chore(Scope): short-circuit after dirty-checking last dirty watcher

Stop dirty-checking during $digest after the last dirty watcher has been re-checked.

This prevents unneeded re-checking of the remaining watchers (They were already
checked in the previous iteration), bringing a substantial performance improvement
to the average case run time of $digest.

Closes #5272
Closes #5287

Author: kseamon

src/ng/rootScope.js

@@ -71,6 +71,7 @@
 function $RootScopeProvider(){
   var TTL = 10;
   var $rootScopeMinErr = minErr('$rootScope');
+  var lastDirtyWatch = null;
 
   this.digestTtl = function(value) {
     if (arguments.length) {
@@ -325,6 +326,8 @@ function $RootScopeProvider(){
               eq: !!objectEquality
             };
 
+        lastDirtyWatch = null;
+
         // in the case user pass string, we need to compile it, do we really need this ?
         if (!isFunction(listener)) {
           var listenFn = compileToFn(listener || noop, 'listener');
@@ -553,6 +556,8 @@ function $RootScopeProvider(){
 
         beginPhase('$digest');
 
+        lastDirtyWatch = null;
+
         do { // "while dirty" loop
           dirty = false;
           current = target;
@@ -565,8 +570,10 @@ function $RootScopeProvider(){
               clearPhase();
               $exceptionHandler(e);
             }
+            lastDirtyWatch = null;
           }
 
+          traverseScopesLoop:
           do { // "traverse the scopes" loop
             if ((watchers = current.$$watchers)) {
               // process our watches
@@ -576,22 +583,30 @@ function $RootScopeProvider(){
                   watch = watchers[length];
                   // Most common watches are on primitives, in which case we can short
                   // circuit it with === operator, only when === fails do we use .equals
-                  if (watch && (value = watch.get(current)) !== (last = watch.last) &&
-                      !(watch.eq
-                          ? equals(value, last)
-                          : (typeof value == 'number' && typeof last == 'number'
-                             && isNaN(value) && isNaN(last)))) {
-                    dirty = true;
-                    watch.last = watch.eq ? copy(value) : value;
-                    watch.fn(value, ((last === initWatchVal) ? value : last), current);
-                    if (ttl < 5) {
-                      logIdx = 4 - ttl;
-                      if (!watchLog[logIdx]) watchLog[logIdx] = [];
-                      logMsg = (isFunction(watch.exp))
-                          ? 'fn: ' + (watch.exp.name || watch.exp.toString())
-                          : watch.exp;
-                      logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);
-                      watchLog[logIdx].push(logMsg);
+                  if (watch) {
+                    if ((value = watch.get(current)) !== (last = watch.last) &&
+                        !(watch.eq
+                            ? equals(value, last)
+                            : (typeof value == 'number' && typeof last == 'number'
+                               && isNaN(value) && isNaN(last)))) {
+                      dirty = true;
+                      lastDirtyWatch = watch;
+                      watch.last = watch.eq ? copy(value) : value;
+                      watch.fn(value, ((last === initWatchVal) ? value : last), current);
+                      if (ttl < 5) {
+                        logIdx = 4 - ttl;
+                        if (!watchLog[logIdx]) watchLog[logIdx] = [];
+                        logMsg = (isFunction(watch.exp))
+                            ? 'fn: ' + (watch.exp.name || watch.exp.toString())
+                            : watch.exp;
+                        logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);
+                        watchLog[logIdx].push(logMsg);
+                      }
+                    } else if (watch === lastDirtyWatch) {
+                      // If the most recently dirty watcher is now clean, short circuit since the remaining watchers
+                      // have already been tested.
+                      dirty = false;
+                      break traverseScopesLoop;
                     }
                   }
                 } catch (e) {
@@ -604,20 +619,24 @@ 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 || (current !== target && current.$$nextSibling)))) {
+            if (!(next = (current.$$childHead ||
+                (current !== target && current.$$nextSibling)))) {
               while(current !== target && !(next = current.$$nextSibling)) {
                 current = current.$parent;
               }
             }
           } while ((current = next));
 
+          // `break traverseScopesLoop;` takes us to here
+
           if(dirty && !(ttl--)) {
             clearPhase();
             throw $rootScopeMinErr('infdig',
                 '{0} $digest() iterations reached. Aborting!\n' +
                 'Watchers fired in the last 5 iterations: {1}',
                 TTL, toJson(watchLog));
           }
+
         } while (dirty || asyncQueue.length);
 
         clearPhase();

test/ng/rootScopeSpec.js

@@ -577,6 +577,82 @@ describe('Scope', function() {
         });
       });
     });
+
+
+    describe('optimizations', function() {
+
+      function setupWatches(scope, log) {
+        scope.$watch(function() { log('w1'); return scope.w1; }, log.fn('w1action'));
+        scope.$watch(function() { log('w2'); return scope.w2; }, log.fn('w2action'));
+        scope.$watch(function() { log('w3'); return scope.w3; }, log.fn('w3action'));
+        scope.$digest();
+        log.reset();
+      }
+
+
+      it('should check watches only once during an empty digest', inject(function(log, $rootScope) {
+        setupWatches($rootScope, log);
+        $rootScope.$digest();
+        expect(log).toEqual(['w1', 'w2', 'w3']);
+      }));
+
+
+      it('should quit digest early after we check the last watch that was previously dirty',
+          inject(function(log, $rootScope) {
+        setupWatches($rootScope, log);
+        $rootScope.w1 = 'x';
+        $rootScope.$digest();
+        expect(log).toEqual(['w1', 'w1action', 'w2', 'w3', 'w1']);
+      }));
+
+
+      it('should not quit digest early if a new watch was added from an existing watch action',
+          inject(function(log, $rootScope) {
+        setupWatches($rootScope, log);
+        $rootScope.$watch(log.fn('w4'), function() {
+          log('w4action');
+          $rootScope.$watch(log.fn('w5'), log.fn('w5action'));
+        });
+        $rootScope.$digest();
+        expect(log).toEqual(['w1', 'w2', 'w3', 'w4', 'w4action',
+                             'w1', 'w2', 'w3', 'w4', 'w5', 'w5action',
+                             'w1', 'w2', 'w3', 'w4', 'w5']);
+      }));
+
+
+      it('should not quit digest early if an evalAsync task was scheduled from a watch action',
+          inject(function(log, $rootScope) {
+        setupWatches($rootScope, log);
+        $rootScope.$watch(log.fn('w4'), function() {
+          log('w4action');
+          $rootScope.$evalAsync(function() {
+            log('evalAsync')
+          });
+        });
+        $rootScope.$digest();
+        expect(log).toEqual(['w1', 'w2', 'w3', 'w4', 'w4action', 'evalAsync',
+                             'w1', 'w2', 'w3', 'w4']);
+      }));
+
+
+      it('should quit digest early but not too early when various watches fire', inject(function(log, $rootScope) {
+        setupWatches($rootScope, log);
+        $rootScope.$watch(function() { log('w4'); return $rootScope.w4; }, function(newVal) {
+          log('w4action');
+          $rootScope.w2 = newVal;
+        });
+
+        $rootScope.$digest();
+        log.reset();
+
+        $rootScope.w1 = 'x';
+        $rootScope.w4 = 'x';
+        $rootScope.$digest();
+        expect(log).toEqual(['w1', 'w1action', 'w2', 'w3', 'w4', 'w4action',
+                             'w1', 'w2', 'w2action', 'w3', 'w4',
+                             'w1', 'w2']);
+      }));
+    });
   });