This gist is a sample of Futurebuilder, search with highlighting

main.dart

import 'package:flutter/material.dart';
import 'dart:core';
import 'dart:math';


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

class MyApp extends StatelessWidget {
  
// This widget is the root of your application.
@override
Widget build(BuildContext context) {
    return  MaterialApp(
  debugShowCheckedModeBanner: false,
  title: 'Search Delegate example',
  theme: ThemeData(
  primaryColor: Colors.black,
),
//We are building our home according to how our app have started
home: HomePage());
}

}

const chars = "abcdefghijklmnopqrstuvwxyz0123456789";

 String RandomString(int strlen) {
  Random rnd = new Random(new DateTime.now().millisecondsSinceEpoch);
  String result = "";
  for (var i = 0; i < strlen; i++) {
    result += chars[rnd.nextInt(chars.length)];
  }
  return result;
}


class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  
  Future<List<Product>> productsFuture;
  ProductRepository productRepo;
  
  @override
  initState(){
    super.initState();
    productRepo = new ProductRepository();
    productsFuture = productRepo.fetchAll();
  }
  
    
  @override
  Widget build(BuildContext context){
    return Scaffold(
      backgroundColor:Colors.white,
      appBar:AppBar(
        title:Text('Home'),
        elevation:0,
        centerTitle:true,
        actions:[
          IconButton(icon:Icon(Icons.search),onPressed:(){
            showSearch(context:context,delegate:MySearchDelegate(productRepo:this.productRepo));
            
          }),
        ]
       
      ),
       body:MyFutureBuilder<List<Product>>(
         future:productsFuture,
         successWidget:(List<Product> products){
              return ListView.separated(
                itemCount:products.length,
                 separatorBuilder:(context,index){
                   return Divider(height:.1);
                 },
                itemBuilder:(BuildContext context,int index){
                  Product p = products[index];
                  return Padding(
                    padding:EdgeInsets.symmetric(horizontal:8,vertical:1),
                    child:Container(
                      padding:EdgeInsets.all(8),
                    decoration:BoxDecoration(
                    color:Colors.grey.withOpacity(.2),),
                    child:Column(
                    children:[
                      Row(children:[
                         Text(' ${p.name}',style:TextStyle(fontWeight: FontWeight.w600)),
                      ]),
                      Container(
                      child:Text('${p.description}',),),
                    ]
                  ),
                  ),);
       });
         }));
          
}
}


class MySearchDelegate extends SearchDelegate{
  
  ProductRepository productRepo;
  MySearchDelegate({this.productRepo});
  
  List<Widget> buildActions(BuildContext context){
    return [
      IconButton(
      icon:Icon(Icons.cancel),
        onPressed:(){
          this.query = "";
        }
      ),
    ];
  }
  
  Widget buildResults(BuildContext context){
     if(query=='')return Container();
    return MyFutureBuilder<List<Product>>(
        future:this.productRepo.searchProducts(this.query),
      successWidget:(List<Product> products){
              return ListView.separated(
                itemCount:products.length,
                 separatorBuilder:(context,index){
                   return Divider(height:.1);
                 },
                itemBuilder:(BuildContext context,int index){
                  Product p = products[index];
                  return Padding(
                    padding:EdgeInsets.symmetric(horizontal:8,vertical:1),
                    child:Container(
                      padding:EdgeInsets.all(8),
                    decoration:BoxDecoration(
                    color:Colors.grey.withOpacity(.2),),
                    child:Column(
                    children:[
                      Row(children:[
                         Text(' ${p.name}',style:TextStyle(fontWeight: FontWeight.w600)),
                      ]),
                      Container(
                      child:Text('${p.description}',),),
                    ]
                  ),
                  ),);
       });
         }
      
    );
  }
  buildLeading(BuildContext context){
    return  IconButton(
      icon:Icon(Icons.arrow_back),
        onPressed:(){
          this.close(context,null);
        }
      );
    
  }
  
  buildSuggestions(BuildContext context){
    if(query=='')return Container();
    return MyFutureBuilder<List<Product>>(
        future:this.productRepo.searchProducts(this.query),
      successWidget:(List<Product> products){
              return ListView.separated(
                itemCount:products.length,
                 separatorBuilder:(context,index){
                   return Divider(height:.1);
                 },
                itemBuilder:(BuildContext context,int index){
                  Product p = products[index];
                  return Padding(
                    padding:EdgeInsets.symmetric(horizontal:8,vertical:1),
                    child:Container(
                      padding:EdgeInsets.all(8),
                    decoration:BoxDecoration(
                    color:Colors.grey.withOpacity(.2),),
                    child:Column(
                    children:[
                      Row(children:[
                         buildMatch(query,p.name,context),
                      ]),
                      Container(
                      child:  buildMatch(query,p.description,context),),
                    ]
                  ),
                  ),);
       });
         }
      
    );
  }
  
}

buildMatch(String query, String found,BuildContext context){
  var tabs = found.toLowerCase().split(query.toLowerCase());
  List<TextSpan> list=[];
   for(var i =0;i<tabs.length;i++){
      if(i%2==1){
       list.add(TextSpan(text:query,style:TextStyle(color:Colors.green,fontWeight: FontWeight.w600,fontSize:15)));
     }
     list.add(TextSpan(text:tabs[i]));
    
   }
  return  RichText(                      
    text: TextSpan(
      style:TextStyle(color:Colors.black),
      children:list
    ),
  );
  
}

//Common widget

class MyFutureBuilder<T> extends StatelessWidget{
  final Future future;
   Widget Function(dynamic error) errorWidget;
  Widget Function(T data) successWidget;
  
  MyFutureBuilder({
    this.future,
    this.errorWidget,
    this.successWidget,
  });
  
  @override
  build(BuildContext context){
    return FutureBuilder<T>(
       future:this.future,
       builder:(BuildContext context,AsyncSnapshot snapshotData){
         switch(snapshotData.connectionState){
           case ConnectionState.active:
           case ConnectionState.waiting:
             return Center(child:Container(child:CircularProgressIndicator()));
             
           case ConnectionState.none:
             return Center(child:Text('Check Your Internet Connection'));
             break;
           case ConnectionState.done:
             if(snapshotData.hasError){
              return this.errorWidget!=null?this.errorWidget(snapshotData.error):Center(child:Text('${snapshotData.error}')); 
             }else{
              
               return this.successWidget(snapshotData.data);
             }
             break;
           default:
             return Container();
         }
       });
  }
  
}


// Repository file

class Product{
  final int id;
  final String name;
  final String description;
  final int price;
  Product({this.id,this.name,this.description,this.price});
  
  toString(){
    return '''
     $id:  $name
    ''';
  }
}

class ProductRepository {
  final List<Product> products = List.generate(20,(int index){
      return            Product(id:index,name:"Product $index",description:RandomString(100),price:index*20);
      }); 
  
  Future<List<Product>> fetchAll(){
    
    return Future.delayed(Duration(seconds:2),(){
        return products;
    });
    
  }
  
  Future<List<Product>> searchProducts(String query){
    
    return Future.delayed(Duration(seconds:1),(){
    return products.where((p){
       return (p.name.toLowerCase().contains(query.toLowerCase()) || p.description.toLowerCase().contains(query.toLowerCase()));
    }).toList();
    });
    
  }
  
}