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

src/operator/concatMapTo.ts

@@ -1,5 +1,5 @@
 import { Observable, ObservableInput } from '../Observable';
-import { MergeMapToOperator } from './mergeMapTo';
+import { concatMapTo as higherOrder } from '../operators';
 
 /* tslint:disable:max-line-length */
 export function concatMapTo<T, R>(this: Observable<T>, observable: ObservableInput<R>): Observable<R>;
@@ -64,5 +64,5 @@ export function concatMapTo<T, I, R>(this: Observable<T>, observable: Observable
  */
 export function concatMapTo<T, I, R>(this: Observable<T>, innerObservable: Observable<I>,
                                      resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R): Observable<R> {
-  return this.lift(new MergeMapToOperator(innerObservable, resultSelector, 1));
+  return higherOrder(innerObservable, resultSelector)(this);
 }

src/operators/concatMapTo.ts

@@ -0,0 +1,71 @@
+import { Observable, ObservableInput } from '../Observable';
+import { concatMap } from './concatMap';
+import { OperatorFunction } from '../interfaces';
+
+/* tslint:disable:max-line-length */
+export function concatMapTo<T, R>(observable: ObservableInput<R>): OperatorFunction<T, R>;
+export function concatMapTo<T, I, R>(observable: 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 the same Observable which is merged multiple
+ * times in a serialized fashion on the output Observable.
+ *
+ * <span class="informal">It's like {@link concatMap}, but maps each value
+ * always to the same inner Observable.</span>
+ *
+ * <img src="./img/concatMapTo.png" width="100%">
+ *
+ * Maps each source value to the given Observable `innerObservable` regardless
+ * of the source value, and then flattens those resulting Observables into one
+ * single Observable, which is the output Observable. Each new `innerObservable`
+ * instance emitted on the output Observable is concatenated with the previous
+ * `innerObservable` instance.
+ *
+ * __Warning:__ if source values arrive endlessly and faster than their
+ * corresponding inner Observables can complete, it will result in memory issues
+ * as inner Observables amass in an unbounded buffer waiting for their turn to
+ * be subscribed to.
+ *
+ * Note: `concatMapTo` is equivalent to `mergeMapTo` with concurrency parameter
+ * set to `1`.
+ *
+ * @example <caption>For each click event, tick every second from 0 to 3, with no concurrency</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var result = clicks.concatMapTo(Rx.Observable.interval(1000).take(4));
+ * result.subscribe(x => console.log(x));
+ *
+ * // Results in the following:
+ * // (results are not concurrent)
+ * // For every click on the "document" it will emit values 0 to 3 spaced
+ * // on a 1000ms interval
+ * // one click = 1000ms-> 0 -1000ms-> 1 -1000ms-> 2 -1000ms-> 3
+ *
+ * @see {@link concat}
+ * @see {@link concatAll}
+ * @see {@link concatMap}
+ * @see {@link mergeMapTo}
+ * @see {@link switchMapTo}
+ *
+ * @param {ObservableInput} innerObservable An Observable to replace each value from
+ * the source 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 of values merged together by joining the
+ * passed observable with itself, one after the other, for each value emitted
+ * from the source.
+ * @method concatMapTo
+ * @owner Observable
+ */
+export function concatMapTo<T, I, R>(
+  innerObservable: Observable<I>,
+  resultSelector?: (outerValue: T, innerValue: I, outerIndex: number, innerIndex: number) => R
+): OperatorFunction<T, R> {
+  return concatMap(() => innerObservable, resultSelector);
+}

src/operators/index.ts

@@ -9,6 +9,7 @@ export { catchError } from './catchError';
 export { concat } from './concat';
 export { concatAll } from './concatAll';
 export { concatMap } from './concatMap';
+export { concatMapTo } from './concatMapTo';
 export { defaultIfEmpty } from './defaultIfEmpty';
 export { dematerialize } from './dematerialize';
 export { filter } from './filter';