如何从流生成器返回 null

Question

我有一个想以编程方式移除抽屉的场景。
我使用流构建器来获取值并决定显示或删除抽屉，例如，当流值为真时，我返回我的抽屉小部件，否则返回 null 以删除抽屉，但流构建器不允许我返回 null，如果我return empty Container flutter 不会从应用栏中删除抽屉图标，所以我怎样才能实现我的目标并使用流构建器删除抽屉。
这是我的代码

new Scaffold(
      appBar: PreferredSize(
        child: MyAppBar(
          title: "Settings",
        ),
        preferredSize: Size.fromHeight(55),
      ),
      drawer: StreamBuilder(
        stream: settingService.getSettings,
        builder: (BuildContext context, snapshot) {
          if (snapshot.hasData) {
            final drawer= snapshot.data;
            if (drawer.visibility) {
              return myAppDrawerWidget(
                activeIndex: 13,
              );
            }
          }
          
        },
      ),
);

Answer 1

在AppBar()中添加automaticallyImplyLeading

  `automaticallyImplyLeading`: false, // this will hide Drawer hamburger icon

代码：

Scaffold(
      appBar: PreferredSize(
        child: AppBar(
          title: Text('Settings'),
          automaticallyImplyLeading: _isDrawerShowing,
        ),
        preferredSize: Size.fromHeight(55),
      ),
      drawer: StreamBuilder(
        stream: settingService.getSettings,
        builder: (BuildContext context, snapshot) {
          if (snapshot.hasData) {
            final drawer= snapshot.data;
            setState(() {
              _isDrawerShowing = drawer.visibility;
            });
            if (drawer.visibility) {
              return myAppDrawerWidget(
                activeIndex: 13,
              );
            }
          }
        },
      ),
    );

或

当你不必显示Drawer时返回null

  drawer: null,