我有一个我自定义的文本字段小部件，它有一个labelText和一个focusBorder。我希望标签文本在输入字段，当我专注于字段，标签文本应该离开输入框和浮动（添加图像描述）。
我还将在问题中添加我的小部件代码片段。
谢谢你。
但是经过尝试，这是我可以实现的

我想把这个作为我的结局

/*
 * Copyright (c) 2020 .
 */

import 'package:flutter/material.dart';

class TextFieldWidget extends StatelessWidget {
  const TextFieldWidget({
    Key key,
    this.onSaved,
    this.onChanged,
    this.validator,
    this.keyboardType,
    this.initialValue,
    this.hintText,
    this.errorText,
    this.iconData,
    this.labelText,
    this.obscureText,
    this.suffixIcon,
    this.isFirst,
    this.isLast,
    this.style,
    this.textAlign,
    this.suffix,
  }) : super(key: key);

  final FormFieldSetter<String> onSaved;
  final ValueChanged<String> onChanged;
  final FormFieldValidator<String> validator;
  final TextInputType keyboardType;
  final String initialValue;
  final String hintText;
  final String errorText;
  final TextAlign textAlign;
  final String labelText;
  final TextStyle style;
  final IconData iconData;
  final bool obscureText;
  final bool isFirst;
  final bool isLast;
  final Widget suffixIcon;
  final Widget suffix;

  @override
  Widget build(BuildContext context) {
    return Container(
      margin: EdgeInsets.only(
          left: 20, right: 20, top: topMargin, bottom: bottomMargin),
      decoration: BoxDecoration(
        color: Color(0xffF8F8F8),
        borderRadius: BorderRadius.circular(8),
        boxShadow: [
          BoxShadow(
            color: Color(0xffeeeeee),
            blurRadius: 10,
            offset: Offset(0, 2),
          ),
        ],
        border: Border.all(
          color: Colors.black.withOpacity(0.13),
        ),
      ),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: [
          Text(
            labelText ?? "",
            style: TextStyle(
              color: Colors.black54.withOpacity(0.5),
              fontSize: 12,
              fontWeight: FontWeight.normal,
            ),
            textAlign: textAlign ?? TextAlign.start,
          ),
          TextFormField(
            maxLines: keyboardType == TextInputType.multiline ? null : 1,
            key: key,
            keyboardType: keyboardType ?? TextInputType.text,
            onSaved: onSaved,
            onChanged: onChanged,
            validator: validator,
            initialValue: initialValue ?? '',
            style: TextStyle(
              color: Colors.black.withOpacity(0.9),
              fontSize: 12,
              fontWeight: FontWeight.normal,
            ),
            obscureText: obscureText ?? false,
            textAlign: textAlign ?? TextAlign.start,
            decoration: InputDecoration(
              contentPadding: EdgeInsets.only(bottom: 15, left: 0),
              border: InputBorder.none,
              focusedBorder: OutlineInputBorder(
                borderSide: BorderSide(color: Color(0xff3498DB), width: 2.0),
                borderRadius: BorderRadius.circular(8),
              ),
              hintText: hintText ?? '',
              hintStyle: TextStyle(fontSize: 12, color: Colors.black),
              suffixIcon: suffixIcon,
              suffix: suffix,
              errorText: errorText,
            ),
          ),
        ],
      ),
    );
  }

  BorderRadius get buildBorderRadius {
    if (isFirst != null && isFirst) {
      return BorderRadius.vertical(top: Radius.circular(10));
    }
    if (isLast != null && isLast) {
      return BorderRadius.vertical(bottom: Radius.circular(10));
    }
    if (isFirst != null && !isFirst && isLast != null && !isLast) {
      return BorderRadius.all(Radius.circular(0));
    }
    return BorderRadius.all(Radius.circular(10));
  }

  double get topMargin {
    if ((isFirst != null && isFirst)) {
      return 20;
    } else if (isFirst == null) {
      return 20;
    } else {
      return 0;
    }
  }

  double get bottomMargin {
    if ((isLast != null && isLast)) {
      return 10;
    } else if (isLast == null) {
      return 10;
    } else {
      return 0;
    }
  }
}