feat(TestScheduler): Add subscription schedule to expectObservable (#3997)

Author: gstamac

doc/marble-testing.md

@@ -44,7 +44,7 @@ testScheduler.run(helpers => {
 
 - `hot(marbleDiagram: string, values?: object, error?: any)` - creates a ["hot" observable](https://medium.com/@benlesh/hot-vs-cold-observables-f8094ed53339) (like a subject) that will behave as though it's already "running" when the test begins. An interesting difference is that `hot` marbles allow a `^` character to signal where the "zero frame" is. That is the point at which the subscription to observables being tested begins.
 - `cold(marbleDiagram: string, values?: object, error?: any)` - creates a ["cold" observable](https://medium.com/@benlesh/hot-vs-cold-observables-f8094ed53339) whose subscription starts when the test begins.
-- `expectObservable(actual: Observable<T>).toBe(marbleDiagram: string, values?: object, error?: any)` - schedules an assertion for when the TestScheduler flushes.
+- `expectObservable(actual: Observable<T>).toBe(marbleDiagram: string, values?: object, error?: any)` - schedules an assertion for when the TestScheduler flushes. Give `subscriptionMarbles` as parameter to change the schedule of subscription and unsubscription. If you don't provide the `subscriptionMarbles` parameter it will subscribe at the beginning and never unsubscribe. Read below about subscription marble diagram.
 - `expectSubscriptions(actualSubscriptionLogs: SubscriptionLog[]).toBe(subscriptionMarbles: string)` - like `expectObservable` schedules an assertion for when the testScheduler flushes. Both `cold()` and `hot()` return an observable with a property `subscriptions` of type `SubscriptionLog[]`. Give `subscriptions` as parameter to `expectSubscriptions` to assert whether it matches the `subscriptionsMarbles` marble diagram given in `toBe()`. Subscription marble diagrams are slightly different than Observable marble diagrams. Read more below.
 - `flush()` - immediately starts virtual time. Not often used since `run()` will automatically flush for you when your callback returns, but in some cases you may wish to flush more than once or otherwise have more control.
 

spec/schedulers/TestScheduler-spec.ts

@@ -248,6 +248,13 @@ describe('TestScheduler', () => {
         const expected =      '--a';
         expectObservable(source, unsubscribe).toBe(expected);
       });
+
+      it('should accept a subscription marble diagram', () => {
+        const source = hot('-a-b-c|');
+        const subscribe =  '---^';
+        const expected =   '---b-c|';
+        expectObservable(source, subscribe).toBe(expected);
+      });
     });
 
     describe('expectSubscriptions()', () => {

src/internal/testing/TestScheduler.ts

@@ -92,11 +92,13 @@ export class TestScheduler extends VirtualTimeScheduler {
   }
 
   expectObservable(observable: Observable<any>,
-                   unsubscriptionMarbles: string = null): ({ toBe: observableToBeFn }) {
+                   subscriptionMarbles: string = null): ({ toBe: observableToBeFn }) {
     const actual: TestMessage[] = [];
     const flushTest: FlushableTest = { actual, ready: false };
-    const unsubscriptionFrame = TestScheduler
-      .parseMarblesAsSubscriptions(unsubscriptionMarbles, this.runMode).unsubscribedFrame;
+    const subscriptionParsed = TestScheduler.parseMarblesAsSubscriptions(subscriptionMarbles, this.runMode);
+    const subscriptionFrame = subscriptionParsed.subscribedFrame === Number.POSITIVE_INFINITY ?
+      0 : subscriptionParsed.subscribedFrame;
+    const unsubscriptionFrame = subscriptionParsed.unsubscribedFrame;
     let subscription: Subscription;
 
     this.schedule(() => {
@@ -112,7 +114,7 @@ export class TestScheduler extends VirtualTimeScheduler {
       }, () => {
         actual.push({ frame: this.frame, notification: Notification.createComplete() });
       });
-    }, 0);
+    }, subscriptionFrame);
 
     if (unsubscriptionFrame !== Number.POSITIVE_INFINITY) {
       this.schedule(() => subscription.unsubscribe(), unsubscriptionFrame);