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احتمالا یا chiddlen توی رندر حدف می شه یا یه جایی null بهش برمی گردونه کدتون رو قرار بدید دقیق تر برسی کنیم.

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter_app/pages/camera_page.dart';
import 'package:flutter_app/pages/chanels_page.dart';
import 'package:flutter_app/pages/chats_page.dart';
import 'package:flutter_app/pages/grups_page.dart';
class Waahtasap extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => WahtsappState();
}
class WahtsappState extends State<Waahtasap>with SingleTickerProviderStateMixin{
  TabController tabController;
  Map<String , SliverAppBar> AppBarList;

  String _currentAppBar = "SearchAppBar";

  @override
  // ignore: must_call_super
  void initState() {
    tabController = TabController(length: 4, vsync: this,initialIndex: 1);

    SliverAppBar mainAppBar= SliverAppBar (
      pinned: true,
      floating: true,
      elevation: 9.9,
      titleSpacing: 10,
      backgroundColor: Color(0xff1C8959),
      title: Text("WhatsApp",style: TextStyle(fontFamily: 'SF',fontWeight:FontWeight.w800),),
      actions: <Widget>[
        GestureDetector(
          child: Icon(Icons.search),
          onTap: (){
            setState(() {
              _currentAppBar = 'SearchAppBar';
            });
          },
        ),
        PopupMenuButton(
          tooltip: "more",
          elevation: 5,
          itemBuilder: (BuildContext context){
            return [
              PopupMenuItem(child: Text("Add Contact") ,value: "New group",),
              PopupMenuItem(child: Text("New Group") ,value: "New group",),
              PopupMenuItem(child: Text("Theme") ,),
              PopupMenuItem(child: Text("Setting") ,value: "New group",),];},
        )
      ],

      bottom: TabBar(
        indicatorColor: Colors.white,
        controller: tabController,

        tabs: [
          Icon(Icons.camera_alt),
          Text("Chats"),
          Text("Groups"),
          Text("Chanels"),
        ],),);
    SliverAppBar SearchAppBar= SliverAppBar  (

        pinned: true,
        floating: true,
        elevation: 9.9,
        titleSpacing: 10,
        backgroundColor: Color(0xff1C8959),
        title: TextField(
          decoration: InputDecoration(
            border: InputBorder.none,
            hintText: "Search",

          ),
        ),
        leading: GestureDetector(
      child: Icon(Icons.arrow_back),
      onTap: (){
        setState(() {
          _currentAppBar = 'mainAppBar';
        });
      },
    )
    );

    AppBarList = <String , SliverAppBar>{
      'MainAppBar': mainAppBar,
      'SearchAppBar' : SearchAppBar,

    };
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body:NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled){
            return <Widget>[
              AppBarList[
              _currentAppBar
              ]
            ];
          },

          body: _currentAppBar == "mainAppBar"
              ? TabBarView(
            controller: tabController,
            children:<Widget> [Camera(), Chats(), Groups(), Chanels()
            ],
          ):
          Center(
            child: Text("data"),
          )
      ),
    );
  }
}