feat(component): clear LetDirective view when replaced observable i…

…s in suspense state (#3671)

BREAKING CHANGES:

The `LetDirective` view will be cleared when the replaced observable is in a suspense state. Also, the `suspense` property is removed from the `LetViewContext` because it would always be `false` when the `LetDirective` view is rendered. Instead of `suspense` property, use the suspense template to handle the suspense state.

BEFORE:

The `LetDirective` view will not be cleared when the replaced observable is in a suspense state and the suspense template is not passed:

```ts
@component({
  template: `
    <!-- When button is clicked, the 'LetDirective' view won't be cleared. -->
    <!-- Instead, the value of 'o' will be 'undefined' until the replaced -->
    <!-- observable emits the first value (after 1 second). -->
    <p *ngrxLet="obs$ as o">{{ o }}</p>
    <button (click)="replaceObs()">Replace Observable</button>
  `
})
export class TestComponent {
  obs$ = of(1);

  replaceObs(): void {
    this.obs$ = of(2).pipe(delay(1000));
  }
}
```

AFTER:

The `LetDirective` view will be cleared when the replaced observable is in a suspense state and the suspense template is not passed:

```ts
@component({
  template: `
    <!-- When button is clicked, the 'LetDirective' view will be cleared. -->
    <!-- The view will be created again when the replaced observable -->
    <!-- emits the first value (after 1 second). -->
    <p *ngrxLet="obs$ as o">{{ o }}</p>
    <button (click)="replaceObs()">Replace Observable</button>
  `
})
export class TestComponent {
  obs$ = of(1);

  replaceObs(): void {
    this.obs$ = of(2).pipe(delay(1000));
  }
}
```

modules/component/spec/let/let.directive.spec.ts

@@ -66,9 +66,7 @@ class LetDirectiveTestCompleteComponent {
 
 @Component({
   template: `
-    <ng-container *ngrxLet="value$ as value; suspense as s">{{
-      s ? 'suspense' : value
-    }}</ng-container>
+    <ng-container *ngrxLet="value$ as value">{{ value }}</ng-container>
   `,
 })
 class LetDirectiveTestSuspenseComponent {
@@ -338,13 +336,13 @@ describe('LetDirective', () => {
       expect(componentNativeElement.textContent).toBe('42');
     }));
 
-    it('should render undefined as value when a new observable NEVER was passed (as no value ever was emitted from new observable)', () => {
+    it('should clear the view when a new observable NEVER was passed (as no value ever was emitted from new observable)', () => {
       letDirectiveTestComponent.value$ = of(42);
       fixtureLetDirectiveTestComponent.detectChanges();
       expect(componentNativeElement.textContent).toBe('42');
       letDirectiveTestComponent.value$ = NEVER;
       fixtureLetDirectiveTestComponent.detectChanges();
-      expect(componentNativeElement.textContent).toBe('undefined');
+      expect(componentNativeElement.textContent).toBe('');
     });
 
     it('should render new value when a new observable was passed', () => {
@@ -453,12 +451,12 @@ describe('LetDirective', () => {
       expect(componentNativeElement.textContent).toBe('true');
     }));
 
-    it('should render suspense when next observable is in suspense state', fakeAsync(() => {
+    it('should clear the view when next observable is in suspense state', fakeAsync(() => {
       letDirectiveTestComponent.value$ = of(true);
       fixtureLetDirectiveTestComponent.detectChanges();
       letDirectiveTestComponent.value$ = of(false).pipe(delay(1000));
       fixtureLetDirectiveTestComponent.detectChanges();
-      expect(componentNativeElement.textContent).toBe('suspense');
+      expect(componentNativeElement.textContent).toBe('');
       tick(1000);
       fixtureLetDirectiveTestComponent.detectChanges();
       expect(componentNativeElement.textContent).toBe('false');

modules/component/src/let/let.directive.ts

@@ -31,10 +31,6 @@ export interface LetViewContext<PO> {
    * `*ngrxLet="obs$; let c = complete"` or `*ngrxLet="obs$; complete as c"`
    */
   complete: boolean;
-  /**
-   * `*ngrxLet="obs$; let s = suspense"` or `*ngrxLet="obs$; suspense as s"`
-   */
-  suspense: boolean;
 }
 
 /**
@@ -119,22 +115,19 @@ export class LetDirective<PO> implements OnInit, OnDestroy {
     ngrxLet: undefined,
     error: undefined,
     complete: false,
-    suspense: true,
   };
   private readonly renderEventManager = createRenderEventManager<PO>({
     suspense: () => {
       this.viewContext.$implicit = undefined;
       this.viewContext.ngrxLet = undefined;
       this.viewContext.error = undefined;
       this.viewContext.complete = false;
-      this.viewContext.suspense = true;
 
       this.renderSuspenseView();
     },
     next: (event) => {
       this.viewContext.$implicit = event.value;
       this.viewContext.ngrxLet = event.value;
-      this.viewContext.suspense = false;
 
       if (event.reset) {
         this.viewContext.error = undefined;
@@ -145,7 +138,6 @@ export class LetDirective<PO> implements OnInit, OnDestroy {
     },
     error: (event) => {
       this.viewContext.error = event.error;
-      this.viewContext.suspense = false;
 
       if (event.reset) {
         this.viewContext.$implicit = undefined;
@@ -158,7 +150,6 @@ export class LetDirective<PO> implements OnInit, OnDestroy {
     },
     complete: (event) => {
       this.viewContext.complete = true;
-      this.viewContext.suspense = false;
 
       if (event.reset) {
         this.viewContext.$implicit = undefined;
@@ -224,7 +215,7 @@ export class LetDirective<PO> implements OnInit, OnDestroy {
   }
 
   private renderSuspenseView(): void {
-    if (this.suspenseTemplateRef && this.isMainViewCreated) {
+    if (this.isMainViewCreated) {
       this.isMainViewCreated = false;
       this.viewContainerRef.clear();
     }

projects/ngrx.io/content/guide/migration/v15.md

@@ -168,3 +168,56 @@ AFTER:
   ...
 </ng-container>
 ```
+
+#### LetDirective Behavior on Suspense Event
+
+The `LetDirective` view will be cleared when the replaced observable is in a suspense state.
+Also, the `suspense` property is removed from the `LetViewContext` because it would always be `false` when the `LetDirective` view is rendered.
+Instead of `suspense` property, use [suspense template](guide/component/let#using-suspense-template) to handle the suspense state.
+
+BEFORE:
+
+The `LetDirective` view will not be cleared when the replaced observable is in a suspense state and the suspense template is not passed:
+
+```ts
+@Component({
+  template: `
+    <!-- When button is clicked, the 'LetDirective' view won't be cleared. -->
+    <!-- Instead, the value of 'o' will be 'undefined' until the replaced --> 
+    <!-- observable emits the first value (after 1 second). -->
+    <p *ngrxLet="obs$ as o">{{ o }}</p>
+    <button (click)="replaceObs()">Replace Observable</button>
+  `
+})
+export class TestComponent {
+  obs$ = of(1);
+
+  replaceObs(): void {
+    this.obs$ = of(2).pipe(delay(1000));
+  }
+}
+```
+
+AFTER:
+
+The `LetDirective` view will be cleared when the replaced observable is in a suspense state and the suspense template is not passed:
+
+```ts
+@Component({
+  template: `
+    <!-- When button is clicked, the 'LetDirective' view will be cleared. -->
+    <!-- The view will be created again when the replaced observable -->
+    <!-- emits the first value (after 1 second). -->
+    <p *ngrxLet="obs$ as o">{{ o }}</p>
+    <button (click)="replaceObs()">Replace Observable</button>
+  `
+})
+export class TestComponent {
+  obs$ = of(1);
+
+  replaceObs(): void {
+    this.obs$ = of(2).pipe(delay(1000));
+  }
+}
+```
+