fix(elements): fire custom element output events during component initialization (#37570)

Previously, event listeners for component output events attached on an
Angular custom element before inserting it into the DOM (i.e. before
instantiating the underlying component) didn't fire for events emitted
during initialization lifecycle hooks, such as `ngAfterContentInit`,
`ngAfterViewInit`, `ngOnChanges` (initial call) and `ngOnInit`.
The reason was that `NgElementImpl` [subscribed to events][1] _after_
calling [ngElementStrategy#connect()][2], which is where the
[initial change detection][3] takes place (running the initialization
lifecycle hooks).

This commit fixes this by:
1. Ensuring `ComponentNgElementStrategy#events` is defined and available
   for subscribing to, even before instantiating the component.
2. Changing `NgElementImpl` to subscribe to `NgElementStrategy#events`
   (if available) before calling `NgElementStrategy#connect()` (which
   initializes the component instance) if available.
3. Falling back to the old behavior (subscribing to `events` after
   calling `connect()` for strategies that do not initialize `events`
   before their `connect()` is run).

NOTE:
By falling back to the old behavior when `NgElementStrategy#events` is
not initialized before calling `NgElementStrategy#connect()`, we avoid
breaking existing custom `NgElementStrategy` implementations (with
@remackgeek's [ElementZoneStrategy][4] being a commonly used example).

Jira issue: [FW-2010](https://angular-team.atlassian.net/browse/FW-2010)

[1]: c0143cb2ab/packages/elements/src/create-custom-element.ts (L167-L170)
[2]: c0143cb2ab/packages/elements/src/create-custom-element.ts (L164)
[3]: c0143cb2ab/packages/elements/src/component-factory-strategy.ts (L158)
[4]: f1b6699495/projects/elements-zone-strategy/src/lib/element-zone-strategy.ts

Fixes #36141

PR Close #37570

packages/elements/src/component-factory-strategy.ts

@@ -7,8 +7,8 @@
  */
 
 import {ApplicationRef, ComponentFactory, ComponentFactoryResolver, ComponentRef, EventEmitter, Injector, OnChanges, SimpleChange, SimpleChanges, Type} from '@angular/core';
-import {merge, Observable} from 'rxjs';
-import {map} from 'rxjs/operators';
+import {merge, Observable, ReplaySubject} from 'rxjs';
+import {map, switchMap} from 'rxjs/operators';
 
 import {NgElementStrategy, NgElementStrategyEvent, NgElementStrategyFactory} from './element-strategy';
 import {extractProjectableNodes} from './extract-projectable-nodes';
@@ -43,9 +43,11 @@ export class ComponentNgElementStrategyFactory implements NgElementStrategyFacto
  * @publicApi
  */
 export class ComponentNgElementStrategy implements NgElementStrategy {
+  // Subject of `NgElementStrategyEvent` observables corresponding to the component's outputs.
+  private eventEmitters = new ReplaySubject<Observable<NgElementStrategyEvent>[]>(1);
+
   /** Merged stream of the component's output events. */
-  // TODO(issue/24571): remove '!'.
-  events!: Observable<NgElementStrategyEvent>;
+  readonly events = this.eventEmitters.pipe(switchMap(emitters => merge(...emitters)));
 
   /** Reference to the component that was created on connect. */
   private componentRef: ComponentRef<any>|null = null;
@@ -187,12 +189,13 @@ export class ComponentNgElementStrategy implements NgElementStrategy {
 
   /** Sets up listeners for the component's outputs so that the events stream emits the events. */
   protected initializeOutputs(componentRef: ComponentRef<any>): void {
-    const eventEmitters = this.componentFactory.outputs.map(({propName, templateName}) => {
-      const emitter: EventEmitter<any> = componentRef.instance[propName];
-      return emitter.pipe(map(value => ({name: templateName, value})));
-    });
+    const eventEmitters: Observable<NgElementStrategyEvent>[] =
+        this.componentFactory.outputs.map(({propName, templateName}) => {
+          const emitter: EventEmitter<any> = componentRef.instance[propName];
+          return emitter.pipe(map(value => ({name: templateName, value})));
+        });
 
-    this.events = merge(...eventEmitters);
+    this.eventEmitters.next(eventEmitters);
   }
 
   /** Calls ngOnChanges with all the inputs that have changed since the last call. */

packages/elements/src/create-custom-element.ts

@@ -187,13 +187,30 @@ export function createCustomElement<P>(
     }
 
     connectedCallback(): void {
+      // For historical reasons, some strategies may not have initialized the `events` property
+      // until after `connect()` is run. Subscribe to `events` if it is available before running
+      // `connect()` (in order to capture events emitted suring inittialization), otherwise
+      // subscribe afterwards.
+      //
+      // TODO: Consider deprecating/removing the post-connect subscription in a future major version
+      //       (e.g. v11).
+
+      let subscribedToEvents = false;
+
+      if (this.ngElementStrategy.events) {
+        // `events` are already available: Subscribe to it asap.
+        this.subscribeToEvents();
+        subscribedToEvents = true;
+      }
+
       this.ngElementStrategy.connect(this);
 
-      // Listen for events from the strategy and dispatch them as custom events
-      this.ngElementEventsSubscription = this.ngElementStrategy.events.subscribe(e => {
-        const customEvent = createCustomEvent(this.ownerDocument!, e.name, e.value);
-        this.dispatchEvent(customEvent);
-      });
+      if (!subscribedToEvents) {
+        // `events` were not initialized before running `connect()`: Subscribe to them now.
+        // The events emitted during the component initialization have been missed, but at least
+        // future events will be captured.
+        this.subscribeToEvents();
+      }
     }
 
     disconnectedCallback(): void {
@@ -207,6 +224,14 @@ export function createCustomElement<P>(
         this.ngElementEventsSubscription = null;
       }
     }
+
+    private subscribeToEvents(): void {
+      // Listen for events from the strategy and dispatch them as custom events.
+      this.ngElementEventsSubscription = this.ngElementStrategy.events.subscribe(e => {
+        const customEvent = createCustomEvent(this.ownerDocument!, e.name, e.value);
+        this.dispatchEvent(customEvent);
+      });
+    }
   }
 
   // TypeScript 3.9+ defines getters/setters as configurable but non-enumerable properties (in