feat: new dirty option for $computed

src/observables.ts

@@ -181,9 +181,14 @@ export function isObservable<T>(fn: MaybeObservable<T>): fn is Observable<T> {
  * console.log($c()); // logs 40
  * ```
  */
-export function $computed<T>(fn: () => T, opts?: { id?: string }): Observable<T> {
+export function $computed<T>(
+  fn: () => T,
+  opts?: { id?: string; dirty?: (prev: T, next: T) => boolean },
+): Observable<T> {
   let currentValue;
 
+  const isDirty = opts?.dirty ?? notEqual;
+
   const $computed: Observable<T> = () => {
     if (__DEV__ && _computeStack.includes($computed)) {
       const calls = _callStack.map((c) => c.id ?? '?').join(' --> ');
@@ -198,9 +203,14 @@ export function $computed<T>(fn: () => T, opts?: { id?: string }): Observable<T>
     if (!$computed[DISPOSED] && $computed[DIRTY]) {
       forEachChild($computed, $dispose);
       emptyDisposal($computed);
-      currentValue = compute($computed, fn);
+
+      const nextValue = compute($computed, fn);
       $computed[DIRTY] = false;
-      dirtyNode($computed);
+
+      if (isDirty(currentValue, nextValue)) {
+        currentValue = nextValue;
+        dirtyNode($computed);
+      }
     }
 
     return currentValue;

tests/$computed.test.ts

@@ -1,4 +1,4 @@
-import { $computed, $observable, $tick } from '../src';
+import { $computed, $effect, $observable, $tick } from '../src';
 
 afterEach(() => $tick());
 
@@ -148,3 +148,32 @@ it('should discover new dependencies', async () => {
   await $tick();
   expect($c()).toBe(10);
 });
+
+it('should accept dirty option', async () => {
+  const $a = $observable(0);
+
+  const $b = $computed(() => $a(), {
+    // Skip odd numbers.
+    dirty: (prev, next) => prev + 1 !== next,
+  });
+
+  const effect = vi.fn();
+  $effect(() => {
+    $b();
+    effect();
+  });
+
+  expect($b()).toBe(0);
+  expect(effect).toHaveBeenCalledTimes(1);
+
+  $a.set(2);
+  await $tick();
+  expect($b()).toBe(2);
+  expect(effect).toHaveBeenCalledTimes(2);
+
+  // no-change
+  $a.set(3);
+  await $tick();
+  expect($b()).toBe(2);
+  expect(effect).toHaveBeenCalledTimes(2);
+});