feat(framework): core 响应式 signal 基元(自研薄封装)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cocoscreator_projects/YouleNexus/assets/framework/core/reactive.ts

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+export type Unsubscribe = () => void;
+
+export interface ReadonlyReactive<T> {
+  readonly value: T;
+  subscribe(fn: (value: T, prev: T) => void): Unsubscribe;  // 返回退订函数
+}
+
+export interface Reactive<T> extends ReadonlyReactive<T> {
+  value: T;   // 可写
+}
+
+/** 极小响应式信号：value 读写 + 订阅。Object.is 判等(值未变不通知)；订阅者抛错隔离。 */
+export function signal<T>(initial: T): Reactive<T> {
+  let current = initial;
+  const subs = new Set<(value: T, prev: T) => void>();
+  return {
+    get value(): T { return current; },
+    set value(v: T) {
+      if (Object.is(v, current)) return;
+      const prev = current;
+      current = v;
+      for (const fn of [...subs]) {
+        try { fn(current, prev); } catch (e) { console.error('[reactive] subscriber error:', e); }
+      }
+    },
+    subscribe(fn: (value: T, prev: T) => void): Unsubscribe {
+      subs.add(fn);
+      return () => { subs.delete(fn); };
+    },
+  };
+}

cocoscreator_projects/framework-tests/platform/reactive.test.ts

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+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { signal } from '../../YouleNexus/assets/framework/core/reactive.ts';
+
+test('signal: value 读写', () => {
+  const s = signal(1);
+  assert.equal(s.value, 1);
+  s.value = 2;
+  assert.equal(s.value, 2);
+});
+
+test('signal: subscribe 收到 (value, prev)', () => {
+  const s = signal(1);
+  let got: [number, number] | null = null;
+  s.subscribe((v, p) => { got = [v, p]; });
+  s.value = 5;
+  assert.deepEqual(got, [5, 1]);
+});
+
+test('signal: 退订后不再收到', () => {
+  const s = signal(0);
+  let count = 0;
+  const un = s.subscribe(() => { count++; });
+  s.value = 1;
+  un();
+  s.value = 2;
+  assert.equal(count, 1);
+});
+
+test('signal: Object.is 判等，值未变不通知', () => {
+  const s = signal(7);
+  let count = 0;
+  s.subscribe(() => { count++; });
+  s.value = 7;
+  assert.equal(count, 0);
+});
+
+test('signal: 一个订阅者抛错不影响其它', () => {
+  const s = signal(0);
+  let reached = false;
+  s.subscribe(() => { throw new Error('boom'); });
+  s.subscribe(() => { reached = true; });
+  s.value = 1;
+  assert.equal(reached, true);
+});