Support initializer functions for init values provided to useAsyncIterMulti hook

README.md

@@ -990,11 +990,10 @@ const [nextNum, nextStr, nextArr] = useAsyncIterMulti([numberIter, stringIter, a
   An _optional_ object with properties:
 
   - `initialValues`:
-    An _optional_ array of initial values. The values here will be the starting points for all the async iterables from `values` (by corresponding array positions) while they are rendered by the `children` render function __for the first time__ and for each while it is __pending its first yield__. Async iterables from `values` that have no initial value corresponding to them will assume `undefined` as initial value.
+    An _optional_ array of initial values or functions that return initial values. The values will be the starting points for all the async iterables from `values` (by corresponding array positions) __for the first time__ and for each while it is __pending its first yield__. For async iterables from `values` that have no corresponding item here the provided `opts.defaultInitialValue` will be used as fallback.
 
   - `defaultInitialValue`:
-    An _optional_ default starting value for every new async iterable in `values` if there is no corresponding one for it in `opts.initialValues`, defaults to `undefined`.
-
+    An _optional_ default starting value for every new async iterable in `values` if there is no corresponding one for it in `opts.initialValues`, defaults to `undefined`. You can pass an actual value, or a function that returns a value (which the hook will call for every new iterable added).
 
 ### Returns
 

spec/tests/useAsyncIterMulti.spec.ts

@@ -1,4 +1,4 @@
-import { it, describe, expect, afterEach } from 'vitest';
+import { it, describe, expect, afterEach, vi } from 'vitest';
 import { gray } from 'colorette';
 import { cleanup as cleanupMountedReactTrees, act, renderHook } from '@testing-library/react';
 import { iterateFormatted, useAsyncIterMulti } from '../../src/index.js';
@@ -165,6 +165,99 @@
     }
   );
 
+  it(
+    gray(
+      'When given multiple iterables with a default initial value as a function, calls it once on every added source iterable'
+    ),
+    async () => {
+      const channels = [
+        new IteratorChannelTestHelper<string>(),
+        new IteratorChannelTestHelper<string>(),
+      ];
+      const initialValueFn = vi.fn(() => '___');
+      let timesRerendered = 0;
+
+      const renderedHook = renderHook(() => {
+        timesRerendered++;
+        return useAsyncIterMulti(channels, { defaultInitialValue: initialValueFn });
+      });
+
+      await act(() => {});
+      expect(timesRerendered).toStrictEqual(1);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: '___', pendingFirst: true, done: false, error: undefined },
+        { value: '___', pendingFirst: true, done: false, error: undefined },
+      ]);
+
+      await act(() => {
+        channels[0].put('a');
+        channels[1].put('b');
+      });
+      expect(timesRerendered).toStrictEqual(2);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: 'a', pendingFirst: false, done: false, error: undefined },
+        { value: 'b', pendingFirst: false, done: false, error: undefined },
+      ]);
+
+      await act(() => {
+        channels.push(new IteratorChannelTestHelper());
+        renderedHook.rerender();
+      });
+      expect(timesRerendered).toStrictEqual(3);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: 'a', pendingFirst: false, done: false, error: undefined },
+        { value: 'b', pendingFirst: false, done: false, error: undefined },
+        { value: '___', pendingFirst: true, done: false, error: undefined },
+      ]);
+      expect(initialValueFn).toHaveBeenCalledTimes(3);
+    }
+  );
+
+  it(
+    gray(
+      'When given multiple iterables with initial values as a functions, calls each once when a corresponding iterable is added'
+    ),
+    async () => {
+      const channels = [new IteratorChannelTestHelper<string>()];
+      const [initialValueFn1, initialValueFn2] = [vi.fn(), vi.fn()];
+      let timesRerendered = 0;
+
+      const renderedHook = renderHook(() => {
+        timesRerendered++;
+        return useAsyncIterMulti(channels, {
+          initialValues: [
+            initialValueFn1.mockImplementation(() => '_1_'),
+            initialValueFn2.mockImplementation(() => '_2_'),
+          ],
+        });
+      });
+
+      await act(() => {});
+      expect(timesRerendered).toStrictEqual(1);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: '_1_', pendingFirst: true, done: false, error: undefined },
+      ]);
+
+      await act(() => channels[0].put('a'));
+      expect(timesRerendered).toStrictEqual(2);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: 'a', pendingFirst: false, done: false, error: undefined },
+      ]);
+
+      await act(() => {
+        channels.push(new IteratorChannelTestHelper());
+        renderedHook.rerender();
+      });
+      expect(timesRerendered).toStrictEqual(3);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: 'a', pendingFirst: false, done: false, error: undefined },
+        { value: '_2_', pendingFirst: true, done: false, error: undefined },
+      ]);
+      expect(initialValueFn1).toHaveBeenCalledOnce();
+      expect(initialValueFn2).toHaveBeenCalledOnce();
+    }
+  );
+
   it(
     gray(
       "When given multiple iterables with corresponding initial values for some and a default initial value, reflects each's state correctly, starting with its corresponding initial value or the default initial value if not present"
@@ -173,14 +266,19 @@
       const channels = [
         new IteratorChannelTestHelper<string>(),
         new IteratorChannelTestHelper<string>(),
-      ] as const;
+      ];
       let timesRerendered = 0;
 
       const renderedHook = renderHook(() => {
         timesRerendered++;
-        return useAsyncIterMulti(channels, { initialValues: ['_1_'], defaultInitialValue: '___' });
+        return useAsyncIterMulti(channels, {
+          initialValues: [() => '_1_' as const] as const,
+          defaultInitialValue: () => '___' as const,
+        });
       });
 
+      renderedHook.result.current[0].value;
+
       await act(() => {});
       expect(timesRerendered).toStrictEqual(1);
       expect(renderedHook.result.current).toStrictEqual([
@@ -201,6 +299,17 @@
         { value: 'a', pendingFirst: false, done: false, error: undefined },
         { value: 'b', pendingFirst: false, done: false, error: undefined },
       ]);
+
+      await act(() => {
+        channels.push(new IteratorChannelTestHelper());
+        renderedHook.rerender();
+      });
+      expect(timesRerendered).toStrictEqual(4);
+      expect(renderedHook.result.current).toStrictEqual([
+        { value: 'a', pendingFirst: false, done: false, error: undefined },
+        { value: 'b', pendingFirst: false, done: false, error: undefined },
+        { value: '___', pendingFirst: true, done: false, error: undefined },
+      ]);
     }
   );
 

src/useAsyncIterMulti/index.ts

@@ -4,6 +4,8 @@ import { useRefWithInitialValue } from '../common/hooks/useRefWithInitialValue.j
 import { isAsyncIter } from '../common/isAsyncIter.js';
 import { type IterationResult } from '../useAsyncIter/index.js';
 import { type AsyncIterableSubject } from '../AsyncIterableSubject/index.js';
+import { type MaybeFunction } from '../common/MaybeFunction.js';
+import { callOrReturn } from '../common/callOrReturn.js';
 import { asyncIterSyncMap } from '../common/asyncIterSyncMap.js';
 import { parseReactAsyncIterable } from '../common/ReactAsyncIterable.js';
 import { iterateAsyncIterWithCallbacks } from '../common/iterateAsyncIterWithCallbacks.js';
@@ -79,8 +81,8 @@ export { useAsyncIterMulti, type IterationResult, type IterationResultSet };
  *
  * @param inputs An array of zero or more async iterable or plain values (mixable).
  * @param {object} opts An _optional_ object with options.
- * @param opts.initialValues An _optional_ array of initial values, each item of which is a starting value for the async iterable from `inputs` on the same array position. For every async iterable that has no corresponding item in this array, it would use the provided `opts.defaultInitialValue` as fallback.
- * @param opts.defaultInitialValue An _optional_ default starting value for every new async iterable in `inputs` if there is no corresponding one for it in `opts.initialValues`, defaults to `undefined`.
+ * @param opts.initialValues An _optional_ array of initial values or functions that return initial values, each item of which is a starting value for the async iterable from `inputs` on the same array position. For every async iterable that has no corresponding item here, the provided `opts.defaultInitialValue` will be used as fallback.
+ * @param opts.defaultInitialValue An _optional_ default starting value for every new async iterable in `inputs` if there is no corresponding one for it in `opts.initialValues`, defaults to `undefined`. You can pass an actual value, or a function that returns a value (which the hook will call for every new iterable added).
  *
  * @returns An array of objects that provide up-to-date information about each input's current value, completion status, whether it's still waiting for its first value and so on, correspondingly with the order in which they appear on `inputs` (see {@link IterationResultSet `IterationResultSet`}).
  *
@@ -206,12 +208,12 @@ function useAsyncIterMulti<
     initialValues?: TInitValues;
     defaultInitialValue?: TDefaultInitValue;
   }
-): IterationResultSet<TValues, TInitValues, TDefaultInitValue> {
+): IterationResultSet<TValues, MaybeFunctions<TInitValues>, TDefaultInitValue> {
   const update = useSimpleRerender();
 
   const ref = useRefWithInitialValue(() => ({
     currDiffCompId: 0,
-    prevResults: [] as IterationResultSet<TValues, TInitValues, TDefaultInitValue>,
+    prevResults: [] as IterationResultSet<TValues, MaybeFunctions<TInitValues>, TDefaultInitValue>,
     activeItersMap: new Map<
       AsyncIterable<unknown>,
       {
@@ -233,8 +235,10 @@ function useAsyncIterMulti<
     };
   }, []);
 
-  const initialValues = opts?.initialValues ?? [];
-  const defaultInitialValue = opts?.defaultInitialValue;
+  const optsNormed = {
+    initialValues: opts?.initialValues ?? [],
+    defaultInitialValue: opts?.defaultInitialValue,
+  };
 
   const nextDiffCompId = (ref.current.currDiffCompId = ref.current.currDiffCompId === 0 ? 1 : 0);
   let numOfPrevRunItersPreserved = 0;
@@ -279,9 +283,11 @@ function useAsyncIterMulti<
       startingValue =
         i < prevResults.length
           ? prevResults[i].value
-          : i < initialValues.length
-            ? initialValues[i]
-            : defaultInitialValue;
+          : callOrReturn(
+              i < optsNormed.initialValues.length
+                ? optsNormed.initialValues[i]
+                : optsNormed.defaultInitialValue
+            );
       pendingFirst = true;
     }
 
@@ -305,7 +311,7 @@ function useAsyncIterMulti<
     activeItersMap.set(baseIter, newIterState);
 
     return newIterState.currState;
-  }) as IterationResultSet<TValues, TInitValues, TDefaultInitValue>;
+  }) as IterationResultSet<TValues, MaybeFunctions<TInitValues>, TDefaultInitValue>;
 
   const numOfPrevRunItersDisappeared = numOfPrevRunIters - numOfPrevRunItersPreserved;
 
@@ -322,7 +328,9 @@ function useAsyncIterMulti<
     }
   }
 
-  return (ref.current.prevResults = nextResults);
+  ref.current.prevResults = nextResults;
+
+  return nextResults;
 }
 
 type IterationResultSet<
@@ -335,3 +343,7 @@ type IterationResultSet<
     I extends keyof TInitValues ? TInitValues[I] : TDefaultInitValue
   >;
 };
+
+type MaybeFunctions<T extends readonly unknown[]> = {
+  [I in keyof T]: T[I] extends MaybeFunction<infer J> ? J : T[I];
+};