feat(timeline): implement animated title transition in TimelineHeader

- Added AnimatedTitle component to handle cross-fade animation for title changes in the TimelineHeader.
- Utilized useSharedValue and useAnimatedStyle from react-native-reanimated for smooth opacity transitions.
- Improved user experience by visually indicating title changes during navigation.

Signed-off-by: Innei <tukon479@gmail.com>

apps/mobile/src/modules/screen/TimelineSelectorProvider.tsx

@@ -1,8 +1,17 @@
-import { useMemo } from "react"
+import type { FC } from "react"
+import { useEffect, useMemo, useRef, useState } from "react"
 import { View } from "react-native"
+import Animated, {
+  runOnJS,
+  useAnimatedStyle,
+  useSharedValue,
+  withSpring,
+} from "react-native-reanimated"
 
+import { FakeNativeHeaderTitle } from "@/src/components/layouts/header/FakeNativeHeaderTitle"
 import { DefaultHeaderBackButton } from "@/src/components/layouts/header/NavigationHeader"
 import { NavigationBlurEffectHeader } from "@/src/components/layouts/views/SafeNavigationScrollView"
+import { gentleSpringPreset } from "@/src/constants/spring"
 import { TIMELINE_VIEW_SELECTOR_HEIGHT } from "@/src/constants/ui"
 import {
   ActionGroup,
@@ -27,7 +36,7 @@ export function TimelineHeader({ feedId }: { feedId?: string }) {
 
   return (
     <NavigationBlurEffectHeader
-      title={viewTitle}
+      headerTitle={<AnimatedTitle title={viewTitle} />}
       isLoading={(isFeed || isTimeline) && isFetching}
       headerLeft={useMemo(
         () =>
@@ -52,3 +61,81 @@ export function TimelineHeader({ feedId }: { feedId?: string }) {
     />
   )
 }
+
+const AnimatedTitle: FC<{
+  title: string | undefined
+}> = ({ title }) => {
+  // Track titles and animation state
+  const [displayedTitle, setDisplayedTitle] = useState(title)
+  const [oldTitle, setOldTitle] = useState<string | undefined>()
+  const [isAnimating, setIsAnimating] = useState(false)
+  const timeoutRef = useRef<NodeJS.Timeout | undefined>(undefined)
+
+  // Animation values - only opacity for cross-fade effect
+  const newTitleOpacity = useSharedValue(1)
+  const oldTitleOpacity = useSharedValue(0)
+
+  // Handle title changes with cross-fade animation
+  useEffect(() => {
+    if (title !== displayedTitle && title !== undefined && !isAnimating) {
+      setIsAnimating(true)
+      setOldTitle(displayedTitle)
+
+      // Start cross-fade animation
+      oldTitleOpacity.value = 1
+      newTitleOpacity.value = 0
+
+      // Fade out old title and fade in new title
+      oldTitleOpacity.value = withSpring(0, gentleSpringPreset)
+
+      // Update displayed title and fade in
+      timeoutRef.current = setTimeout(() => {
+        setDisplayedTitle(title)
+        newTitleOpacity.value = withSpring(1, gentleSpringPreset, () => {
+          runOnJS(setIsAnimating)(false)
+          runOnJS(setOldTitle)(void 0)
+        })
+      }, 150)
+    }
+  }, [title, displayedTitle, isAnimating, newTitleOpacity, oldTitleOpacity])
+
+  // Cleanup timeout on unmount
+  useEffect(() => {
+    return () => {
+      if (timeoutRef.current) {
+        clearTimeout(timeoutRef.current)
+      }
+    }
+  }, [])
+
+  // Initialize displayed title
+  useEffect(() => {
+    if (displayedTitle === undefined && title !== undefined) {
+      setDisplayedTitle(title)
+    }
+  }, [title, displayedTitle])
+
+  const newTitleStyle = useAnimatedStyle(() => ({
+    opacity: newTitleOpacity.value,
+  }))
+
+  const oldTitleStyle = useAnimatedStyle(() => ({
+    opacity: oldTitleOpacity.value,
+  }))
+
+  return (
+    <View className="relative">
+      {/* Current/New Title */}
+      <Animated.View style={newTitleStyle}>
+        <FakeNativeHeaderTitle>{displayedTitle}</FakeNativeHeaderTitle>
+      </Animated.View>
+
+      {/* Old Title (during transition) */}
+      {oldTitle && isAnimating && (
+        <Animated.View className="absolute inset-0" style={oldTitleStyle}>
+          <FakeNativeHeaderTitle>{oldTitle}</FakeNativeHeaderTitle>
+        </Animated.View>
+      )}
+    </View>
+  )
+}