fix(change detector): fix the handling of NaN & string values for maps

Author: vicb

lib/change_detection/dirty_checking_change_detector.dart

@@ -451,9 +451,13 @@ class _MapChangeRecord<K, V> implements MapChangeRecord<K, V> {
       if (oldSeqRecord != null && key == oldSeqRecord.key) {
         newSeqRecord = oldSeqRecord;
         if (!identical(value, oldSeqRecord._currentValue)) {
-          oldSeqRecord._previousValue = oldSeqRecord._currentValue;
+          var prev = oldSeqRecord._previousValue = oldSeqRecord._currentValue;
           oldSeqRecord._currentValue = value;
-          _addToChanges(oldSeqRecord);
+          if (!((value is String && prev is String && value == prev) ||
+                (value is num && value.isNaN && prev is num && prev.isNaN))) {
+            // Check string by value rather than reference
+            _addToChanges(oldSeqRecord);
+          }
         }
       } else {
         seqChanged = true;

test/change_detection/dirty_checking_change_detector_spec.dart

@@ -395,6 +395,31 @@ main() => describe('DirtyCheckingChangeDetector', () {
           changes: [],
           removals: ['a[A -> null]', 'd[D -> null]']));
     });
+
+    it('should test string keys by value rather than by reference', () {
+      var map = {'foo': 0};
+      detector..watch(map, null, null)..collectChanges();
+
+      map['f' + 'oo'] = 0;
+
+      expect(detector.collectChanges()).toEqual(null);
+    });
+
+    it('should test string values by value rather than by reference', () {
+      var map = {'foo': 'bar'};
+      detector..watch(map, null, null)..collectChanges();
+
+      map['foo'] = 'b' + 'ar';
+
+      expect(detector.collectChanges()).toEqual(null);
+    });
+
+    it('should not see a NaN value as a change', () {
+      var map = {'foo': double.NAN};
+      var record = detector..watch(map, null, null)..collectChanges();
+
+      expect(detector.collectChanges()).toEqual(null);
+    });
   });
 
   describe('DuplicateMap', () {