в моем снимке нет данных в futurebuilder

Вы можете скопировать и вставить полный код ниже
Шаг 1. Вам не нужно isLoading, вы можете напрямую использовать проверку ConnectionState
Шаг 2. Вам не нужно объявлять List<RecentPage> wallpapers, snapshot.data уже сохраните это

фрагмент кода

builder: (context, AsyncSnapshot<List<RecentPage>> snapshot) {
      switch (snapshot.connectionState) {
        case ConnectionState.none:
          return Text('none');
        case ConnectionState.waiting:
          return Center(child: CircularProgressIndicator());
        case ConnectionState.active:
          return Text('');
        case ConnectionState.done:
          if (snapshot.hasError) {
            return Text(
              '${snapshot.error}',
              style: TextStyle(color: Colors.red),
            );
          } else {
            return ListView.builder(
              itemCount: snapshot.data.length,
              itemBuilder: (context, index) {
                return Container(
                  padding: EdgeInsets.symmetric(horizontal: 16),
                  child: GridView.count(
                    shrinkWrap: true,
                    crossAxisCount: 2,
                    physics: ClampingScrollPhysics(),
                    childAspectRatio: 0.6,
                    mainAxisSpacing: 6.0,
                    crossAxisSpacing: 6.0,
                    children: snapshot.data.map((wallpapers) {
                      return GridTile(
                          child: GestureDetector(
                        onTap: () {
                          //            Navigator.push(context, MaterialPageRoute(builder: (context)=>ImageView(
                          //              imageUrl: wallpapers.src.potrait,
                          //            )));
                        },
                        child: Container(
                          child: ClipRRect(
                            borderRadius: BorderRadius.circular(10),
                            child: Image.network(
                              wallpapers.imageUrl,
                              fit: BoxFit.cover,

рабочая демонстрация

полный код

import 'package:flutter/material.dart';
import 'dart:convert';
import 'package:http/http.dart' as http;

List<RecentPage> recentPageFromJson(String str) =>
    List<RecentPage>.from(json.decode(str).map((x) => RecentPage.fromJson(x)));

String recentPageToJson(List<RecentPage> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class RecentPage {
  RecentPage({
    this.no,
    this.imageId,
    this.imageUpload,
    this.imageUrl,
    this.type,
    this.viewCount,
    this.downloadCount,
    this.featured,
    this.tags,
    this.categoryId,
    this.categoryName,
  });

  int no;
  String imageId;
  String imageUpload;
  String imageUrl;
  Type type;
  String viewCount;
  String downloadCount;
  Featured featured;
  String tags;
  String categoryId;
  String categoryName;

  factory RecentPage.fromJson(Map<String, dynamic> json) => RecentPage(
        no: json["no"],
        imageId: json["image_id"],
        imageUpload: json["image_upload"],
        imageUrl: json["image_url"],
        type: typeValues.map[json["type"]],
        viewCount: json["view_count"],
        downloadCount: json["download_count"],
        featured: featuredValues.map[json["featured"]],
        tags: json["tags"],
        categoryId: json["category_id"],
        categoryName: json["category_name"],
      );

  Map<String, dynamic> toJson() => {
        "no": no,
        "image_id": imageId,
        "image_upload": imageUpload,
        "image_url": imageUrl,
        "type": typeValues.reverse[type],
        "view_count": viewCount,
        "download_count": downloadCount,
        "featured": featuredValues.reverse[featured],
        "tags": tags,
        "category_id": categoryId,
        "category_name": categoryName,
      };
}

enum Featured { YES, NO }

final featuredValues = EnumValues({"no": Featured.NO, "yes": Featured.YES});

enum Type { UPLOAD }

final typeValues = EnumValues({"upload": Type.UPLOAD});

class EnumValues<T> {
  Map<String, T> map;
  Map<T, String> reverseMap;

  EnumValues(this.map);

  Map<T, String> get reverse {
    if (reverseMap == null) {
      reverseMap = map.map((k, v) => new MapEntry(v, k));
    }
    return reverseMap;
  }
}

class FrontPage extends StatefulWidget {
  @override
  _FrontPageState createState() => _FrontPageState();
}

class _FrontPageState extends State<FrontPage> {
  Future<List<RecentPage>> recentPage;
  RecentPage recentPagee;
  bool isLoading = true;
  List<RecentPage> wallpapers = new List();

  Future<List<RecentPage>> getTrendingWallpapers() async {
    // final url ="http://wallpaper.pkappstudio.info/api/api.php?action=get_recent";
    String url =
        'http://wallpaper.pkappstudio.info/api/api.php?action=get_recent';
    final response =
        await http.get(url, headers: {"Accept": "application/json"});
    if (response.statusCode == 200) {
      final Map = jsonDecode(response.body);
      print("${response.body}");
      isLoading = false;

      final recentPaget = recentPageFromJson(response.body).asMap();

      recentPaget.entries.forEach((element) => wallpapers.add(element.value));

      return wallpapers;
    } else {
      throw Exception('Failed to load post');
    }
  }

  @override
  void initState() {
    // TODO: implement initState
    recentPage = getTrendingWallpapers();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: Colors.white,
        appBar: AppBar(
          title: Text("brandName()"),
          elevation: 0.0,
        ),
        body: FutureBuilder(
            future: recentPage,
            builder: (context, AsyncSnapshot<List<RecentPage>> snapshot) {
              switch (snapshot.connectionState) {
                case ConnectionState.none:
                  return Text('none');
                case ConnectionState.waiting:
                  return Center(child: CircularProgressIndicator());
                case ConnectionState.active:
                  return Text('');
                case ConnectionState.done:
                  if (snapshot.hasError) {
                    return Text(
                      '${snapshot.error}',
                      style: TextStyle(color: Colors.red),
                    );
                  } else {
                    return ListView.builder(
                      itemCount: snapshot.data.length,
                      itemBuilder: (context, index) {
                        return Container(
                          padding: EdgeInsets.symmetric(horizontal: 16),
                          child: GridView.count(
                            shrinkWrap: true,
                            crossAxisCount: 2,
                            physics: ClampingScrollPhysics(),
                            childAspectRatio: 0.6,
                            mainAxisSpacing: 6.0,
                            crossAxisSpacing: 6.0,
                            children: snapshot.data.map((wallpapers) {
                              return GridTile(
                                  child: GestureDetector(
                                onTap: () {
                                  //            Navigator.push(context, MaterialPageRoute(builder: (context)=>ImageView(
                                  //              imageUrl: wallpapers.src.potrait,
                                  //            )));
                                },
                                child: Container(
                                  child: ClipRRect(
                                    borderRadius: BorderRadius.circular(10),
                                    child: Image.network(
                                      wallpapers.imageUrl,
                                      fit: BoxFit.cover,
                                    ),
                                  ),
                                ),
                              ));
                            }).toList(),
                          ),
                        );
                      },
                    );
                  }
              }
            }));
  }
}

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

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

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

Есть хорошая реализация виджета FutureBuilder

Future<List<dynamic>> getTrendingWallpapers() async {
  String url = 'http://wallpaper.pkappstudio.info/api/api.php?action=get_recent';
  final response = await http.get(url, headers: {"Accept": "application/json"});
  if (response.statusCode == 200) {
    List<dynamic> wallpapers = List();
    wallpapers.add(response.body);

    return wallpapers;
  } else {
    throw Exception('Failed to load post');
  }
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    body: FutureBuilder(
      future: getTrendingWallpapers(),
      builder: (context, snapshot) {
        if (snapshot.connectionState == ConnectionState.waiting || snapshot.connectionState == ConnectionState.none) {
          return Center(child: CircularProgressIndicator());
        }

        if (snapshot.hasError) {
          return Center(
            child: Text('Future error'),
          );
        }
        if (snapshot.hasData) {
          return Center(
            child: Text('Do something with your snapshot.data : \n ${snapshot.data}'),
          );
        }
      },
    ),
  );
}