fix(core): Switch emitDistinctChangesOnlyDefaultValue to true

The previous implementation would fire changes `QueryList.changes.subscribe`
whenever the `QueryList` was recomputed. This resulted in an artificially
high number of change notifications, as it is possible that recomputing
`QueryList` results in the same list. When the `QueryList` gets recomputed
is an implementation detail, and it should not be the thing that determines
how often change event should fire.

Unfortunately, fixing the behavior outright caused too many existing
applications to fail. For this reason, Angular considers this fix a
breaking fix and has introduced a flag in `@ContentChildren` and
`@ViewChildren`, that controls the behavior.

```
export class QueryCompWithStrictChangeEmitParent {
  @ContentChildren('foo', {
    // This option is the new default with this change.
    emitDistinctChangesOnly: true,
  })
  foos!: QueryList<any>;
}
```
For backward compatibility before v12
`emitDistinctChangesOnlyDefaultValue` was set to `false. This change
changes the default to `true`.

BREAKING CHANGE: Switching default of `emitDistinctChangesOnlyDefaultValue`
which changes the default behavior and may cause some applications which
rely on the incorrect behavior to fail.

packages/compiler-cli/ngcc/test/rendering/renderer_spec.ts

@@ -570,7 +570,7 @@ A.ɵdir = /*@__PURE__*/ ɵngcc0.ɵɵdefineDirective({ type: A, selectors: [["",
 UndecoratedBase.ɵfac = function UndecoratedBase_Factory(t) { return new (t || UndecoratedBase)(); };
 // TRANSPILED
 UndecoratedBase.ɵdir = /*@__PURE__*/ ɵngcc0.ɵɵdefineDirective({ type: UndecoratedBase, viewQuery: function UndecoratedBase_Query(rf, ctx) { if (rf & 1) {
-        ɵngcc0.ɵɵviewQuery(_c0, 3);
+        ɵngcc0.ɵɵviewQuery(_c0, 7);
     } if (rf & 2) {
         let _t;
         ɵngcc0.ɵɵqueryRefresh(_t = ɵngcc0.ɵɵloadQuery()) && (ctx.test = _t.first);

packages/compiler-cli/test/ngtsc/ngtsc_spec.ts

@@ -3122,10 +3122,10 @@ function allTests(os: string) {
       expect(jsContents).toMatch(varRegExp('test1'));
       expect(jsContents).toMatch(varRegExp('test2'));
       expect(jsContents).toMatch(varRegExp('accessor'));
-      // match `i0.ɵɵcontentQuery(dirIndex, _c1, 1, TemplateRef)`
-      expect(jsContents).toMatch(contentQueryRegExp('\\w+', 1, 'TemplateRef'));
-      // match `i0.ɵɵviewQuery(_c2, 1, null)`
-      expect(jsContents).toMatch(viewQueryRegExp('\\w+', 1));
+      // match `i0.ɵɵcontentQuery(dirIndex, _c1, 5, TemplateRef)`
+      expect(jsContents).toMatch(contentQueryRegExp('\\w+', 5, 'TemplateRef'));
+      // match `i0.ɵɵviewQuery(_c2, 5, null)`
+      expect(jsContents).toMatch(viewQueryRegExp('\\w+', 5));
     });
 
     it('should generate queries for directives', () => {
@@ -3154,14 +3154,14 @@ function allTests(os: string) {
       expect(jsContents).toMatch(varRegExp('test1'));
       expect(jsContents).toMatch(varRegExp('test2'));
       expect(jsContents).toMatch(varRegExp('accessor'));
-      // match `i0.ɵɵcontentQuery(dirIndex, _c1, 1, TemplateRef)`
-      expect(jsContents).toMatch(contentQueryRegExp('\\w+', 1, 'TemplateRef'));
+      // match `i0.ɵɵcontentQuery(dirIndex, _c1, 5, TemplateRef)`
+      expect(jsContents).toMatch(contentQueryRegExp('\\w+', 5, 'TemplateRef'));
 
-      // match `i0.ɵɵviewQuery(_c2, 1)`
+      // match `i0.ɵɵviewQuery(_c2, 5)`
       // Note that while ViewQuery doesn't necessarily make sense on a directive,
       // because it doesn't have a view, we still need to handle it because a component
       // could extend the directive.
-      expect(jsContents).toMatch(viewQueryRegExp('\\w+', 1));
+      expect(jsContents).toMatch(viewQueryRegExp('\\w+', 5));
     });
 
     it('should handle queries that use forwardRef', () => {
@@ -3183,13 +3183,13 @@ function allTests(os: string) {
 
       env.driveMain();
       const jsContents = env.getContents('test.js');
-      // match `i0.ɵɵcontentQuery(dirIndex, TemplateRef, 1, null)`
-      expect(jsContents).toMatch(contentQueryRegExp('TemplateRef', 1));
-      // match `i0.ɵɵcontentQuery(dirIndex, ViewContainerRef, 1, null)`
-      expect(jsContents).toMatch(contentQueryRegExp('ViewContainerRef', 1));
-      // match `i0.ɵɵcontentQuery(dirIndex, _c0, 1, null)`
+      // match `i0.ɵɵcontentQuery(dirIndex, TemplateRef, 5, null)`
+      expect(jsContents).toMatch(contentQueryRegExp('TemplateRef', 5));
+      // match `i0.ɵɵcontentQuery(dirIndex, ViewContainerRef, 5, null)`
+      expect(jsContents).toMatch(contentQueryRegExp('ViewContainerRef', 5));
+      // match `i0.ɵɵcontentQuery(dirIndex, _c0, 5, null)`
       expect(jsContents).toContain('_c0 = ["parens"];');
-      expect(jsContents).toMatch(contentQueryRegExp('_c0', 1));
+      expect(jsContents).toMatch(contentQueryRegExp('_c0', 5));
     });
 
     it('should handle queries that use an InjectionToken', () => {
@@ -3210,10 +3210,10 @@ function allTests(os: string) {
 
       env.driveMain();
       const jsContents = env.getContents('test.js');
-      // match `i0.ɵɵviewQuery(TOKEN, 1, null)`
-      expect(jsContents).toMatch(viewQueryRegExp('TOKEN', 1));
-      // match `i0.ɵɵcontentQuery(dirIndex, TOKEN, 1, null)`
-      expect(jsContents).toMatch(contentQueryRegExp('TOKEN', 1));
+      // match `i0.ɵɵviewQuery(TOKEN, 5, null)`
+      expect(jsContents).toMatch(viewQueryRegExp('TOKEN', 5));
+      // match `i0.ɵɵcontentQuery(dirIndex, TOKEN, 5, null)`
+      expect(jsContents).toMatch(contentQueryRegExp('TOKEN', 5));
     });
 
     it('should compile expressions that write keys', () => {

packages/compiler/src/core.ts

@@ -28,9 +28,8 @@ export const createAttribute =
     makeMetadataFactory<Attribute>('Attribute', (attributeName: string) => ({attributeName}));
 
 // Stores the default value of `emitDistinctChangesOnly` when the `emitDistinctChangesOnly` is not
-// explicitly set. This value will be changed to `true` in v12.
-// TODO(misko): switch the default in v12 to `true`. See: packages/core/src/metadata/di.ts
-export const emitDistinctChangesOnlyDefaultValue = false;
+// explicitly set.
+export const emitDistinctChangesOnlyDefaultValue = true;
 
 
 export interface Query {

packages/compiler/src/render3/partial/api.ts

@@ -237,9 +237,6 @@ export interface R3DeclareQueryMetadata {
   /**
    * True to only fire changes if there are underlying changes to the query.
    */
-  // TODO(misko):  This will become `true` be default in v12. `QueryList.changes` would fire even if
-  // no changes to the query list were detected. This is not ideal, as changes should only fire if
-  // the `QueryList` actually materially changed.
   emitDistinctChangesOnly?: boolean;
 
   /**

packages/core/src/linker/query_list.ts

@@ -62,8 +62,7 @@ export class QueryList<T> implements Iterable<T> {
   /**
    * @param emitDistinctChangesOnly Whether `QueryList.changes` should fire only when actual change
    *     has occurred. Or if it should fire when query is recomputed. (recomputing could resolve in
-   *     the same result) This is set to `false` for backwards compatibility but will be changed to
-   *     true in v12.
+   *     the same result)
    */
   constructor(private _emitDistinctChangesOnly: boolean = false) {
     // This function should be declared on the prototype, but doing so there will cause the class

packages/core/src/metadata/di.ts

@@ -107,9 +107,8 @@ export interface Query {
 }
 
 // Stores the default value of `emitDistinctChangesOnly` when the `emitDistinctChangesOnly` is not
-// explicitly set. This value will be changed to `true` in v12.
-// TODO(misko): switch the default in v12 to `true`. See: packages/compiler/src/core.ts
-export const emitDistinctChangesOnlyDefaultValue = false;
+// explicitly set.
+export const emitDistinctChangesOnlyDefaultValue = true;
 
 
 /**
@@ -148,8 +147,8 @@ export interface ContentChildrenDecorator {
    * * **selector** - The directive type or the name used for querying.
    * * **descendants** - True to include all descendants, otherwise include only direct children.
    * * **emitDistinctChangesOnly** - The ` QueryList#changes` observable will emit new values only
-   *   if the QueryList result has changed. The default value will change from `false` to `true` in
-   *   v12. When `false` the `changes` observable might emit even if the QueryList has not changed.
+   *   if the QueryList result has changed. When `false` the `changes` observable might emit even
+   *   if the QueryList has not changed.
    * * **read** - Used to read a different token from the queried elements.
    *
    * @usageNotes
@@ -287,8 +286,8 @@ export interface ViewChildrenDecorator {
    * * **selector** - The directive type or the name used for querying.
    * * **read** - Used to read a different token from the queried elements.
    * * **emitDistinctChangesOnly** - The ` QueryList#changes` observable will emit new values only
-   *   if the QueryList result has changed. The default value will change from `false` to `true` in
-   *   v12. When `false` the `changes` observable might emit even if the QueryList has not changed.
+   *   if the QueryList result has changed. When `false` the `changes` observable might emit even
+   *   if the QueryList has not changed.
    *
    * @usageNotes
    *

packages/core/test/acceptance/query_spec.ts

@@ -1298,7 +1298,7 @@ describe('query logic', () => {
            vc.move(viewRef, 0);
            fixture.detectChanges();
            expect(queryList.length).toBe(1);
-           expect(fixture.componentInstance.queryListNotificationCounter).toBe(2);
+           expect(fixture.componentInstance.queryListNotificationCounter).toBe(1);
          });
 
       it('should support a mix of content queries from the declaration and embedded view', () => {