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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 | 1x 1x 1x 65x 65x 119x 65x 65x 65x | import { createSignal } from './signal';
import { createEffect } from './effect';
import type { Memo, ReactiveOptions } from '../types';
/**
* Creates a derived, memoized signal that caches its computed value.
*
* Memos are read-only signals that compute their value from other signals.
* They automatically track dependencies and only re-compute when those dependencies change.
* The computed value is cached and returned on subsequent reads until dependencies change.
*
* @param fn The function to compute the memoized value - runs when dependencies change
* @param options Optional configuration including debug name
* @returns A read-only memo that can be called to get the current computed value
*
* @example
* ```typescript
* const [firstName, setFirstName] = createSignal('John');
* const [lastName, setLastName] = createSignal('Doe');
*
* const fullName = createMemo(() => `${firstName()} ${lastName()}`);
*
* console.log(fullName()); // "John Doe"
* setFirstName('Jane'); // Triggers re-computation
* console.log(fullName()); // "Jane Doe"
*
* // Memos are lazy - they only compute when read
* const expensive = createMemo(() => {
* console.log('Computing...');
* return heavyComputation();
* });
*
* // Nothing computed yet
* expensive(); // Logs "Computing..." and returns result
* expensive(); // Returns cached result (no re-computation)
* ```
*/
export function createMemo<T>(fn: () => T, options?: ReactiveOptions): Memo<T> {
// A memo is essentially a signal that is updated by an effect.
// The signal can hold either the computed value T or be undefined initially.
const [memo, setMemo] = createSignal<T | undefined>(undefined, options);
// This effect tracks the dependencies of the memo function and updates the signal's value.
// Pass the name to the underlying effect for better debugging
createEffect(() => {
setMemo(fn());
}, options);
// The type cast here is safe and intentional.
// The internal effect runs synchronously upon creation, so the `memo` signal
// is guaranteed to have a value of type `T` before it's returned to the user.
// This hides the initial `undefined` state from the public API.
return memo as Memo<T>;
}
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