优化：卡片播放视频增加淡入效果

lib/features/home/widgets/video_card.dart

@@ -1,3 +1,4 @@
+import 'dart:async' show unawaited;
 import 'dart:io' show File;
 
 import 'package:flutter/material.dart';
@@ -39,6 +40,8 @@ class _VideoCardState extends State<VideoCard> {
   VideoPlayerController? _controller;
   /// 递增以作废过期的异步 [ _loadAndPlay ]，避免多路初始化互相抢同一个 State
   int _loadGen = 0;
+  /// 视频层是否已淡入（先 0 再置 true，触发 [AnimatedOpacity]）
+  bool _videoOpacityTarget = false;
   /// 从网络拉取视频时显示底部细进度条（缓存命中则不会出现）
   bool _showBottomProgress = false;
   /// 0–1 为确定进度；null 为不确定（无 Content-Length 等）
@@ -71,7 +74,50 @@ class _VideoCardState extends State<VideoCard> {
   void _stop() {
     _controller?.removeListener(_onVideoUpdate);
     _controller?.pause();
-    if (mounted) setState(() {});
+    if (mounted) {
+      setState(() {
+        _videoOpacityTarget = false;
+      });
+    }
+  }
+
+  /// 淡入略长 + easeInOut，避免「闪一下」的感觉
+  static const Duration _videoFadeDuration = Duration(milliseconds: 820);
+  /// 在 [play] 之后最多等到首帧再开始淡入，否则解码器突然出画会盖过 opacity 动画
+  static const Duration _videoFadeLeadInDeadline = Duration(milliseconds: 1200);
+
+  void _scheduleVideoFadeIn(int gen) {
+    unawaited(_runVideoFadeInSequence(gen));
+  }
+
+  Future<void> _runVideoFadeInSequence(int gen) async {
+    final c = _controller;
+    if (c == null || !mounted || gen != _loadGen || !widget.isActive) return;
+
+    // 与「视频层首帧 build」错开，保证 AnimatedOpacity 从 0 开始做完整动画
+    await Future<void>.delayed(const Duration(milliseconds: 16));
+    if (!mounted || gen != _loadGen || !widget.isActive) return;
+
+    final leadInEnd = DateTime.now().add(_videoFadeLeadInDeadline);
+    while (mounted && gen == _loadGen && widget.isActive && DateTime.now().isBefore(leadInEnd)) {
+      final v = c.value;
+      if (v.isInitialized && v.isPlaying) {
+        final likelyHasFrame = v.position > const Duration(milliseconds: 56) ||
+            (!v.isBuffering && v.size.width > 1 && v.size.height > 1);
+        if (likelyHasFrame) break;
+      }
+      await Future<void>.delayed(const Duration(milliseconds: 24));
+    }
+    if (!mounted || gen != _loadGen || !widget.isActive) return;
+
+    // 再给 Texture 一瞬提交首帧，减轻与透明度不同步的突兀感
+    await Future<void>.delayed(const Duration(milliseconds: 52));
+    if (!mounted || gen != _loadGen || !widget.isActive) return;
+
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      if (!mounted || gen != _loadGen || !widget.isActive) return;
+      setState(() => _videoOpacityTarget = true);
+    });
   }
 
   void _disposeController() {
@@ -113,7 +159,8 @@ class _VideoCardState extends State<VideoCard> {
         _stop();
         return;
       }
-      setState(() {});
+      setState(() => _videoOpacityTarget = false);
+      _scheduleVideoFadeIn(gen);
       return;
     }
 
@@ -175,12 +222,14 @@ class _VideoCardState extends State<VideoCard> {
         _disposeController();
         return;
       }
-      setState(() {});
+      setState(() => _videoOpacityTarget = false);
+      _scheduleVideoFadeIn(gen);
     } catch (e) {
       if (mounted) {
         _disposeController();
         setState(() {
           _clearBottomProgress();
+          _videoOpacityTarget = false;
         });
         widget.onStopRequested();
       }
@@ -193,7 +242,12 @@ class _VideoCardState extends State<VideoCard> {
 
   @override
   Widget build(BuildContext context) {
-    final showVideo = widget.isActive && _controller != null && _controller!.value.isInitialized;
+    final hasVideoUrl =
+        widget.videoUrl != null && widget.videoUrl!.isNotEmpty;
+    final videoInitialized =
+        _controller != null && _controller!.value.isInitialized;
+    final showVideoLayer =
+        widget.isActive && hasVideoUrl && videoInitialized;
 
     return LayoutBuilder(
       builder: (context, constraints) {
@@ -216,8 +270,30 @@ class _VideoCardState extends State<VideoCard> {
             child: Stack(
               fit: StackFit.expand,
               children: [
-                if (showVideo)
                 Positioned.fill(
+                  child: GestureDetector(
+                    onTap: widget.onGenerateSimilar,
+                    behavior: HitTestBehavior.opaque,
+                    child: CachedNetworkImage(
+                      imageUrl: widget.imageUrl,
+                      fit: BoxFit.cover,
+                      placeholder: (_, __) => Container(
+                        color: AppColors.surfaceAlt,
+                      ),
+                      errorWidget: (_, __, ___) => Container(
+                        color: AppColors.surfaceAlt,
+                      ),
+                    ),
+                  ),
+                ),
+                if (showVideoLayer)
+                  Positioned.fill(
+                    child: IgnorePointer(
+                      ignoring: !_videoOpacityTarget,
+                      child: AnimatedOpacity(
+                        opacity: _videoOpacityTarget ? 1.0 : 0.0,
+                        duration: _videoFadeDuration,
+                        curve: Curves.easeInOutCubic,
                         child: GestureDetector(
                           onTap: widget.onGenerateSimilar,
                           behavior: HitTestBehavior.opaque,
@@ -234,21 +310,6 @@ class _VideoCardState extends State<VideoCard> {
                             ),
                           ),
                         ),
-                  )
-                else
-                  Positioned.fill(
-                    child: GestureDetector(
-                      onTap: widget.onGenerateSimilar,
-                      behavior: HitTestBehavior.opaque,
-                      child: CachedNetworkImage(
-                        imageUrl: widget.imageUrl,
-                        fit: BoxFit.cover,
-                        placeholder: (_, __) => Container(
-                          color: AppColors.surfaceAlt,
-                        ),
-                        errorWidget: (_, __, ___) => Container(
-                          color: AppColors.surfaceAlt,
-                        ),
                       ),
                     ),
                   ),