feat(zones)!: deliver Observable read values as microtasks

src/zones.spec.ts

@@ -0,0 +1,43 @@
+import { EnvironmentInjector, NgZone, runInInjectionContext } from '@angular/core';
+import { TestBed } from '@angular/core/testing';
+import { ɵAngularFireSchedulers, ɵzoneWrap } from '@angular/fire';
+import { Observable, Subject, asapScheduler } from 'rxjs';
+import { AsapScheduler } from 'rxjs/internal/scheduler/AsapScheduler';
+import { take } from 'rxjs/operators';
+
+describe('ɵzoneWrap delivery timing', () => {
+
+  let source: Subject<number>;
+  let stream: Observable<number>;
+  let ngZone: NgZone;
+
+  beforeEach(() => {
+    source = new Subject<number>();
+    ngZone = TestBed.inject(NgZone);
+    const read = ɵzoneWrap(() => source.asObservable(), true);
+    stream = runInInjectionContext(TestBed.inject(EnvironmentInjector), () => ngZone.run(() => read()));
+  });
+
+  it('delivers an Observable value asynchronously, inside the Angular zone, before a zero-delay timer registered when the source emitted', async () => {
+    const delivery = await new Promise<{ delivered: boolean; afterEmit: boolean; inAngularZone: boolean }>(resolve => {
+      let emitted = false;
+      stream.pipe(take(1)).subscribe(() => resolve({ delivered: true, afterEmit: emitted, inAngularZone: NgZone.isInAngularZone() }));
+      ngZone.runOutsideAngular(() => {
+        // The timer races the delivery, so a dropped or late value fails here, not on the spec timeout.
+        setTimeout(() => resolve({ delivered: false, afterEmit: emitted, inAngularZone: false }), 0);
+        source.next(1);
+        emitted = true;
+      });
+    });
+    expect(delivery.delivered).withContext('delivered before the zero-delay timer').toBe(true);
+    expect(delivery.afterEmit).withContext('never synchronous').toBe(true);
+    expect(delivery.inAngularZone).withContext('delivered inside the Angular zone').toBe(true);
+  });
+
+  it('delivers through its own asap scheduler, not the one rxjs shares with every other caller', () => {
+    const delegate = Reflect.get(TestBed.inject(ɵAngularFireSchedulers).insideAngular, 'delegate');
+    expect(delegate).toBeInstanceOf(AsapScheduler);
+    expect(delegate).not.toBe(asapScheduler);
+  });
+
+});

src/zones.ts

@@ -14,13 +14,16 @@ import {
   SchedulerAction,
   SchedulerLike,
   Subscription,
-  asyncScheduler,
   queueScheduler
 } from 'rxjs';
+import { AsapAction } from 'rxjs/internal/scheduler/AsapAction';
+import { AsapScheduler } from 'rxjs/internal/scheduler/AsapScheduler';
 import { observeOn, subscribeOn } from 'rxjs/operators';
 
 declare const Zone: {current: unknown} | undefined;
 
+const microtaskScheduler = new AsapScheduler(AsapAction);
+
 export enum LogLevel {
   "SILENT" = 0, 
   "WARN" = 1,
@@ -57,9 +60,11 @@ export class ɵZoneScheduler implements SchedulerLike {
       }
     };
 
-    // Scheduling itself needs to be run in zone to ensure setInterval calls for async scheduling are done
-    // inside the correct zone. This scheduler needs to schedule asynchronously always to ensure that
-    // firebase emissions are never synchronous. Specifying a delay causes issues with the queueScheduler delegate.
+    /* Delivery re-enters the target zone through runGuarded above, whatever zone the delegate's
+     * microtask or timer runs in. The delegate must always schedule asynchronously, so firebase
+     * emissions are never synchronous, and with the asap delegate a positive delay falls back to a
+     * timer, so callers must keep the delay at zero to keep microtask delivery. Specifying a delay
+     * causes issues with the queueScheduler delegate. */
     return this.delegate.schedule(workInZone, delay, state);
   }
 }
@@ -79,7 +84,7 @@ export class ɵAngularFireSchedulers {
     this.insideAngular = ngZone.run(
       () => new ɵZoneScheduler(
         typeof Zone === 'undefined' ? undefined : Zone.current,
-        asyncScheduler
+        microtaskScheduler
       )
     );
   }