feat(cdk): enhance rxIfList with a template to render in falsy case

libs/cdk/template/src/lib/rx-if-list/rx-if-list.directive.ts

@@ -1,20 +1,121 @@
-import {Directive, Input, NgModule, TemplateRef, ViewContainerRef} from '@angular/core';
+import {
+  Directive,
+  EmbeddedViewRef,
+  Input,
+  NgModule,
+  TemplateRef,
+  ViewContainerRef,
+  ɵstringify as stringify
+} from '@angular/core';
 import {CommonModule} from '@angular/common';
 
 @Directive({
+  // eslint-disable-next-line @angular-eslint/directive-selector
   selector: '[rxIfList]',
 })
 export class RxIfListDirective {
-  @Input() set rxIfList(value: ArrayLike<unknown> | null | undefined) {
-    this.vcr.clear();
-    if ((value?.length ?? []) > 0) {
-      this.vcr.createEmbeddedView(this.tpl);
+  private _context: RxIfListContext = new RxIfListContext();
+  private _thenTemplateRef: TemplateRef<RxIfListContext>|null = null;
+  private _elseTemplateRef: TemplateRef<RxIfListContext>|null = null;
+  private _thenViewRef: EmbeddedViewRef<RxIfListContext>|null = null;
+  private _elseViewRef: EmbeddedViewRef<RxIfListContext>|null = null;
+  /** @internal */
+  static rxIfListUseIfTypeGuard: void;
+
+  /**
+   * Assert the correct type of the expression bound to the `rxIfList` input within the template.
+   *
+   * The presence of this static field is a signal to the Ivy template type check compiler that
+   * when the `rxIfList` structural directive renders its template, the type of the expression bound
+   * to `rxIfList` should be narrowed in some way. For `rxIfList`, the binding expression itself is used to
+   * narrow its type, which allows the strictNullChecks feature of TypeScript to work with `rxIfList`.
+   */
+  static ngTemplateGuard_rxIfList: 'binding';
+
+  /**
+   * Asserts the correct type of the context for the template that `rxIfList` will render.
+   *
+   * The presence of this method is a signal to the Ivy template type-check compiler that the
+   * `rxIfList` structural directive renders its template with a specific context type.
+   */
+  static ngTemplateContextGuard<T>(dir: RxIfListDirective, ctx: any):
+    ctx is RxIfListContext{
+    return true;
+  }
+
+  constructor(private _viewContainer: ViewContainerRef, templateRef: TemplateRef<RxIfListContext>) {
+    this._thenTemplateRef = templateRef;
+  }
+
+  /**
+   * The Boolean expression to evaluate as the condition for showing a template.
+   */
+  @Input()
+  set rxIfList(value: ArrayLike<unknown> | null | undefined) {
+    this._context.$implicit = this._context.rxIfList = ((value?.length ?? []) > 0);
+    this._updateView();
+  }
+
+  /**
+   * A template to show if the condition expression evaluates to true.
+   */
+  @Input()
+  set rxIfListThen(templateRef: TemplateRef<RxIfListContext>|null) {
+    assertTemplate('rxIfListThen', templateRef);
+    this._thenTemplateRef = templateRef;
+    this._thenViewRef = null;  // clear previous view if any.
+    this._updateView();
+  }
+
+  /**
+   * A template to show if the condition expression evaluates to false.
+   */
+  @Input()
+  set rxIfListElse(templateRef: TemplateRef<RxIfListContext>|null) {
+    assertTemplate('rxIfListElse', templateRef);
+    this._elseTemplateRef = templateRef;
+    this._elseViewRef = null;  // clear previous view if any.
+    this._updateView();
+  }
+
+  private _updateView() {
+    if (this._context.$implicit) {
+      if (!this._thenViewRef) {
+        this._viewContainer.clear();
+        this._elseViewRef = null;
+        if (this._thenTemplateRef) {
+          this._thenViewRef =
+            this._viewContainer.createEmbeddedView(this._thenTemplateRef, this._context);
+        }
+      }
+    } else {
+      if (!this._elseViewRef) {
+        this._viewContainer.clear();
+        this._thenViewRef = null;
+        if (this._elseTemplateRef) {
+          this._elseViewRef =
+            this._viewContainer.createEmbeddedView(this._elseTemplateRef, this._context);
+        }
+      }
     }
   }
-  constructor(
-    private readonly vcr: ViewContainerRef,
-    private readonly tpl: TemplateRef<unknown>
-  ) {}
+
+
+}
+
+/**
+ * @publicApi
+ */
+export class RxIfListContext {
+   $implicit = false;
+   rxIfList = false;
+}
+
+function assertTemplate(property: string, templateRef: TemplateRef<any>|null): void {
+  const isTemplateRefOrNull = !!(!templateRef || templateRef.createEmbeddedView);
+  if (!isTemplateRefOrNull) {
+    throw new Error(`${property} must be a TemplateRef, but received '${stringify(templateRef)}'.`);
+  }
 }
 
 @NgModule({