fix(kida): keep a child write in the caller's context

packages/agera/src/signal.spec.ts

@@ -9,6 +9,7 @@ import {
   computed,
   signal,
   effect,
+  untracked,
   isSignal,
   createSignal,
   trigger
@@ -77,6 +78,50 @@ describe('agera', () => {
       stop()
     })
 
+    it('should not rerun an effect that writes its own dependency directly', () => {
+      const $tick = signal(0)
+      const $data = signal(0)
+      let runs = 0
+      const stop = effect(() => {
+        $tick()
+
+        const data = $data()
+
+        runs++
+
+        $data(data + 1)
+      })
+
+      $tick(1)
+
+      expect(runs).toBe(2)
+      expect($data()).toBe(2)
+
+      stop()
+    })
+
+    it('should settle an effect that feeds itself instead of running away', () => {
+      const $tick = signal(0)
+      const $data = signal(0)
+      let runs = 0
+      const stop = effect(() => {
+        $tick()
+
+        const data = $data()
+
+        runs++
+
+        untracked(() => $data(data + 1))
+      })
+
+      $tick(1)
+
+      expect(runs).toBe(2)
+      expect($data()).toBe(2)
+
+      stop()
+    })
+
     describe('computed', () => {
       it('should correctly propagate changes through computed signals', () => {
         const src = signal(0)

packages/kida/src/internals/child.spec.ts

@@ -6,9 +6,11 @@ import {
 } from 'vitest'
 import {
   signal,
+  mountable,
   effect
 } from 'agera'
 import { assignKey } from './utils.js'
+import { onMount } from './lifecycle.js'
 import { child } from './child.js'
 
 describe('kida', () => {
@@ -152,6 +154,81 @@ describe('kida', () => {
 
         off()
       })
+
+      it('should not rerun an effect that writes the child it reads', () => {
+        const $map = signal({
+          a: 'clean'
+        })
+        const $a = child($map, 'a', assignKey)
+        let runs = 0
+        // an effect that normalises the child it reads settles: a writer is
+        // never woken by its own write
+        const stop = effect(() => {
+          const value = $a()
+
+          runs++
+
+          if (runs > 10) {
+            throw new Error('runaway')
+          }
+
+          if (value !== value.trim()) {
+            $a(value.trim())
+          }
+        })
+
+        runs = 0
+
+        $map({
+          a: '  dirty  '
+        })
+
+        expect(runs).toBe(1)
+        expect($map()).toEqual({
+          a: 'dirty'
+        })
+
+        stop()
+      })
+
+      it('should not fire mount listeners from inside an effect that writes a child', () => {
+        const log: string[] = []
+        const $map = signal({
+          a: 1
+        })
+        const $a = child($map, 'a', assignKey)
+        const $mountable = mountable(signal('x'))
+
+        onMount($mountable, () => {
+          log.push('mounted')
+        })
+
+        // reaches the mountable signal only once the write has landed, so
+        // the flush the write triggers is what makes it live
+        const stopReader = effect(() => {
+          if ($map().a === 2) {
+            $mountable()
+          }
+        })
+        const stopWriter = effect(() => {
+          log.push('start')
+
+          if ($a() === 1) {
+            $a(2)
+          }
+
+          log.push('end')
+        })
+
+        expect(log).toEqual([
+          'start',
+          'end',
+          'mounted'
+        ])
+
+        stopWriter()
+        stopReader()
+      })
     })
   })
 })

packages/kida/src/internals/child.ts

@@ -39,12 +39,18 @@ function childCompute(this: ChildNode) {
 
 function childOper(this: ChildNode, ...value: [NewValue<unknown>]) {
   if (value.length) {
-    untracked(() => {
-      const parent = this.p()
-      const key = $get(this.k)
+    // Only the reads are untracked: the write itself stays in the caller's
+    // context, so an effect that writes a child it reads is exempted from
+    // its own write instead of being re-run by it
+    let parent!: AnyObject
+    let key!: PropertyKey
 
-      this.p(this.sv(parent, key, nextValue(parent[key], value[0])))
+    untracked(() => {
+      parent = this.p()
+      key = $get(this.k)
     })
+
+    this.p(this.sv(parent, key, nextValue(parent[key], value[0])))
   } else {
     return computedOper.call(this)
   }

packages/nanoviews/.size-limit.json

@@ -23,6 +23,6 @@
     "name": "Average usage (Brotli)",
     "path": "dist/index.js",
     "import": "{ fragment, div, form, input, button, label, classList$, if_, for_, value$, $$children, effect }",
-    "limit": "4 kB"
+    "limit": "4.05 kB"
   }
 ]