终于找到了解决办法。需要将AlertDialog Package 在SingleChildScrollView中。这将打开屏幕顶部的AlertDialog，并且在键盘打开/关闭时不会移动它。它还确保如果内容太长（垂直），它可以在键盘下滑动，用户可以使用滚动手势查看/与隐藏的内容交互。

return SingleChildScrollView(
    child: AlertDialog(
      title: alertTitle,
      content: Column(
        mainAxisSize: MainAxisSize.min,
        children: <Widget> [
          DropdownButton<String>(
            .......
          ),
          TextField(
            autofocus: true,
            .......
          ),
        ],
      ),
    )
  );

使用dropdown_button2的最佳解决方案

FocusNode focusNode = FocusNode();
  bool hasFocus = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Dropdown Fix'),
      ),
      body: Center(
        child: ListView(
          children: <Widget>[
            DropdownButton2(
              onMenuStateChange: (isOpen) {
                hasFocus = false;

                // this allows us to track whether or not you need to focus
                if (focusNode.hasFocus && isOpen && MediaQuery.of(context).viewInsets.bottom != 0) {
                  hasFocus = true;
                }

                if (hasFocus) {
                  // it should be called twice to provide focusNode being focused before DropdownButton2 inner calculations and after build method
                  focusNode.requestFocus();
                  WidgetsBinding.instance.addPostFrameCallback((_) => focusNode.requestFocus());
                }
              },
              isExpanded: true,
              hint: const Text('associatedHint'),
              disabledHint: const Text('associatedHint'),
              // don't forget to remove `const` when providing a new value
              items: const [
                DropdownMenuItem<String>(
                  value: 'value1',
                  child: Text('value1'),
                ),
                DropdownMenuItem<String>(
                  value: 'value2',
                  child: Text('value2'),
                ),
              ],
              onChanged: (_) {
                // 
              },
              iconStyleData: const IconStyleData(),
              dropdownStyleData: const DropdownStyleData(),
              menuItemStyleData: const MenuItemStyleData(),
            ),
            const SizedBox(height: 200),
            TextField(
              focusNode: focusNode,
              // autofocus: true,
              textCapitalization: TextCapitalization.sentences,
              decoration: const InputDecoration(hintText: "Entry"),
              maxLines: 1,
              onChanged: (value) {},
            ),
          ],
        ),
      ),
    );