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import { Subject } from "rxjs"; | ||
import { mapArrayWithCaching } from "./map-with-caching"; | ||
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describe("mapArrayWithCaching()", () => { | ||
it("maps over the array using the given function", () => { | ||
const source = new Subject<number[]>(); | ||
let lastEmittedThing: number[] = []; | ||
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source | ||
.asObservable() | ||
.pipe(mapArrayWithCaching((item) => item.toString(), (item) => item * 3)) | ||
.subscribe((value) => { | ||
lastEmittedThing = value; | ||
}); | ||
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const array = [1, 2, 3, 4, 5, 6]; | ||
source.next(array); | ||
expect(lastEmittedThing).toEqual([3, 6, 9, 12, 15, 18]); | ||
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array.splice(2, 2); // array = [1, 2, 5, 6] | ||
source.next(array); | ||
expect(lastEmittedThing).toEqual([3, 6, 15, 18]); | ||
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array.push(10); // array = [1, 2, 5, 6, 10] | ||
source.next(array); | ||
expect(lastEmittedThing).toEqual([3, 6, 15, 18, 30]); | ||
}); | ||
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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 }> = []; | ||
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source | ||
.asObservable() | ||
.pipe( | ||
mapArrayWithCaching( | ||
(item) => item.index.toString(), | ||
(item) => ({ index: item.index + 1 }), | ||
), | ||
) | ||
.subscribe((value) => { | ||
lastEmittedThing = value; | ||
}); | ||
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source.next([{ index: 1 }]); | ||
expect(lastEmittedThing).toEqual([{ index: 2 }]); | ||
const mappedItem1 = lastEmittedThing[0]; | ||
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source.next([{ index: 1 }, { index: 2 }]); | ||
expect(lastEmittedThing).toEqual([{ index: 2 }, { index: 3 }]); | ||
expect(lastEmittedThing[0]).toBe(mappedItem1); | ||
}); | ||
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it("handles the upstream source completing", () => { | ||
const source = new Subject<number[]>(); | ||
const complete = jasmine.createSpy(); | ||
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source | ||
.pipe(mapArrayWithCaching((item) => item.toString(), (item) => item + 1)) | ||
.subscribe(undefined, undefined, complete); | ||
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source.complete(); | ||
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expect(source.observers.length).toBe(0); | ||
expect(complete).toHaveBeenCalledTimes(1); | ||
}); | ||
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it("handles errors", () => { | ||
const source = new Subject<number[]>(); | ||
const error = jasmine.createSpy(); | ||
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source | ||
.pipe(mapArrayWithCaching((item) => item.toString(), (item) => item + 1)) | ||
.subscribe(undefined, error); | ||
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source.error("fire!!"); | ||
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expect(source.observers.length).toBe(0); | ||
expect(error).toHaveBeenCalledTimes(1); | ||
expect(error).toHaveBeenCalledWith("fire!!"); | ||
}); | ||
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it("handles unsubscribes", () => { | ||
const source = new Subject<number[]>(); | ||
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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); | ||
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subscription1.unsubscribe(); | ||
expect(source.observers.length).toBe(1); | ||
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subscription2.unsubscribe(); | ||
expect(source.observers.length).toBe(0); | ||
}); | ||
}); |
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import { ObjectWith } from "micro-dash"; | ||
import { map } from "rxjs/operators"; | ||
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/** | ||
* Applies a given function to each item in the upstream array and emits the result. Each item is then cached using the key generated by `buildCacheKey` so that the next emission contains references to the matching objects from previous emission, without running `buildDownstreamItem` again. The cache is only held between successive emissions. | ||
* | ||
* @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> = {}; | ||
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return map((upstreamItems: U[]) => { | ||
const nextCache: ObjectWith<D> = {}; | ||
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const downstreamItems = upstreamItems.map((upstreamItem) => { | ||
const cacheKey = buildCacheKey(upstreamItem); | ||
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let downstreamItem: D; | ||
if (cache.hasOwnProperty(cacheKey)) { | ||
downstreamItem = cache[cacheKey]; | ||
} else { | ||
downstreamItem = buildDownstreamItem(upstreamItem); | ||
} | ||
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nextCache[cacheKey] = downstreamItem; | ||
return downstreamItem; | ||
}); | ||
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cache = nextCache; | ||
return downstreamItems; | ||
}); | ||
} |