Dropdown Using RenderObject In Flutter

Dropdown Using RenderObject In Flutter :

Screenshot 1 :

Screenshot 2 :

custom_dropdown.dart

import 'dart:io';
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:flutterapp/screen_adapter.dart';

class CustomDropDown<T> extends StatefulWidget {
  final String keyName;
  final ValueChanged<T> onChanged;
  final List<String> items;
  final int showItemNum;

  const CustomDropDown(
      {Key key,
      @required this.keyName,
      @required this.onChanged,
      @required this.items,
      this.showItemNum = 2})
      : super(key: key);

  @override
  _CustomDropDownState createState() => _CustomDropDownState();
}

class _CustomDropDownState extends State<CustomDropDown> {
  GlobalKey actionKey;
  double width, height, xPosition, yPosition;
  bool isDropDownOpened = false;
  OverlayEntry floatingDropdown;
  OverlayEntry middleLayer;
  String content;

  @override
  void initState() {
    actionKey = LabeledGlobalKey(widget.keyName);
    super.initState();
  }

  void findDropDownData() {
    RenderBox renderBox = actionKey.currentContext.findRenderObject();
    width = renderBox.size.width;
    height = renderBox.size.height;
    Offset offset = renderBox.localToGlobal(Offset.zero);
    xPosition = offset.dx;
    yPosition = offset.dy;
  }

  OverlayEntry _createFloatingDropdown() {
    return OverlayEntry(builder: (context) {
      return Positioned(
          left: xPosition + ScreenAdapter.width(10),
          width: width - 2 * ScreenAdapter.width(10),
          top: yPosition + height,
          height: widget.showItemNum * height + 2 * ScreenAdapter.height(10),
          child: dropDownWidget());
    });
  }

  OverlayEntry _createMiddleLayer() {
    return OverlayEntry(builder: (context) {
      return Positioned(
          left: 0,
          top: 0,
          child: GestureDetector(
              behavior: HitTestBehavior.opaque,
              onTap: () {
                setState(() {
                  floatingDropdown.remove();
                  middleLayer.remove();
                  isDropDownOpened = !isDropDownOpened;
                });
              },
              child: Container(
                  width: ScreenAdapter.getScreenWidth(),
                  height: ScreenAdapter.getScreenHeight())));
    });
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
        key: actionKey,
        onTap: () {
          setState(() {
            if (isDropDownOpened) {
              floatingDropdown.remove();
              middleLayer.remove();
            } else {
              middleLayer = _createMiddleLayer();
              Overlay.of(context).insert(middleLayer);
              findDropDownData();
              floatingDropdown = _createFloatingDropdown();
              Overlay.of(context).insert(floatingDropdown);
            }

            isDropDownOpened = !isDropDownOpened;
          });
        },
        child: Container(
            width: ScreenAdapter.getScreenWidth(),
            decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(ScreenAdapter.width(8)),
              border:
                  Border.all(color: Colors.grey, width: ScreenAdapter.width(1)),
            ),
            padding: EdgeInsets.symmetric(
                horizontal: ScreenAdapter.width(20),
                vertical: ScreenAdapter.height(20)),
            child: Row(children: <Widget>[
              Spacer(),
              Text(content ?? widget.keyName,
                  style: TextStyle(fontSize: ScreenAdapter.size(30))),
              Spacer(),
              Icon(Icons.arrow_drop_down, size: ScreenAdapter.width(40))
            ])));
  }

  Widget dropDownWidget() {
    return MediaQuery.removePadding(
        removeTop: true,
        context: context,
        child: Container(
            height: widget.showItemNum * height + 2 * ScreenAdapter.height(20),
            decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(ScreenAdapter.width(16))),
            child: ScrollConfiguration(
                behavior: MyBehavior(),
                child: ListView.builder(
                    shrinkWrap: true,
                    itemCount: widget.items.length,
                    itemBuilder: (_, index) {
                      if (index == widget.items.length - 1) {
                        return DropDownItem.last(
                            text: widget.items[index],
                            onPress: (value) {
                              setState(() {
                                content = value;
                              });
                              widget.onChanged(value);
                              floatingDropdown.remove();
                              middleLayer.remove();
                              isDropDownOpened = !isDropDownOpened;
                            });
                      } else {
                        return DropDownItem(
                            text: widget.items[index],
                            onPress: (value) {
                              setState(() {
                                content = value;
                              });
                              widget.onChanged(value);
                              floatingDropdown.remove();
                              middleLayer.remove();
                              isDropDownOpened = !isDropDownOpened;
                            });
                      }
                    }))));
  }
}

class DropDownItem extends StatelessWidget {
  final String text;

  final bool isFirstItem;

  final bool isLastItem;

  final Function(String) onPress;

  const DropDownItem(
      {Key key,
      this.text,
      this.isFirstItem = false,
      this.isLastItem = false,
      this.onPress})
      : super(key: key);

  factory DropDownItem.last({String text, Function(String) onPress}) {
    return DropDownItem(text: text, onPress: onPress, isLastItem: true);
  }

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
        onTap: () {
          onPress(text);
        },
        child: Container(
            alignment: Alignment.center,
            width: ScreenAdapter.width(100),
            decoration: BoxDecoration(
                border: Border.all(
                    color: Colors.grey, width: ScreenAdapter.width(0.5)),
                borderRadius: BorderRadius.vertical(
                    top: isFirstItem
                        ? Radius.circular(ScreenAdapter.width(16))
                        : Radius.zero,
                    bottom: isLastItem
                        ? Radius.circular(ScreenAdapter.width(16))
                        : Radius.zero),
                color: Colors.white),
            padding: EdgeInsets.symmetric(
                horizontal: ScreenAdapter.width(20),
                vertical: ScreenAdapter.height(20)),
            child: Text(text,
                style: TextStyle(
                    fontWeight: FontWeight.normal,
                    decoration: TextDecoration.none,
                    color: Colors.black,
                    fontSize: ScreenAdapter.size(30)))));
  }
}

class MyBehavior extends ScrollBehavior {
  @override
  Widget buildViewportChrome(
      BuildContext context, Widget child, AxisDirection axisDirection) {
    if (Platform.isAndroid || Platform.isFuchsia) {
      return child;
    } else {
      return super.buildViewportChrome(context, child, axisDirection);
    }
  }
}

screen_adapter.dart

import 'package:flutter_screenutil/flutter_screenutil.dart';

class ScreenAdapter {
  static init(context) {
    //ScreenUtil.instance = ScreenUtil(width: 750, height: 1334)..init(context);
    ScreenUtil.init(context, width: 750, height: 1334,allowFontScaling: false);
  }

  static height(double value) {
    return ScreenUtil().setHeight(value.toDouble());
  }

  static width(double value) {
    return ScreenUtil().setWidth(value.toDouble());
  }

  static getScreenHeight() {
    return ScreenUtil.screenHeightDp;
  }

  static getPixelRatio(){
    return ScreenUtil.pixelRatio;
  }

  static getScreenWidth() {
    return ScreenUtil.screenWidthDp;
  }

  static getScreenPxHeight() {
    return ScreenUtil.screenHeight;
  }

  static getScreenPxWidth() {
    return ScreenUtil.screenWidth;
  }

  static size(double value){
   return ScreenUtil().setSp(value.toDouble());  
  }

  static instance()=> ScreenUtil;
}

main.dart

import 'package:flutter/material.dart';
import 'custom_dropdown.dart';
import 'screen_adapter.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(visualDensity: VisualDensity.adaptivePlatformDensity),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    ScreenAdapter.init(context);

    return Scaffold(
        appBar: AppBar(
          title: Text('Dropdown using RenderObject'),
        ),
        body: CustomDropDown(
            keyName: "Gender",
            items: ["Male", "Female", "None"],
            showItemNum: 20,
            onChanged: (value) {}));
  }
}

pubspec.yaml

name: flutterapp
description: A new Flutter application.

publish_to: 'none'
version: 1.0.0+1

environment:
  sdk: ">=2.7.0 <3.0.0"

dependencies:
  flutter:
    sdk: flutter

  cupertino_icons: ^0.1.3
  flutter_screenutil:

dev_dependencies:
  flutter_test:
    sdk: flutter

flutter:
  uses-material-design: true

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