerAsyncQueue,
-        length,
-        dirty, _ttlLeft = _ttl;
+    var innerAsyncQueue = _innerAsyncQueue;
+    num length;
+    _Watch lastDirtyWatch = null;
+    _Watch lastLoopLastDirtyWatch;
+    num _ttlLeft = _ttl;
     List<List<String>> watchLog = [];
     List<_Watch> watchers;
     _Watch watch;
@@ -412,7 +414,8 @@ class Scope implements Map {
       var watcherCount;
       var scopeCount;
       do { // "while dirty" loop
-        dirty = false;
+        lastLoopLastDirtyWatch = lastDirtyWatch;
+        lastDirtyWatch = null;
         current = target;
         //asyncQueue = current._asyncQueue;
         //dump('aQ: ${asyncQueue.length}');
@@ -433,6 +436,7 @@ class Scope implements Map {
         watcherCount = 0;
         scopeCount = 0;
         assert((timerId = _perf.startTimer('ng.dirty_check', _ttl-_ttlLeft)) != false);
+        digestLoop:
         do { // "traverse the scopes" loop
           scopeCount++;
           if ((watchers = current._watchers) != null) {
@@ -442,10 +446,14 @@ class Scope implements Map {
             while (length-- > 0) {
               try {
                 watch = watchers[length];
+                if (identical(lastLoopLastDirtyWatch, watch)) {
+                  break digestLoop;
+                }
                 var value = watch.get(current);
                 var last = watch.last;
                 if (!_identical(value, last)) {
-                  dirty = true;
+                  lastDirtyWatch = watch;
+                  lastLoopLastDirtyWatch = null;
                   watch.last = value;
                   var fireTimer;
                   assert((fireTimer = _perf.startTimer('ng.fire', watch.exp)) != false);
@@ -483,11 +491,11 @@ class Scope implements Map {
         } while ((current = next) != null);
 
         assert(_perf.stopTimer(timerId) != false);
-        if(dirty && (_ttlLeft--) == 0) {
+        if(lastDirtyWatch != null && (_ttlLeft--) == 0) {
           throw '$_ttl \$digest() iterations reached. Aborting!\n' +
               'Watchers fired in the last 5 iterations: ${_toJson(watchLog)}';
         }
-      } while (dirty || innerAsyncQueue.length > 0);
+      } while (lastDirtyWatch != null || innerAsyncQueue.length > 0);
       _perf.counters['ng.scope.watchers'] = watcherCount;
       _perf.counters['ng.scopes'] = scopeCount;
 

test/core/scope_spec.dart

@@ -212,7 +212,7 @@ main() {
 
         // init
         $rootScope.$digest();
-        expect(log.join('')).toEqual('rabrab');
+        expect(log.join('')).toEqual('rabra');
 
         log.removeWhere((e) => true);
         childA.$digest();
@@ -246,7 +246,7 @@ main() {
         $rootScope.$watch((a) { log += 'a'; });
         child.$watch((a) { log += 'b'; });
         $rootScope.$digest();
-        expect(log).toEqual('abab');
+        expect(log).toEqual('aba');
       }));
 
 
@@ -364,14 +364,14 @@ main() {
           eagerScope.$watch(() {log += 'eager;';});
 
           rootScope.$digest();
-          expect(log).toEqual('lazy;eager;eager;');
+          expect(log).toEqual('lazy;eager;');
 
           rootScope.$digest();
-          expect(log).toEqual('lazy;eager;eager;eager;');
+          expect(log).toEqual('lazy;eager;eager;');
 
           lazyScope.$dirty();
           rootScope.$digest();
-          expect(log).toEqual('lazy;eager;eager;eager;lazy;eager;');
+          expect(log).toEqual('lazy;eager;eager;lazy;eager;');
         });
       });
 
@@ -388,17 +388,17 @@ main() {
           childScope.$watch(() {log += 'digest;';});
 
           rootScope.$digest();
-          expect(log).toEqual('digest;digest;');
+          expect(log).toEqual('digest;');
 
           childScope.$disabled = true;
           expect(childScope.$disabled).toEqual(true);
           rootScope.$digest();
-          expect(log).toEqual('digest;digest;');
+          expect(log).toEqual('digest;');
 
           childScope.$disabled = false;
           expect(childScope.$disabled).toEqual(false);
           rootScope.$digest();
-          expect(log).toEqual('digest;digest;digest;');
+          expect(log).toEqual('digest;digest;');
         });
       });
     });
@@ -592,7 +592,7 @@ main() {
         $rootScope.$evalAsync(() => $rootScope.log += 'eval;', outsideDigest: true);
         $rootScope.$watch(() { $rootScope.log += 'digest;'; });
         $rootScope.$digest();
-        expect($rootScope.log).toEqual('digest;digest;eval;');
+        expect($rootScope.log).toEqual('digest;eval;');
       }));
 
       it(r'should allow running after digest in issolate scope', inject((Scope $rootScope) {
@@ -601,7 +601,7 @@ main() {
         isolateScope.$evalAsync(() => isolateScope.log += 'eval;', outsideDigest: true);
         isolateScope.$watch(() { isolateScope.log += 'digest;'; });
         isolateScope.$digest();
-        expect(isolateScope.log).toEqual('digest;digest;eval;');
+        expect(isolateScope.log).toEqual('digest;eval;');
       }));
 
     });
@@ -1085,7 +1085,6 @@ main() {
 
 
     describe('perf', () {
-
       describe('counters', () {
 
         it('should expose scope count', inject((Profiler perf, Scope scope) {
@@ -1147,7 +1146,41 @@ main() {
           expect(perf.counters['ng.scope.watchers']).toEqual(0);
         }));
       });
+    });
+
+
+    describe('optimizations', () {
+      var scope;
+      var log;
+      beforeEach(inject((Scope _scope, Logger _log) {
+        scope = _scope;
+        log = _log;
+        scope['a'] = 1;
+        scope['b'] = 2;
+        scope['c'] = 3;
+        scope.$watch(() {log('a'); return scope['a'];}, (value) => log('fire:a'));
+        scope.$watch(() {log('b'); return scope['b'];}, (value) => log('fire:b'));
+        scope.$watch(() {log('c'); return scope['c'];}, (value) {log('fire:c'); scope['b']++; });
+        scope.$digest();
+        log.clear();
+      }));
+
+      it('should loop once on no dirty', () {
+        scope.$digest();
+        expect(log.result()).toEqual('a; b; c');
+      });
 
+      it('should exit early on second loop', () {
+        scope['b']++;
+        scope.$digest();
+        expect(log.result()).toEqual('a; b; fire:b; c; a');
+      });
+
+      it('should continue checking if second loop dirty', () {
+        scope['c']++;
+        scope.$digest();
+        expect(log.result()).toEqual('a; b; c; fire:c; a; b; fire:b; c; a');
+      });
     });
   });
 }