feat(observeOn): add higher-order lettable version of observeOn

src/ReplaySubject.ts

@@ -3,7 +3,7 @@ import { IScheduler } from './Scheduler';
 import { queue } from './scheduler/queue';
 import { Subscriber } from './Subscriber';
 import { Subscription } from './Subscription';
-import { ObserveOnSubscriber } from './operator/observeOn';
+import { ObserveOnSubscriber } from './operators/observeOn';
 import { ObjectUnsubscribedError } from './util/ObjectUnsubscribedError';
 import { SubjectSubscription } from './SubjectSubscription';
 /**

src/observable/FromObservable.ts

@@ -10,7 +10,7 @@ import { IScheduler } from '../Scheduler';
 import { iterator as Symbol_iterator } from '../symbol/iterator';
 import { Observable, ObservableInput } from '../Observable';
 import { Subscriber } from '../Subscriber';
-import { ObserveOnSubscriber } from '../operator/observeOn';
+import { ObserveOnSubscriber } from '../operators/observeOn';
 import { observable as Symbol_observable } from '../symbol/observable';
 
 /**

src/operator/observeOn.ts

@@ -1,11 +1,6 @@
 import { Observable } from '../Observable';
 import { IScheduler } from '../Scheduler';
-import { Operator } from '../Operator';
-import { PartialObserver } from '../Observer';
-import { Subscriber } from '../Subscriber';
-import { Notification } from '../Notification';
-import { TeardownLogic } from '../Subscription';
-import { Action } from '../scheduler/Action';
+import { observeOn as higherOrder } from '../operators';
 
 /**
  *
@@ -54,59 +49,5 @@ import { Action } from '../scheduler/Action';
  * @owner Observable
  */
 export function observeOn<T>(this: Observable<T>, scheduler: IScheduler, delay: number = 0): Observable<T> {
-  return this.lift(new ObserveOnOperator(scheduler, delay));
-}
-
-export class ObserveOnOperator<T> implements Operator<T, T> {
-  constructor(private scheduler: IScheduler, private delay: number = 0) {
-  }
-
-  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
-    return source.subscribe(new ObserveOnSubscriber(subscriber, this.scheduler, this.delay));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-export class ObserveOnSubscriber<T> extends Subscriber<T> {
-  static dispatch(this: Action<ObserveOnMessage>, arg: ObserveOnMessage) {
-    const { notification, destination } = arg;
-    notification.observe(destination);
-    this.unsubscribe();
-  }
-
-  constructor(destination: Subscriber<T>,
-              private scheduler: IScheduler,
-              private delay: number = 0) {
-    super(destination);
-  }
-
-  private scheduleMessage(notification: Notification<any>): void {
-    this.add(this.scheduler.schedule(
-      ObserveOnSubscriber.dispatch,
-      this.delay,
-      new ObserveOnMessage(notification, this.destination)
-    ));
-  }
-
-  protected _next(value: T): void {
-    this.scheduleMessage(Notification.createNext(value));
-  }
-
-  protected _error(err: any): void {
-    this.scheduleMessage(Notification.createError(err));
-  }
-
-  protected _complete(): void {
-    this.scheduleMessage(Notification.createComplete());
-  }
-}
-
-export class ObserveOnMessage {
-  constructor(public notification: Notification<any>,
-              public destination: PartialObserver<any>) {
-  }
+  return higherOrder(scheduler, delay)(this);
 }

src/operators/index.ts

@@ -20,6 +20,7 @@ export { mergeAll } from './mergeAll';
 export { mergeMap } from './mergeMap';
 export { min } from './min';
 export { multicast } from './multicast';
+export { observeOn } from './observeOn';
 export { publish } from './publish';
 export { race } from './race';
 export { reduce } from './reduce';

src/operators/observeOn.ts

@@ -0,0 +1,115 @@
+import { Observable } from '../Observable';
+import { IScheduler } from '../Scheduler';
+import { Operator } from '../Operator';
+import { PartialObserver } from '../Observer';
+import { Subscriber } from '../Subscriber';
+import { Notification } from '../Notification';
+import { TeardownLogic } from '../Subscription';
+import { Action } from '../scheduler/Action';
+import { MonoTypeOperatorFunction } from '../interfaces';
+
+/**
+ *
+ * Re-emits all notifications from source Observable with specified scheduler.
+ *
+ * <span class="informal">Ensure a specific scheduler is used, from outside of an Observable.</span>
+ *
+ * `observeOn` is an operator that accepts a scheduler as a first parameter, which will be used to reschedule
+ * notifications emitted by the source Observable. It might be useful, if you do not have control over
+ * internal scheduler of a given Observable, but want to control when its values are emitted nevertheless.
+ *
+ * Returned Observable emits the same notifications (nexted values, complete and error events) as the source Observable,
+ * but rescheduled with provided scheduler. Note that this doesn't mean that source Observables internal
+ * scheduler will be replaced in any way. Original scheduler still will be used, but when the source Observable emits
+ * notification, it will be immediately scheduled again - this time with scheduler passed to `observeOn`.
+ * An anti-pattern would be calling `observeOn` on Observable that emits lots of values synchronously, to split
+ * that emissions into asynchronous chunks. For this to happen, scheduler would have to be passed into the source
+ * Observable directly (usually into the operator that creates it). `observeOn` simply delays notifications a
+ * little bit more, to ensure that they are emitted at expected moments.
+ *
+ * As a matter of fact, `observeOn` accepts second parameter, which specifies in milliseconds with what delay notifications
+ * will be emitted. The main difference between {@link delay} operator and `observeOn` is that `observeOn`
+ * will delay all notifications - including error notifications - while `delay` will pass through error
+ * from source Observable immediately when it is emitted. In general it is highly recommended to use `delay` operator
+ * for any kind of delaying of values in the stream, while using `observeOn` to specify which scheduler should be used
+ * for notification emissions in general.
+ *
+ * @example <caption>Ensure values in subscribe are called just before browser repaint.</caption>
+ * const intervals = Rx.Observable.interval(10); // Intervals are scheduled
+ *                                               // with async scheduler by default...
+ *
+ * intervals
+ * .observeOn(Rx.Scheduler.animationFrame)       // ...but we will observe on animationFrame
+ * .subscribe(val => {                           // scheduler to ensure smooth animation.
+ *   someDiv.style.height = val + 'px';
+ * });
+ *
+ * @see {@link delay}
+ *
+ * @param {IScheduler} scheduler Scheduler that will be used to reschedule notifications from source Observable.
+ * @param {number} [delay] Number of milliseconds that states with what delay every notification should be rescheduled.
+ * @return {Observable<T>} Observable that emits the same notifications as the source Observable,
+ * but with provided scheduler.
+ *
+ * @method observeOn
+ * @owner Observable
+ */
+export function observeOn<T>(scheduler: IScheduler, delay: number = 0): MonoTypeOperatorFunction<T> {
+  return function observeOnOperatorFunction(source: Observable<T>): Observable<T> {
+    return source.lift(new ObserveOnOperator(scheduler, delay));
+  };
+}
+
+export class ObserveOnOperator<T> implements Operator<T, T> {
+  constructor(private scheduler: IScheduler, private delay: number = 0) {
+  }
+
+  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
+    return source.subscribe(new ObserveOnSubscriber(subscriber, this.scheduler, this.delay));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+export class ObserveOnSubscriber<T> extends Subscriber<T> {
+  static dispatch(this: Action<ObserveOnMessage>, arg: ObserveOnMessage) {
+    const { notification, destination } = arg;
+    notification.observe(destination);
+    this.unsubscribe();
+  }
+
+  constructor(destination: Subscriber<T>,
+              private scheduler: IScheduler,
+              private delay: number = 0) {
+    super(destination);
+  }
+
+  private scheduleMessage(notification: Notification<any>): void {
+    this.add(this.scheduler.schedule(
+      ObserveOnSubscriber.dispatch,
+      this.delay,
+      new ObserveOnMessage(notification, this.destination)
+    ));
+  }
+
+  protected _next(value: T): void {
+    this.scheduleMessage(Notification.createNext(value));
+  }
+
+  protected _error(err: any): void {
+    this.scheduleMessage(Notification.createError(err));
+  }
+
+  protected _complete(): void {
+    this.scheduleMessage(Notification.createComplete());
+  }
+}
+
+export class ObserveOnMessage {
+  constructor(public notification: Notification<any>,
+              public destination: PartialObserver<any>) {
+  }
+}