试试这个

Container(
            height:40.0,
            padding: EdgeInsets.only(left:10.0,right:10.0),
            child: InputDecorator(
              decoration: InputDecoration(
                enabledBorder: InputBorder.none,//add this Disabling Underline/Border only when Enabled
focusedBorder: InputBorder.none, //Add this Removing the Underline/Border only when Focused
                border: InputBorder.none, //Add this when
                prefixIcon: const Icon(Icons.search, size: 25.0,),
                border: OutlineInputBorder(
                  borderRadius: BorderRadius.circular(50.0),
                  borderSide: const BorderSide(
                    width: 0,
                    style: BorderStyle.none,
                  ),
                ),
                filled: true,
                hintStyle: TextStyle(color: Colors.grey[800]),
                hintText: "Type in your text",
                fillColor: Colors.white,
                isDense: true,
                // contentPadding: EdgeInsets.all(0.0),
              ),
              child: Autocomplete<String>(
                optionsBuilder:
                    (TextEditingValue textEditingValue) {
                  if (textEditingValue.text == '') {
                    return const Iterable<String>.empty();
                  }
                  return _kOptions.where((String option) {
                    return option.contains(
                        textEditingValue.text.toLowerCase());
                  });
                },
                optionsViewBuilder: ((context, onSelected, options){
                  return Align(
                    alignment: Alignment.topLeft,
                    child: Material(
                      elevation: 4.0,
                      child: ConstrainedBox(
                        constraints: const BoxConstraints(maxHeight: 200, maxWidth: 300), //RELEVANT CHANGE: added maxWidth
                        child: ListView.builder(
                          padding: EdgeInsets.zero,
                          shrinkWrap: true,
                          itemCount: options.length,
                          itemBuilder: (BuildContext context, int index) {
                            final String option = _kOptions.elementAt(index);
                            return InkWell(
                              onTap: () {
                                onSelected(option);
                              },
                              child: Builder(builder: (BuildContext context) {
                                final bool highlight = AutocompleteHighlightedOption.of(context) == index;
                                return Container(
                                  color: highlight ? Theme.of(context).focusColor : null,
                                  padding: const EdgeInsets.all(16.0),
                                  child: Text(option),
                                );
                              }),
                            );
                          },
                        ),
                      ),
                    ),
                  );
                }
                ),
              ),
            ),
          ),

你可以试试这个。这将创建一个没有下划线的矩形边框的自动补全。

Autocomplete<String>(
  fieldViewBuilder: (BuildContext context,
      TextEditingController fieldTextEditingController,
      FocusNode fieldFocusNode,
      VoidCallback onFieldSubmitted) {
    return TextFormField(
      cursorColor: Colors.white,
      validator: (value) {
        if (value == null || value.isEmpty) {
          return 'Please enter item name';
        }
        return null;
      },
      decoration: const InputDecoration(
        focusedBorder: OutlineInputBorder(
          borderSide: BorderSide(width: 1, color: Colors.white),
        ),
        border: OutlineInputBorder(
          borderSide: BorderSide(width: 1, color: Colors.white),
        ),
        labelText: "Enter Item Name",
        labelStyle: TextStyle(color: Colors.white),
      ),
      controller: fieldTextEditingController,
      focusNode: fieldFocusNode,
    );
  },
  optionsBuilder: (TextEditingValue textEditingValue) {
    if (textEditingValue.text == '') {
      return const Iterable<String>.empty();
    }
    return ["One", "Two", "Three", "Four"];
  },
  onSelected: (selectedValue) {},
);