divide screen into two equal parts in flutter

You can Use - MediaQuery to get the size of screen then divide it by 2 to get the First half.

@override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: title,
//        theme: ThemeData.light().copyWith(
//          platform: _platform ?? Theme.of(context).platform,
//        ),
        home: DefaultTabController(
            length: 3,
            child: Scaffold(
//                appBar: AppBar(
//                  title: Text(title),
//                ),
                body: SafeArea(
                    child: Column(children: <Widget>[
                  Container(
                    color: Colors.greenAccent,
                    height: MediaQuery.of(context).size.height / 2.2,  // Also Including Tab-bar height.
//                        child: Chewie(
//                          controller: _chewieController,
//                        ),
                  ),
                  PreferredSize(
                    preferredSize: Size.fromHeight(50.0),
                    child: TabBar(
                      labelColor: Colors.black,
                      tabs: [
                        Tab(
                          text: 'One',
                        ),
                        Tab(
                          text: 'Two',
                        ),
                        Tab(
                          text: 'Three',
                        )
                      ], // list of tabs
                    ),
                  ),
                  //TabBarView(children: [ImageList(),])
                  Expanded(
                    child: TabBarView(
                      children: [
                        Container(
                          color: Colors.deepOrange,
                          child: Center(child: Text('Tab1')),
                        ),
                        Container(
                          color: Colors.red,
                          child: Center(child: Text('Tab2')),
                        ),
                        Container(
                          color: Colors.yellowAccent,
                          child: Center(child: Text('Tab3')),
                        ) // class name
                      ],
                    ),
                  ),
                ])))));
  }

Output:

with AppBar - height: MediaQuery.of(context).size.height / 2.5,

with GridView.builder in - TabBarView

Expanded(
                    child: TabBarView(
                      children: [
                        GridView.builder(
                          itemBuilder: (context, int) {
                            return CircleAvatar(
                              backgroundImage: NetworkImage(
                                  'https://placeimg.com/640/480/any'),
                            );
                          },
                          itemCount: 20,
                          gridDelegate:
                              SliverGridDelegateWithFixedCrossAxisCount(
                                  crossAxisCount: 3),
                          shrinkWrap: true,
                        ),
                        Container(
                          color: Colors.red,
                          child: Center(child: Text('Tab2')),
                        ),
                        Container(
                          color: Colors.yellowAccent,
                          child: Center(child: Text('Tab3')),
                        ) // class name
                      ],
                    ),
                  ),

to fetch async data - use - FutureBuilder

    @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      builder: (context,snap){
        if(snap.hasData){
          return Expanded(
            child: GridView.count(
              shrinkWrap: true,
              childAspectRatio: 2,
              scrollDirection: Axis.vertical,
              crossAxisCount: 2,
              children: new List<Widget>.generate(images.length, (index) {
                return buildImage(images[index], context, index);
              },
              ).toList(),
            ),
          );

        }
        return Center(child: CircularProgressIndicator())
      },
      future: fetchSubCategoryContentlist(context, 20),
    );

Using an Expanded widget makes a child of a Row, Column, or Flex expand to fill the available space in the main axis (e.g., horizontally for a Row or vertically for a Column). If multiple children are expanded, the available space is divided among them according to the flex factor.

https://docs.flutter.io/flutter/widgets/Expanded-class.html

Column(
  children: <Widget>[
    Expanded(
      child: TopWidget()
    ),
    CenterWidget(),
    Expanded(
      child: BottomWidget()
    ),
  ]
)

Edit: full source code here

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {
    return Column(
        children: <Widget>[
          Expanded(
              child: Container(
                color: Colors.green,
              )
          ),
          Container(
              height: 40,
              color: Colors.grey
          ),
          Expanded(
              child: Container(
                color: Colors.blueGrey,
              )
          ),
        ]
    );
  }
}

Edit 2: and result here