Flutter 为什么FloatingActionButton未显示

我试着在列表视图上长按显示浮动按钮，但浮动按钮没有显示. 知道为什么吗？

我使用以下代码:

return GestureDetector(
          onLongPress: () {
            print('test');
            FloatingActionButton.extended(
              onPressed: () {
                print('test1');
              },
              icon: const Icon(Icons.add),
              label: const Text('Usuń'),
            );

更新1

我已经更新了帖子与更大的代码视图，以更好地了解构建过程… 我正在添加"Dodaj"按钮，到目前为止工作正常，并想添加另一个，"usuń"显示后，列表视图项长按. 你能看一下这段代码并指出我哪里做错了吗？

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.grey[100],
      floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
      floatingActionButton: FloatingActionButton.extended(
        onPressed: () {
          //addItem();
        },
        icon: const Icon(Icons.add),
        label: const Text('Dodaj'),
      ),
      appBar: AppBar(
        backgroundColor: Colors.grey[900],
        title: const Text('Lista zakupowa'),
        actions: [
          IconButton(
            onPressed: signUserOut,
            icon: const Icon(Icons.logout),
          )
        ],
      ),
      body: Center(
        child: FutureBuilder<List<PostListDetails>>(
          future: postsFuture,
          builder: (context, snapshot) {
            if (snapshot.connectionState == ConnectionState.waiting) {
              return const CircularProgressIndicator();
            } else if (snapshot.hasData) {
              return buildPosts(snapshot.data!);
            } else {
              return const Text("Brak danych...");
            }
          },
        ),
      ),
    );
  }


  // function to display fetched data on screen
  @override
  Widget buildPosts(List<PostListDetails> posts) {
    return ListView.builder(
      itemCount: posts.length,
      itemBuilder: (context, index) {
        final post = posts[index];
        int customCompare(PostListDetails a, PostListDetails b) {
          if (a.status != b.status) {
            return a.status!.toInt() - b.status!.toInt();
          }
          final int categoryCompare = a.kategoria.toString().compareTo(b.kategoria.toString());
          if (categoryCompare != 0) {
            return categoryCompare;
          }
          return a.nazwa.toString().compareTo(b.nazwa.toString());
        }
        return GestureDetector(
          onLongPress: () {
            print('test1');
            FloatingActionButton.extended(
              onPressed: () {
                print('test2');
              },
              icon: const Icon(Icons.add),
              label: const Text('Usuń'),
            );
          },
          child: Container(
            color: post.status! % 2 == 1 ? Colors.grey[100] : Colors.white,
            margin: const EdgeInsets.symmetric(vertical: 0.5, horizontal: 0),
            padding: const EdgeInsets.symmetric(vertical: 5, horizontal: 15),
            height: 50,
            width: double.maxFinite,
            child: Row(
              children: [
                Expanded(
                  flex: 8,
                    child: Column(
                    ...
                    ),
                  ),
                   Expanded(
                    flex: 2,
                    child: Column(
                    ...
                    ),
                  ),
            ],
            ),
            ),
            );
          },
        );
    }

更新2

我已经相应地修改了代码，但这次没有运气...长时间按下Listview项目后，OnLong Press按钮不显示. 这段代码应该创建一个Listview，其中包含一个项目列表和一个始终可见的浮动按钮"+Dodaj"(这部分工作得很好). 我也想创建一个项目后，长按一个"usuń"按钮…此部件不起作用，我的意思是按钮不可见.(未打印"测试1"和"测试2").

代码现在如下所示:

bool showButton = false;

@override
    Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.grey[100],
      floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
      floatingActionButton: FloatingActionButton.extended(
        onPressed: () {
          //addList();
        },
        icon: const Icon(Icons.add),
        label: const Text('Dodaj'),
      ),
      appBar: AppBar(
        backgroundColor: Colors.grey[900],
        title: const Text('Lista zakupowa'),
        actions: [
          IconButton(
            onPressed: signUserOut,
            icon: const Icon(Icons.logout),
          )
        ],
      ),
      body: Center(
        child: FutureBuilder<List<PostListDetails>>(
          future: postsFuture,
          builder: (context, snapshot) {
            if (snapshot.connectionState == ConnectionState.waiting) {
              return const CircularProgressIndicator();
            } else if (snapshot.hasData) {
              return buildPosts(snapshot.data!);
            } else {
              return const Text("Brak danych...");
            }
          },
        ),
      ),
    );
  }


  // function to display fetched data on screen
  @override
  Widget buildPosts(List<PostListDetails> posts) {
    return ListView.builder(
      itemCount: posts.length,
      itemBuilder: (context, index) {
        final post = posts[index];
        int customCompare(PostListDetails a, PostListDetails b) {
          if (a.status != b.status) {
            return a.status!.toInt() - b.status!.toInt();
          }
          final int categoryCompare = a.kategoria.toString().compareTo(b.kategoria.toString());
          if (categoryCompare != 0) {
            return categoryCompare;
          }
          return a.nazwa.toString().compareTo(b.nazwa.toString());
        }
        GestureDetector(
          onLongPress: () {
            print('test1');
            setState(() => showButton = true);
          },
        );
        showButton ? FloatingActionButton.extended(
          onPressed: () {
            print('test2');
            //removeList();
          },
          icon: const Icon(Icons.add),
          label: const Text('Usuń'),
        ) : null;
        return Container(
          color: post.status! % 2 == 1 ? Colors.grey[100] : Colors.white,
          margin: const EdgeInsets.symmetric(vertical: 0.5, horizontal: 0),
          padding: const EdgeInsets.symmetric(vertical: 5, horizontal: 15),
          height: 50,
          width: double.maxFinite,
          child: Row(
            children: [
              Expanded(
                flex: 8,
                child: Column(
                    crossAxisAlignment: CrossAxisAlignment.start,
                    children: [
                      Text(post.nazwa!, textAlign: TextAlign.left, style: const TextStyle(fontWeight: FontWeight.bold,fontSize: 15,color: Colors.black)),
                      Text('${post.waga!.toString()} g.', textAlign: TextAlign.left, style: const TextStyle(fontWeight: FontWeight.normal,fontSize: 15,color: Colors.grey))
                    ]
                    ),
                  ),
                   Expanded(
                    flex: 2,
                    child: Column(
                        crossAxisAlignment: CrossAxisAlignment.start,
                        children: [
                          const Expanded(
                            flex: 0,
                            child: Row(
                            ),
                          ),
                          Expanded(
                            flex: 10,
                            child: Row(
                              children: [
                                CatIcon(imagePath: (Cat2Ico(post.kategoria!.toString()))),
                                Checkbox(
                                  value: Int2Bool(post.status!.toInt()),
                                  onChanged: (value) {
                                    setState(() {post.status = Bool2Int(value!);});
                                    saveStatus(post.nazwa!, post.status!, index);
                                    posts.sort(customCompare);
                                  },
                                ),
                              ]
                            ),
                          )
                      ]
                    ),
                  ),
            ],
            ),
          );
      },
    );
  }