refactor: move apply-dark-mode hook into other directory

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/hooks/dark-mode/use-apply-dark-mode.ts

@@ -0,0 +1,51 @@
+/*
+ * SPDX-FileCopyrightText: 2023 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import { DarkModePreference } from '../../redux/dark-mode/types'
+import { isClientSideRendering } from '../../utils/is-client-side-rendering'
+import { Logger } from '../../utils/logger'
+import { useApplicationState } from '../common/use-application-state'
+import useMediaQuery from '@restart/hooks/useMediaQuery'
+import { useEffect } from 'react'
+
+const logger = new Logger('useApplyDarkMode')
+
+export const DARK_MODE_LOCAL_STORAGE_KEY = 'forcedDarkMode'
+
+/**
+ * Applies the `dark` css class to the body tag according to the dark mode state.
+ */
+export const useApplyDarkMode = (): void => {
+  const preference = useApplicationState((state) => state.darkMode.darkModePreference)
+  const isBrowserPreferringDark = useMediaQuery('(prefers-color-scheme: dark)')
+
+  useEffect(() => saveToLocalStorage(preference), [preference])
+  useEffect(() => {
+    if (preference === DarkModePreference.DARK || (preference === DarkModePreference.AUTO && isBrowserPreferringDark)) {
+      window.document.body.classList.add('dark')
+    } else {
+      window.document.body.classList.remove('dark')
+    }
+  }, [isBrowserPreferringDark, preference])
+
+  useEffect(() => () => window.document.body.classList.remove('dark'), [])
+}
+
+export const saveToLocalStorage = (preference: DarkModePreference): void => {
+  if (!isClientSideRendering()) {
+    return
+  }
+  try {
+    if (preference === DarkModePreference.DARK) {
+      window.localStorage.setItem(DARK_MODE_LOCAL_STORAGE_KEY, 'dark')
+    } else if (preference === DarkModePreference.LIGHT) {
+      window.localStorage.setItem(DARK_MODE_LOCAL_STORAGE_KEY, 'light')
+    } else if (preference === DarkModePreference.AUTO) {
+      window.localStorage.removeItem(DARK_MODE_LOCAL_STORAGE_KEY)
+    }
+  } catch (error) {
+    logger.error('Saving to local storage failed', error)
+  }
+}