Merge 171f80c into 35857d7

projects/s-rxjs-utils/src/lib/map-with-caching.spec.ts

@@ -0,0 +1,100 @@
+import { Subject } from "rxjs";
+import { mapArrayWithCaching } from "./map-with-caching";
+
+describe("mapArrayWithCaching()", () => {
+  it("maps over the array using the given function", () => {
+    const source = new Subject<number[]>();
+    let lastEmittedThing: number[] = [];
+
+    source
+      .asObservable()
+      .pipe(mapArrayWithCaching((item) => item.toString(), (item) => item * 3))
+      .subscribe((value) => {
+        lastEmittedThing = value;
+      });
+
+    const array = [1, 2, 3, 4, 5, 6];
+    source.next(array);
+    expect(lastEmittedThing).toEqual([3, 6, 9, 12, 15, 18]);
+
+    array.splice(2, 2); // array = [1, 2, 5, 6]
+    source.next(array);
+    expect(lastEmittedThing).toEqual([3, 6, 15, 18]);
+
+    array.push(10); // array = [1, 2, 5, 6, 10]
+    source.next(array);
+    expect(lastEmittedThing).toEqual([3, 6, 15, 18, 30]);
+  });
+
+  it("emits the same object reference for items that have the same cache key", async () => {
+    const source = new Subject<Array<{ index: number }>>();
+    let lastEmittedThing: Array<{ index: number }> = [];
+
+    source
+      .asObservable()
+      .pipe(
+        mapArrayWithCaching(
+          (item) => item.index.toString(),
+          (item) => ({ index: item.index + 1 }),
+        ),
+      )
+      .subscribe((value) => {
+        lastEmittedThing = value;
+      });
+
+    source.next([{ index: 1 }]);
+    expect(lastEmittedThing).toEqual([{ index: 2 }]);
+    const mappedItem1 = lastEmittedThing[0];
+
+    source.next([{ index: 1 }, { index: 2 }]);
+    expect(lastEmittedThing).toEqual([{ index: 2 }, { index: 3 }]);
+    expect(lastEmittedThing[0]).toBe(mappedItem1);
+  });
+
+  it("handles the upstream source completing", () => {
+    const source = new Subject<number[]>();
+    const complete = jasmine.createSpy();
+
+    source
+      .pipe(mapArrayWithCaching((item) => item.toString(), (item) => item + 1))
+      .subscribe(undefined, undefined, complete);
+
+    source.complete();
+
+    expect(source.observers.length).toBe(0);
+    expect(complete).toHaveBeenCalledTimes(1);
+  });
+
+  it("handles errors", () => {
+    const source = new Subject<number[]>();
+    const error = jasmine.createSpy();
+
+    source
+      .pipe(mapArrayWithCaching((item) => item.toString(), (item) => item + 1))
+      .subscribe(undefined, error);
+
+    source.error("fire!!");
+
+    expect(source.observers.length).toBe(0);
+    expect(error).toHaveBeenCalledTimes(1);
+    expect(error).toHaveBeenCalledWith("fire!!");
+  });
+
+  it("handles unsubscribes", () => {
+    const source = new Subject<number[]>();
+
+    const subscription1 = source
+      .pipe(mapArrayWithCaching((item) => item.toString(), (item) => item + 1))
+      .subscribe();
+    const subscription2 = source
+      .pipe(mapArrayWithCaching((item) => item.toString(), (item) => item + 1))
+      .subscribe();
+    expect(source.observers.length).toBe(2);
+
+    subscription1.unsubscribe();
+    expect(source.observers.length).toBe(1);
+
+    subscription2.unsubscribe();
+    expect(source.observers.length).toBe(0);
+  });
+});

projects/s-rxjs-utils/src/lib/map-with-caching.ts

@@ -0,0 +1,46 @@
+import { ObjectWith } from "micro-dash";
+import { map } from "rxjs/operators";
+
+/**
+ * Applies a given function to each value in the array emitted by the source Observable, and emits the resulting array as an Observable. Each item in the array is cached using key generated by the given function so that the resulting arrays contain references to the exact same objects included in the previous emission, when possible.
+ *
+ * @param buildCacheKey A function that converts an upstream object into a string to use as the cache key. Needs to return a unique key for each item in the source array.
+ * @param buildDownstreamItem A function that converts an upstream object into a downstream object
+ *
+ * ```
+ * const mapWithCaching = mapArrayWithCaching(
+ *   (item) => item.toString(),
+ *   (item) => item + 1
+ * )
+ *
+ * source:         -[1, 2]---[1, 2, 3]---[2]--|
+ * mapWithCaching: -[2, 3]---[2, 3, 4]---[3]--|
+ * ```
+ */
+export function mapArrayWithCaching<U, D>(
+  buildCacheKey: (upstreamItem: U) => string,
+  buildDownstreamItem: (upstreamItem: U) => D,
+) {
+  let cache: ObjectWith<D> = {};
+
+  return map((upstreamItems: U[]) => {
+    const nextCache: ObjectWith<D> = {};
+
+    const downstreamItems = upstreamItems.map((upstreamItem) => {
+      const cacheKey = buildCacheKey(upstreamItem);
+
+      let downstreamItem: D;
+      if (cache.hasOwnProperty(cacheKey)) {
+        downstreamItem = cache[cacheKey];
+      } else {
+        downstreamItem = buildDownstreamItem(upstreamItem);
+      }
+
+      nextCache[cacheKey] = downstreamItem;
+      return downstreamItem;
+    });
+
+    cache = nextCache;
+    return downstreamItems;
+  });
+}