feat(scope): add support to skip auto digest in a turn

Closes #235

lib/core/scope.dart

@@ -55,6 +55,7 @@ class Scope implements Map {
   Map<String, List<Function>> _listeners = {};
   Scope _nextSibling, _prevSibling, _childHead, _childTail;
   bool _isolate = false;
+  bool _skipAutoDigest = false;
   Profiler _perf;
 
 
@@ -68,7 +69,7 @@ class Scope implements Map {
     _outerAsyncQueue = [];
 
     // Set up the zone to auto digest this scope.
-    _zone.onTurnDone = $digest;
+    _zone.onTurnDone = _autoDigestOnTurnDone;
     _zone.onError = (e, s, ls) => _exceptionHandler(e, s);
   }
 
@@ -92,6 +93,14 @@ class Scope implements Map {
     }
   }
 
+  _autoDigestOnTurnDone() {
+    if (_skipAutoDigest) {
+      _skipAutoDigest = false;
+    } else {
+      $digest();
+    }
+  }
+
   _identical(a, b) =>
     identical(a, b) ||
     (a is String && b is String && a == b) ||
@@ -260,6 +269,7 @@ class Scope implements Map {
    * auto-digesting scope.
    */
   $$verifyDigestWillRun() {
+    assert(!_skipAutoDigest);
     _zone.assertInTurn();
   }
 
@@ -395,6 +405,31 @@ class Scope implements Map {
   }
 
 
+  /**
+   * Skip running a $digest at the end of this turn.
+   * The primary use case is to skip the digest in the current VM turn because
+   * you just scheduled or are otherwise certain of an impending VM turn and the
+   * digest at the end of that turn is sufficient.  You should be able to answer
+   * "No" to the question "Is there any other code that is aware that this VM
+   * turn occured and therefore expected a digest?".  If your answer is "Yes",
+   * then you run the risk that the very next VM turn is not for your event and
+   * now that other code runs in that turn and sees stale values.
+   *
+   * You might call this function, for instance, from an event listener where,
+   * though the event occured, you need to wait for another event before you can
+   * perform something meaningful.  You might schedule that other event,
+   * set a flag for the handler of the other event to recognize, etc. and then
+   * call this method to skip the digest this cycle.  Note that you should call
+   * this function *after* you have successfully confirmed that the expected VM
+   * turn will occur (perhaps by scheduling it) to ensure that the digest
+   * actually does take place on that turn.
+   */
+  $skipAutoDigest() {
+    _zone.assertInTurn();
+    _skipAutoDigest = true;
+  }
+
+
   $apply([expr]) {
     return _zone.run(() {
       var timerId;

lib/directive/ng_model.dart

@@ -109,7 +109,10 @@ abstract class _InputTextlikeDirective {
       inputElement.selectionEnd = end;
     };
     inputElement.onChange.listen(relaxFnArgs(processValue));
-    inputElement.onKeyDown.listen((e) => new async.Timer(Duration.ZERO, processValue));
+    inputElement.onKeyDown.listen((e) {
+      new async.Timer(Duration.ZERO, processValue);
+      scope.$skipAutoDigest();
+    });
   }
 
   processValue() {

test/core/scope_spec.dart

@@ -6,8 +6,13 @@ import 'dart:convert' show JSON;
 
 main() {
   describe(r'Scope', () {
+    NgZone zone;
+
+    noop() {}
+
     beforeEach(module(() {
-      return (NgZone zone) {
+      return (NgZone _zone) {
+        zone = _zone;
         zone.onError = (e, s, l) => null;
       };
     }));
@@ -74,6 +79,46 @@ main() {
     });
 
 
+    describe(r'auto digest', () {
+      it(r'should auto digest at the end of the turn', inject((Scope $rootScope) {
+        var digestedValue = 0;
+        $rootScope.a = 1;
+        $rootScope.$watch('a', (newValue, oldValue, _this) {
+          digestedValue = newValue;
+        });
+        expect(digestedValue).toEqual(0);
+        zone.run(noop);
+        expect(digestedValue).toEqual(1);
+      }));
+
+      it(r'should skip auto digest if requested', inject((Scope $rootScope) {
+        var digestedValue = 0;
+        $rootScope.a = 1;
+        $rootScope.$watch('a', (newValue, oldValue, _this) {
+          digestedValue = newValue;
+        });
+        expect(digestedValue).toEqual(0);
+        zone.run(() {
+          $rootScope.$skipAutoDigest();
+        });
+        expect(digestedValue).toEqual(0);
+        zone.run(noop);
+        expect(digestedValue).toEqual(1);
+      }));
+
+      it(r'should throw exception if asked to skip auto digest outside of a turn',
+         inject((Scope $rootScope) {
+        var digestedValue = 0;
+        $rootScope.a = 1;
+        $rootScope.$watch('a', (newValue, oldValue, _this) {
+          digestedValue = newValue;
+        });
+        expect(digestedValue).toEqual(0);
+        expect($rootScope.$skipAutoDigest).toThrow();
+      }));
+    });
+
+
     describe(r'$watch/$digest', () {
       it(r'should watch and fire on simple property change', inject((Scope $rootScope) {
         var log;