fix(query-core): avoid repeated event listener setup without cleanup

.changeset/legal-lamps-repeat.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/query-core': patch
+---
+
+Avoid repeatedly setting up custom focus and online event listeners that do not return a cleanup function when subscribers are added.

packages/query-core/src/__tests__/focusManager.test.tsx

@@ -14,6 +14,43 @@ describe('focusManager', () => {
     vi.useRealTimers()
   })
 
+  it('should initialize a listener without cleanup once until the last unsubscribe', () => {
+    const setup = vi.fn<() => undefined>().mockReturnValue(undefined)
+
+    focusManager.setEventListener(setup)
+    const unsubscribe1 = focusManager.subscribe(() => undefined)
+    const unsubscribe2 = focusManager.subscribe(() => undefined)
+    expect(setup).toHaveBeenCalledTimes(1)
+
+    unsubscribe1()
+    expect(setup).toHaveBeenCalledTimes(1)
+    unsubscribe2()
+
+    const unsubscribe3 = focusManager.subscribe(() => undefined)
+    expect(setup).toHaveBeenCalledTimes(2)
+    unsubscribe3()
+  })
+
+  it('should notify each subscriber once per event when setup returns no cleanup', () => {
+    const source = new EventTarget()
+    const listener1 = vi.fn<(focused: boolean) => void>()
+    const listener2 = vi.fn<(focused: boolean) => void>()
+
+    focusManager.setEventListener((onFocus) => {
+      source.addEventListener('focus', () => onFocus())
+      return undefined
+    })
+    const unsubscribe1 = focusManager.subscribe(listener1)
+    const unsubscribe2 = focusManager.subscribe(listener2)
+
+    source.dispatchEvent(new Event('focus'))
+    expect(listener1).toHaveBeenCalledExactlyOnceWith(true)
+    expect(listener2).toHaveBeenCalledExactlyOnceWith(true)
+
+    unsubscribe1()
+    unsubscribe2()
+  })
+
   it('should call previous remove handler when replacing an event listener', () => {
     const remove1Spy = vi.fn<() => void>()
     const remove2Spy = vi.fn<() => void>()

packages/query-core/src/__tests__/onlineManager.test.tsx

@@ -13,6 +13,23 @@ describe('onlineManager', () => {
     vi.useRealTimers()
   })
 
+  it('should initialize a listener without cleanup once until the last unsubscribe', () => {
+    const setup = vi.fn<() => undefined>().mockReturnValue(undefined)
+
+    onlineManager.setEventListener(setup)
+    const unsubscribe1 = onlineManager.subscribe(() => undefined)
+    const unsubscribe2 = onlineManager.subscribe(() => undefined)
+    expect(setup).toHaveBeenCalledTimes(1)
+
+    unsubscribe1()
+    expect(setup).toHaveBeenCalledTimes(1)
+    unsubscribe2()
+
+    const unsubscribe3 = onlineManager.subscribe(() => undefined)
+    expect(setup).toHaveBeenCalledTimes(2)
+    unsubscribe3()
+  })
+
   it('should return true from isOnline if navigator is undefined', () => {
     const navigatorSpy = vi.spyOn(globalThis, 'navigator', 'get')
 

packages/query-core/src/focusManager.ts

@@ -1,4 +1,5 @@
 import { Subscribable } from './subscribable'
+import { noop } from './utils'
 
 type Listener = (focused: boolean) => void
 
@@ -78,13 +79,14 @@ export class FocusManager extends Subscribable<Listener> {
   setEventListener(setup: SetupFn): void {
     this.#setup = setup
     this.#cleanup?.()
-    this.#cleanup = setup((focused) => {
-      if (typeof focused === 'boolean') {
-        this.setFocused(focused)
-      } else {
-        this.onFocus()
-      }
-    })
+    this.#cleanup =
+      setup((focused) => {
+        if (typeof focused === 'boolean') {
+          this.setFocused(focused)
+        } else {
+          this.onFocus()
+        }
+      }) ?? noop
   }
 
   /**

packages/query-core/src/onlineManager.ts

@@ -1,4 +1,5 @@
 import { Subscribable } from './subscribable'
+import { noop } from './utils'
 
 type Listener = (online: boolean) => void
 type SetupFn = (setOnline: Listener) => (() => void) | undefined
@@ -76,7 +77,7 @@ export class OnlineManager extends Subscribable<Listener> {
   setEventListener(setup: SetupFn): void {
     this.#setup = setup
     this.#cleanup?.()
-    this.#cleanup = setup(this.setOnline.bind(this))
+    this.#cleanup = setup(this.setOnline.bind(this)) ?? noop
   }
 
   /**