feat(materialize): add higher-order lettable materialize operator

src/operator/materialize.ts

@@ -1,7 +1,7 @@
-import { Operator } from '../Operator';
+
 import { Observable } from '../Observable';
-import { Subscriber } from '../Subscriber';
 import { Notification } from '../Notification';
+import { materialize as higherOrder } from '../operators';
 
 /**
  * Represents all of the notifications from the source Observable as `next`
@@ -48,38 +48,5 @@ import { Notification } from '../Notification';
  * @owner Observable
  */
 export function materialize<T>(this: Observable<T>): Observable<Notification<T>> {
-  return this.lift(new MaterializeOperator());
-}
-
-class MaterializeOperator<T> implements Operator<T, Notification<T>> {
-  call(subscriber: Subscriber<Notification<T>>, source: any): any {
-    return source.subscribe(new MaterializeSubscriber(subscriber));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class MaterializeSubscriber<T> extends Subscriber<T> {
-  constructor(destination: Subscriber<Notification<T>>) {
-    super(destination);
-  }
-
-  protected _next(value: T) {
-    this.destination.next(Notification.createNext(value));
-  }
-
-  protected _error(err: any) {
-    const destination = this.destination;
-    destination.next(Notification.createError(err));
-    destination.complete();
-  }
-
-  protected _complete() {
-    const destination = this.destination;
-    destination.next(Notification.createComplete());
-    destination.complete();
-  }
+  return higherOrder()(this);
 }

src/operators/index.ts

@@ -9,6 +9,7 @@ export { dematerialize } from './dematerialize';
 export { filter } from './filter';
 export { ignoreElements } from './ignoreElements';
 export { map } from './map';
+export { materialize } from './materialize';
 export { max } from './max';
 export { mergeAll } from './mergeAll';
 export { mergeMap } from './mergeMap';

src/operators/materialize.ts

@@ -0,0 +1,88 @@
+import { Operator } from '../Operator';
+import { Observable } from '../Observable';
+import { Subscriber } from '../Subscriber';
+import { Notification } from '../Notification';
+import { OperatorFunction } from '../interfaces';
+
+/**
+ * Represents all of the notifications from the source Observable as `next`
+ * emissions marked with their original types within {@link Notification}
+ * objects.
+ *
+ * <span class="informal">Wraps `next`, `error` and `complete` emissions in
+ * {@link Notification} objects, emitted as `next` on the output Observable.
+ * </span>
+ *
+ * <img src="./img/materialize.png" width="100%">
+ *
+ * `materialize` returns an Observable that emits a `next` notification for each
+ * `next`, `error`, or `complete` emission of the source Observable. When the
+ * source Observable emits `complete`, the output Observable will emit `next` as
+ * a Notification of type "complete", and then it will emit `complete` as well.
+ * When the source Observable emits `error`, the output will emit `next` as a
+ * Notification of type "error", and then `complete`.
+ *
+ * This operator is useful for producing metadata of the source Observable, to
+ * be consumed as `next` emissions. Use it in conjunction with
+ * {@link dematerialize}.
+ *
+ * @example <caption>Convert a faulty Observable to an Observable of Notifications</caption>
+ * var letters = Rx.Observable.of('a', 'b', 13, 'd');
+ * var upperCase = letters.map(x => x.toUpperCase());
+ * var materialized = upperCase.materialize();
+ * materialized.subscribe(x => console.log(x));
+ *
+ * // Results in the following:
+ * // - Notification {kind: "N", value: "A", error: undefined, hasValue: true}
+ * // - Notification {kind: "N", value: "B", error: undefined, hasValue: true}
+ * // - Notification {kind: "E", value: undefined, error: TypeError:
+ * //   x.toUpperCase is not a function at MapSubscriber.letters.map.x
+ * //   [as project] (http://1…, hasValue: false}
+ *
+ * @see {@link Notification}
+ * @see {@link dematerialize}
+ *
+ * @return {Observable<Notification<T>>} An Observable that emits
+ * {@link Notification} objects that wrap the original emissions from the source
+ * Observable with metadata.
+ * @method materialize
+ * @owner Observable
+ */
+export function materialize<T>(): OperatorFunction<T, Notification<T>> {
+  return function materializeOperatorFunction(source: Observable<T>) {
+    return source.lift(new MaterializeOperator());
+  };
+}
+
+class MaterializeOperator<T> implements Operator<T, Notification<T>> {
+  call(subscriber: Subscriber<Notification<T>>, source: any): any {
+    return source.subscribe(new MaterializeSubscriber(subscriber));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class MaterializeSubscriber<T> extends Subscriber<T> {
+  constructor(destination: Subscriber<Notification<T>>) {
+    super(destination);
+  }
+
+  protected _next(value: T) {
+    this.destination.next(Notification.createNext(value));
+  }
+
+  protected _error(err: any) {
+    const destination = this.destination;
+    destination.next(Notification.createError(err));
+    destination.complete();
+  }
+
+  protected _complete() {
+    const destination = this.destination;
+    destination.next(Notification.createComplete());
+    destination.complete();
+  }
+}