feat(change_detection): do not register a change from switching from null to null

Author: vsavkin

modules/change_detection/src/array_changes.js

@@ -52,6 +52,10 @@ export class ArrayChanges {
     return isListLikeIterable(obj);
   }
 
+  supportsObj(obj):boolean {
+    return ArrayChanges.supports(obj);
+  }
+
   get collection() {
     return this._collection;
   }

modules/change_detection/src/change_detection_util.js

@@ -85,6 +85,10 @@ function _changeRecord(bindingMemento, change) {
 
 var _singleElementList = [null];
 
+function _isBlank(val):boolean {
+  return isBlank(val) || val === uninitialized;
+}
+
 export class ChangeDetectionUtil {
   static unitialized() {
     return uninitialized;
@@ -146,37 +150,28 @@ export class ChangeDetectionUtil {
   }
 
   static structuralCheck(self, context) {
-    if (isBlank(self) || self === uninitialized) {
+    if (_isBlank(self) && _isBlank(context)) {
+      return null;
+    } else if (_isBlank(context)) {
+      return new SimpleChange(null, null);
+    }
+
+    if (_isBlank(self)) {
       if (ArrayChanges.supports(context)) {
         self = new ArrayChanges();
       } else if (KeyValueChanges.supports(context)) {
         self = new KeyValueChanges();
       }
     }
 
-    if (isBlank(context) || context === uninitialized) {
-      return new SimpleChange(null, null);
+    if (isBlank(self) || !self.supportsObj(context)) {
+      throw new BaseException(`Unsupported type (${context})`);
+    }
 
+    if (self.check(context)) {
+      return new SimpleChange(null, self); // TODO: don't wrap and return self instead
     } else {
-      if (ArrayChanges.supports(context)) {
-
-        if (self.check(context)) {
-          return new SimpleChange(null, self); // TODO: don't wrap and return self instead
-        } else {
-          return null;
-        }
-
-      } else if (KeyValueChanges.supports(context)) {
-
-        if (self.check(context)) {
-          return new SimpleChange(null, self); // TODO: don't wrap and return self instead
-        } else {
-          return null;
-        }
-
-      } else {
-        throw new BaseException(`Unsupported type (${context})`);
-      }
+      return null;
     }
   }
 

modules/change_detection/src/keyvalue_changes.js

@@ -30,6 +30,10 @@ export class KeyValueChanges {
     return obj instanceof Map || isJsObject(obj);
   }
 
+  supportsObj(obj):boolean {
+    return KeyValueChanges.supports(obj);
+  }
+
   get isDirty():boolean {
     return this._additionsHead !== null ||
            this._changesHead !== null ||

modules/change_detection/test/change_detection_spec.js

@@ -283,16 +283,23 @@ export function main() {
           });
 
           describe("collections", () => {
-            it("should support null values", () => {
+            it("should not register a change when going from null to null", () => {
               var context = new TestData(null);
 
               var c = createChangeDetector('a', 'a', context, null, true);
               var cd = c["changeDetector"];
               var dispatcher = c["dispatcher"];
 
               cd.detectChanges();
-              expect(dispatcher.log).toEqual(['a=null']);
-              dispatcher.clear();
+              expect(dispatcher.log).toEqual([]);
+            });
+
+            it("should register changes when switching from null to collection and back", () => {
+              var context = new TestData(null);
+
+              var c = createChangeDetector('a', 'a', context, null, true);
+              var cd = c["changeDetector"];
+              var dispatcher = c["dispatcher"];
 
               context.a = [0];
               cd.detectChanges();