fix: 修复底部导航栏被覆盖和FAB消失问题

lib/screens/activity_list_screen.dart

@@ -13,6 +13,7 @@ class ActivityListScreen extends StatefulWidget {
 
 class _ActivityListScreenState extends State<ActivityListScreen> {
   int _selectedIndex = 0;
+  bool _showFriendsOverlay = false;
   final _feedKey = GlobalKey<RefreshIndicatorState>();
   String? _selectedTime;
 
@@ -83,10 +84,6 @@ class _ActivityListScreenState extends State<ActivityListScreen> {
   }
 
   void _onItemTapped(int index) {
-    if (index == 0 && _selectedIndex == 0) {
-      _feedKey.currentState?.show();
-      return;
-    }
     if (index == 2) {
       // 中间按钮 - 跳转到发布页面
       Navigator.push(
@@ -95,24 +92,26 @@ class _ActivityListScreenState extends State<ActivityListScreen> {
       );
       return;
     }
+
     if (index == 1) {
-      Navigator.push(
-        context,
-        MaterialPageRoute(builder: (context) => const FriendsScreen()),
-      );
+      setState(() {
+        _showFriendsOverlay = !_showFriendsOverlay;
+        if (_showFriendsOverlay) {
+          _selectedIndex = 0;
+        }
+      });
       return;
     }
-    if (index == 3 || index == 4) {
-      if (_selectedIndex != index) {
-        setState(() {
-          _selectedIndex = index;
-        });
-      }
+
+    if (index == 0 && _selectedIndex == 0 && !_showFriendsOverlay) {
+      _feedKey.currentState?.show();
       return;
     }
-    if (_selectedIndex != index) {
+
+    if (_selectedIndex != index || _showFriendsOverlay) {
       setState(() {
         _selectedIndex = index;
+        _showFriendsOverlay = false;
       });
     }
   }
@@ -123,27 +122,29 @@ class _ActivityListScreenState extends State<ActivityListScreen> {
     if (_selectedIndex == 3 || _selectedIndex == 4) {
       return Scaffold(
         backgroundColor: const Color(0xFFFFF8F0),
-        body: _buildBody(),
+        body: _buildBodyWithFriendsOverlay(),
         bottomNavigationBar: _buildBottomNavigationBar(),
       );
     }
     return Scaffold(
       backgroundColor: const Color(0xFFFFF8F0),
-      appBar: AppBar(
-        title: Text('伴享', style: TextStyle(fontWeight: FontWeight.bold)),
-        centerTitle: true,
-        actions: [
-          IconButton(
-            icon: Icon(Icons.search),
-            onPressed: () {},
-          ),
-          IconButton(
-            icon: Icon(Icons.notifications_none),
-            onPressed: () {},
-          ),
-        ],
-      ),
-      body: _buildBody(),
+      appBar: _showFriendsOverlay
+          ? null
+          : AppBar(
+              title: Text('伴享', style: TextStyle(fontWeight: FontWeight.bold)),
+              centerTitle: true,
+              actions: [
+                IconButton(
+                  icon: Icon(Icons.search),
+                  onPressed: () {},
+                ),
+                IconButton(
+                  icon: Icon(Icons.notifications_none),
+                  onPressed: () {},
+                ),
+              ],
+            ),
+      body: _buildBodyWithFriendsOverlay(),
       bottomNavigationBar: _buildBottomNavigationBar(),
       floatingActionButton: _buildFloatingActionButton(),
       floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
@@ -161,6 +162,15 @@ class _ActivityListScreenState extends State<ActivityListScreen> {
     }
   }
 
+  Widget _buildBodyWithFriendsOverlay() {
+    return Stack(
+      children: [
+        _buildBody(),
+        if (_showFriendsOverlay) const Positioned.fill(child: FriendsScreen()),
+      ],
+    );
+  }
+
   Widget _buildFeed() {
     final filteredActivities = _getFilteredActivities();
     return RefreshIndicator(
@@ -413,7 +423,7 @@ class _ActivityListScreenState extends State<ActivityListScreen> {
   }
 
   Widget _buildNavItem(int index, IconData icon, String label) {
-    final isSelected = _selectedIndex == index;
+    final isSelected = index == 1 ? _showFriendsOverlay : _selectedIndex == index;
     return Expanded(
       child: GestureDetector(
         onTap: () => _onItemTapped(index),

lib/screens/main_screen.dart

@@ -4,6 +4,7 @@ import 'services_screen.dart';
 import 'ai_chat_screen.dart';
 import 'messages_screen.dart';
 import 'profile_screen.dart';
+import 'create_activity_screen.dart';
 
 class MainScreen extends StatefulWidget {
   const MainScreen({super.key});
@@ -26,6 +27,8 @@ class _MainScreenState extends State<MainScreen> {
   Widget build(BuildContext context) {
     return Scaffold(
       body: IndexedStack(index: _currentIndex, children: _screens),
+      floatingActionButton: _buildGlobalFab(),
+      floatingActionButtonLocation: FloatingActionButtonLocation.endFloat,
       bottomNavigationBar: BottomNavigationBar(
         currentIndex: _currentIndex,
         onTap: (i) => setState(() => _currentIndex = i),
@@ -39,4 +42,23 @@ class _MainScreenState extends State<MainScreen> {
       ),
     );
   }
+
+  Widget? _buildGlobalFab() {
+    if (_currentIndex == 0) {
+      return null;
+    }
+
+    return FloatingActionButton.extended(
+      onPressed: () {
+        Navigator.push(
+          context,
+          MaterialPageRoute(builder: (_) => CreateActivityScreen()),
+        );
+      },
+      icon: const Icon(Icons.add),
+      label: const Text('发起活动', style: TextStyle(fontSize: 16)),
+      backgroundColor: const Color(0xFF1976D2),
+      foregroundColor: Colors.white,
+    );
+  }
 }