feat(kida,store): move latest from kida to @nano_kit/store

packages/kida/.size-limit.js

@@ -15,14 +15,14 @@ export default [
     gzip: true,
     path: 'dist/index.production.js',
     import: '*',
-    limit: '5.5 kB',
+    limit: '5.45 kB',
     modifyRolldownConfig
   },
   {
     name: 'All publics (Brotli)',
     path: 'dist/index.production.js',
     import: '*',
-    limit: '5.1 kB',
+    limit: '5.05 kB',
     modifyRolldownConfig
   },
   {

packages/kida/src/utils.ts

@@ -39,39 +39,6 @@ export function boolean<T>($signal: Accessor<T>) {
   return computed(() => Boolean($signal()))
 }
 
-/**
- * Create a signal with the value of the source that changed last.
- * A change is a change of value: writing a value equal to the current one
- * changes nothing. The last source wins on the first read and whenever
- * several sources changed since the previous read, which is what reading
- * without a subscription or writing inside a `batch` amounts to.
- * @param sources - Sources to merge.
- * @returns A signal with the value of the most recently changed source.
- */
-/* @__NO_SIDE_EFFECTS__ */
-export function latest<T>(...sources: Accessor<T>[]) {
-  const prev: T[] = []
-  let value: T
-
-  return computed(() => {
-    // The first run has nothing to compare against, so every source counts as
-    // changed and the last one wins
-    const init = !prev.length
-
-    // Every source is read on every run: skipping one drops its subscription
-    for (let i = 0, len = sources.length; i < len; i++) {
-      const next = sources[i]()
-
-      if (init || next !== prev[i]) {
-        prev[i] = next
-        value = next
-      }
-    }
-
-    return value
-  })
-}
-
 /**
  * Compose multiple destroy functions into a single destroy function.
  * @param destroys - Destroy functions to compose.

packages/store/src/utils.spec.ts

@@ -7,11 +7,13 @@ import {
   expect
 } from 'vitest'
 import {
+  batch,
   effect,
   signal
 } from 'kida'
 import {
   interval,
+  latest,
   previous,
   signalNodeAssign
 } from './utils.js'
@@ -54,6 +56,104 @@ describe('store', () => {
       })
     })
 
+    describe('latest', () => {
+      it('should return the last source value on the first read', () => {
+        const $a = signal('a')
+        const $b = signal('b')
+        const $latest = latest($a, $b)
+
+        expect($latest()).toBe('b')
+      })
+
+      it('should return the last source value on the first read when it is undefined', () => {
+        const $a = signal<string | undefined>('a')
+        const $b = signal<string | undefined>(undefined)
+        const $latest = latest($a, $b)
+
+        expect($latest()).toBeUndefined()
+      })
+
+      it('should return the value of the source that changed last', () => {
+        const $a = signal('a')
+        const $b = signal('b')
+        const $latest = latest($a, $b)
+        const listener = vi.fn()
+        const off = effect(() => {
+          listener($latest())
+        })
+
+        $a('a1')
+        $b('b1')
+        $a('a2')
+
+        expect(listener.mock.calls).toEqual([['b'], ['a1'], ['b1'], ['a2']])
+
+        off()
+      })
+
+      it('should return the last changed source when several sources changed in a batch', () => {
+        const $a = signal('a')
+        const $b = signal('b')
+        const $latest = latest($a, $b)
+        const off = effect(() => {
+          $latest()
+        })
+
+        batch(() => {
+          $b('b1')
+          $a('a1')
+        })
+
+        expect($latest()).toBe('b1')
+
+        off()
+      })
+
+      it('should return the last changed source when several sources changed between reads', () => {
+        const $a = signal('a')
+        const $b = signal('b')
+        const $latest = latest($a, $b)
+
+        expect($latest()).toBe('b')
+
+        $b('b1')
+        $a('a1')
+
+        expect($latest()).toBe('b1')
+      })
+
+      it('should keep the value when a source is written with an equal value', () => {
+        const $a = signal('a')
+        const $b = signal('b')
+        const $latest = latest($a, $b)
+
+        expect($latest()).toBe('b')
+
+        $a('a')
+
+        expect($latest()).toBe('b')
+      })
+
+      it('should keep the value when nothing changed since the last read', () => {
+        const $a = signal('a')
+        const $b = signal('b')
+        const $latest = latest($a, $b)
+        const off = effect(() => {
+          $latest()
+        })
+
+        $a('a1')
+
+        expect($latest()).toBe('a1')
+
+        // Losing the last subscriber purges the deps and forces the next read
+        // to recompute with no source changed
+        off()
+
+        expect($latest()).toBe('a1')
+      })
+    })
+
     describe('interval', () => {
       beforeEach(() => {
         vi.useFakeTimers()

packages/store/src/utils.ts

@@ -28,6 +28,39 @@ export function previous<T>($signal: Accessor<T>): Accessor<T | undefined> {
   })
 }
 
+/**
+ * Create a signal with the value of the source that changed last.
+ * A change is a change of value: writing a value equal to the current one
+ * changes nothing. The last source wins on the first read and whenever
+ * several sources changed since the previous read, which is what reading
+ * without a subscription or writing inside a `batch` amounts to.
+ * @param sources - Sources to merge.
+ * @returns A signal with the value of the most recently changed source.
+ */
+/* @__NO_SIDE_EFFECTS__ */
+export function latest<T>(...sources: Accessor<T>[]) {
+  const prev: T[] = []
+  let value: T
+
+  return computed(() => {
+    // The first run has nothing to compare against, so every source counts as
+    // changed and the last one wins
+    const init = !prev.length
+
+    // Every source is read on every run: skipping one drops its subscription
+    for (let i = 0, len = sources.length; i < len; i++) {
+      const next = sources[i]()
+
+      if (init || next !== prev[i]) {
+        prev[i] = next
+        value = next
+      }
+    }
+
+    return value
+  })
+}
+
 /**
  * Create a signal that increments on an interval while mounted.
  * @param ms - Interval duration in milliseconds.