feat(exhaustMap): add higher-order lettable exhaustMap

src/operators/exhaustMap.ts

@@ -0,0 +1,152 @@
+import { Operator } from '../Operator';
+import { Observable, ObservableInput } from '../Observable';
+import { Subscriber } from '../Subscriber';
+import { Subscription } from '../Subscription';
+import { OuterSubscriber } from '../OuterSubscriber';
+import { InnerSubscriber } from '../InnerSubscriber';
+import { subscribeToResult } from '../util/subscribeToResult';
+import { OperatorFunction } from '../interfaces';
+
+/* tslint:disable:max-line-length */
+export function exhaustMap<T, R>(project: (value: T, index: number) => ObservableInput<R>): OperatorFunction<T, R>;
+export function exhaustMap<T, I, R>(project: (value: T, index: number) => ObservableInput<I>, resultSelector: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R): OperatorFunction<T, R>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Projects each source value to an Observable which is merged in the output
+ * Observable only if the previous projected Observable has completed.
+ *
+ * <span class="informal">Maps each value to an Observable, then flattens all of
+ * these inner Observables using {@link exhaust}.</span>
+ *
+ * <img src="./img/exhaustMap.png" width="100%">
+ *
+ * Returns an Observable that emits items based on applying a function that you
+ * supply to each item emitted by the source Observable, where that function
+ * returns an (so-called "inner") Observable. When it projects a source value to
+ * an Observable, the output Observable begins emitting the items emitted by
+ * that projected Observable. However, `exhaustMap` ignores every new projected
+ * Observable if the previous projected Observable has not yet completed. Once
+ * that one completes, it will accept and flatten the next projected Observable
+ * and repeat this process.
+ *
+ * @example <caption>Run a finite timer for each click, only if there is no currently active timer</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var result = clicks.exhaustMap((ev) => Rx.Observable.interval(1000).take(5));
+ * result.subscribe(x => console.log(x));
+ *
+ * @see {@link concatMap}
+ * @see {@link exhaust}
+ * @see {@link mergeMap}
+ * @see {@link switchMap}
+ *
+ * @param {function(value: T, ?index: number): ObservableInput} project A function
+ * that, when applied to an item emitted by the source Observable, returns an
+ * Observable.
+ * @param {function(outerValue: T, innerValue: I, outerIndex: number, innerIndex: number): any} [resultSelector]
+ * A function to produce the value on the output Observable based on the values
+ * and the indices of the source (outer) emission and the inner Observable
+ * emission. The arguments passed to this function are:
+ * - `outerValue`: the value that came from the source
+ * - `innerValue`: the value that came from the projected Observable
+ * - `outerIndex`: the "index" of the value that came from the source
+ * - `innerIndex`: the "index" of the value from the projected Observable
+ * @return {Observable} An Observable containing projected Observables
+ * of each item of the source, ignoring projected Observables that start before
+ * their preceding Observable has completed.
+ * @method exhaustMap
+ * @owner Observable
+ */
+export function exhaustMap<T, I, R>(
+  project: (value: T, index: number) => ObservableInput<I>,
+  resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R
+  ): OperatorFunction<T, R> {
+    return (source: Observable<T>) => source.lift(new SwitchFirstMapOperator(project, resultSelector));
+  }
+
+class SwitchFirstMapOperator<T, I, R> implements Operator<T, R> {
+  constructor(private project: (value: T, index: number) => ObservableInput<I>,
+              private resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R) {
+  }
+
+  call(subscriber: Subscriber<R>, source: any): any {
+    return source.subscribe(new SwitchFirstMapSubscriber(subscriber, this.project, this.resultSelector));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class SwitchFirstMapSubscriber<T, I, R> extends OuterSubscriber<T, I> {
+  private hasSubscription: boolean = false;
+  private hasCompleted: boolean = false;
+  private index: number = 0;
+
+  constructor(destination: Subscriber<R>,
+              private project: (value: T, index: number) => ObservableInput<I>,
+              private resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R) {
+    super(destination);
+  }
+
+  protected _next(value: T): void {
+    if (!this.hasSubscription) {
+      this.tryNext(value);
+    }
+  }
+
+  private tryNext(value: T): void {
+    const index = this.index++;
+    const destination = this.destination;
+    try {
+      const result = this.project(value, index);
+      this.hasSubscription = true;
+      this.add(subscribeToResult(this, result, value, index));
+    } catch (err) {
+      destination.error(err);
+    }
+  }
+
+  protected _complete(): void {
+    this.hasCompleted = true;
+    if (!this.hasSubscription) {
+      this.destination.complete();
+    }
+  }
+
+  notifyNext(outerValue: T, innerValue: I,
+             outerIndex: number, innerIndex: number,
+             innerSub: InnerSubscriber<T, I>): void {
+    const { resultSelector, destination } = this;
+    if (resultSelector) {
+      this.trySelectResult(outerValue, innerValue, outerIndex, innerIndex);
+    } else {
+      destination.next(innerValue);
+    }
+  }
+
+  private trySelectResult(outerValue: T, innerValue: I,
+                          outerIndex: number, innerIndex: number): void {
+    const { resultSelector, destination } = this;
+    try {
+      const result = resultSelector(outerValue, innerValue, outerIndex, innerIndex);
+      destination.next(result);
+    } catch (err) {
+      destination.error(err);
+    }
+  }
+
+  notifyError(err: any): void {
+    this.destination.error(err);
+  }
+
+  notifyComplete(innerSub: Subscription): void {
+    this.remove(innerSub);
+
+    this.hasSubscription = false;
+    if (this.hasCompleted) {
+      this.destination.complete();
+    }
+  }
+}

src/operators/index.ts

@@ -22,6 +22,7 @@ export { distinctUntilKeyChanged } from './distinctUntilKeyChanged';
 export { elementAt } from './elementAt';
 export { every } from './every';
 export { exhaust } from './exhaust';
+export { exhaustMap } from './exhaustMap';
 export { filter } from './filter';
 export { ignoreElements } from './ignoreElements';
 export { map } from './map';