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memoize.js
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54 lines (46 loc) · 1.62 KB
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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at <http://mozilla.org/MPL/2.0/>. */
// @flow
import type { ResourceBound } from "./core";
import type { QueryMap } from "./base-query";
import { shallowEqual } from "./compare";
/**
* Wraps a 'mapper' function to create a shallow-equality memoized version
* of the mapped result. The returned function will return the same value
* even if the input object is different, as long as the identity is the same
* and the mapped result is shallow-equal to the most recent mapped value.
*/
export function memoizeResourceShallow<
R: ResourceBound,
Args,
Mapped,
T: QueryMap<R, Args, Mapped>
>(map: T): T {
const cache = new WeakMap();
const fn = (input, identity, args) => {
let existingEntry = cache.get(identity);
if (!existingEntry || existingEntry.input !== input) {
const mapper = (map: any);
const output = mapper(input, identity, args);
if (existingEntry) {
// If the new output is shallow-equal to the old output, we reuse
// the previous object instead to preserve object equality.
const newOutput = shallowEqual(output, existingEntry.output)
? existingEntry.output
: output;
existingEntry.output = newOutput;
existingEntry.input = input;
} else {
existingEntry = {
input,
output
};
cache.set(identity, existingEntry);
}
}
return existingEntry.output;
};
fn.needsArgs = map.needsArgs;
return (fn: any);
}