Do not emit changed event from async-state if same value is set

yuriyyakym · Dec 6, 2023 · d93ac5e · d93ac5e
1 parent 4a55301
commit d93ac5e
Show file tree

Hide file tree

Showing 2 changed files with 31 additions and 4 deletions.
diff --git a/src/async-state/async-state.ts b/src/async-state/async-state.ts
@@ -46,6 +46,7 @@ const asyncState = <T>(
       let newValue = isFunction(nextValueOrResolver)
         ? await nextValueOrResolver(value)
         : await nextValueOrResolver;
+      const isChanged = !Object.is(newValue, value);
 
       if (currentPendingVersion !== lastPendingVersion) {
         events.ignored.emit({ value: newValue, version: currentPendingVersion });
@@ -57,19 +58,27 @@ const asyncState = <T>(
       status = AsyncStatus.FULFILLED;
 
       events.fulfilled.emit(newValue);
-      events.changed.emit(value);
+
+      if (isChanged) {
+        events.changed.emit(value);
+      }
     } catch (e) {
       if (currentPendingVersion !== lastPendingVersion) {
         events.ignored.emit({ error: e, version: currentPendingVersion });
         return;
       }
 
+      const isChanged = !Object.is(e, error);
+
       error = e;
       value = undefined;
       status = AsyncStatus.REJECTED;
 
       events.rejected.emit(error);
-      events.changed.emit(value);
+
+      if (isChanged) {
+        events.changed.emit(value);
+      }
     }
 
     version = lastPendingVersion;

diff --git a/tests/async-state.test.ts b/tests/async-state.test.ts
@@ -1,6 +1,6 @@
-import { expect, test } from 'vitest';
+import { expect, test, vi } from 'vitest';
 
-import { asyncState, AsyncStatus, delay, SystemTag } from '../src';
+import { asyncState, AsyncStatus, delay, flush, scenario, SystemTag } from '../src';
 
 test('resolves immediately if non-async value is set', async () => {
   const greeting = asyncState<string>('Async state');
@@ -80,6 +80,24 @@ test('ignores previous rejected promises if newer promise was set', async () =>
   expect(state.get()).toBe(4);
 });
 
+test('does not emit `changed` event if same value is set', async () => {
+  const onChange = vi.fn();
+  const state = asyncState('test');
+
+  scenario(state.events.changed, onChange);
+
+  await flush();
+  expect(state.get()).toEqual('test');
+  expect(onChange).toBeCalledTimes(0);
+  await state.set(Promise.resolve('Awai'));
+  expect(state.get()).toEqual('Awai');
+  expect(onChange).toBeCalledTimes(1);
+  await state.set(delay(10).then(() => 'Awai'));
+  await delay(20);
+  expect(state.get()).toEqual('Awai');
+  expect(onChange).toBeCalledTimes(1);
+});
+
 test('emits `ignored` event if outdated version promise is settled', async () => {
   const state = asyncState(delay(5).then(() => 'a'));
   state.set(delay(5).then(() => 'b'));