Iterating over an array of objects in JSON with Flutter FutureBuilder

Question

I am having trouble trying to iterate over a JSON array of objects from a remote URL using Flutter's FutureBuilder.

My goal is to:

Fetch JSON data from an API
Output the data into a 2 column gridview layout

The JSON data is an array of objects(or a List of Maps in dart), the objects have simple string data.

I know that I need to build a future to fetch the data from the API and decode the JSON, then I need to create a FutureBuilder to output the List data into my Gridview Builder. That is what I have tried to do in my code below.

import 'dart:convert';
import 'dart:async';
import 'dart:core';

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


class HomeSection extends StatefulWidget {
  @override
  _HomeSectionState createState() => _HomeSectionState();
}

class _HomeSectionState extends State<HomeSection> {
  @override
  void initState() {
    super.initState();
  }

  Future<List<dynamic>> fetchSectionData() async {
    String dataUrl =
        'https://www.thisisthejsonapilink.co.uk/fetch-data';
    var response = await http.get(Uri.parse(dataUrl));
    if (response.statusCode == 200) {
      return jsonDecode(response.body);
    } else {
      throw Exception('Failed to get the data');
    }
  }

  @override
  Widget build(BuildContext context) {

    return FutureBuilder(
      future: fetchSectionData,
      builder: (context, snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          return GridView.builder(
            itemCount: 12,
            gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
              maxCrossAxisExtent: 300,
              crossAxisSpacing: 16.0,
              mainAxisSpacing: 18.0,
              childAspectRatio: MediaQuery.of(context).size.height / 930,
            ),
            itemBuilder: (BuildContext context, int index) => GestureDetector(
              onTap: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(builder: (context) => OtherScreen()),
                );
              },
              child: Column(
                children: [
                  Container(
                    margin: const EdgeInsets.only(bottom: 12.0),
                    height: 144,
                  ),
                  Text(
                    snapshot.data[index].title,
                  ),
                  Text(
                    snapshot.data[index].duration + ' - ' + snapshot.data[index].type,
                  ),
                ],
              ),
            ),
          );
        } else {
          return Center(
            child: CircularProgressIndicator(
              color: Colors.black,
            ),
          );
        }
      },
    );
  }
}

My JSON that returns from the API link is structured like this:

[
  {
    "Title": "Audio 1",
    "Duration": "5 min",
    "type": "audio",
    "bodyText": "lorem ipsum blah blah audio",
    "url": "https://www.silvermansound.com/music/dirty-gertie.mp3"
  },
  {
    "Title": "Video 1",
    "Duration": "5 min",
    "type": "video",
    "bodyText": "lorem ipsum blah blah video",
    "url": "https://assets.mixkit.co/videos/preview/mixkit-woman-wearing-a-mask-while-running-40158-large.mp4"
  },
  {
    "Title": "Audio 2",
    "Duration": "5 min",
    "type": "audio",
    "bodyText": "lorem ipsum blah blah audio",
    "url": "https://www.silvermansound.com/music/dirty-gertie.mp3"
  },
  {
    "Title": "Video 2",
    "Duration": "5 min",
    "type": "video",
    "bodyText": "lorem ipsum blah blah video",
    "url": "https://assets.mixkit.co/videos/preview/mixkit-woman-wearing-a-mask-while-running-40158-large.mp4"
  },
  {
    "Title": "Audio 3",
    "Duration": "5 min",
    "type": "audio",
    "bodyText": "lorem ipsum blah blah audio",
    "url": "https://www.silvermansound.com/music/dirty-gertie.mp3"
  },
  {
    "Title": "Video 3",
    "Duration": "5 min",
    "type": "video",
    "bodyText": "lorem ipsum blah blah video",
    "url": "https://assets.mixkit.co/videos/preview/mixkit-woman-wearing-a-mask-while-running-40158-large.mp4"
  },
]

This is the error I am getting in my VS code debug console:

The argument type 'Future<List<dynamic>> Function()' can't be assigned to the parameter type 'Future<Object?>?'.

The red line appears right where I try to define my future in the FutureBuilder here under fetchSectionData:

return FutureBuilder(
      future: fetchSectionData,
      builder: (context, snapshot) {

I am not sure what this error means. Could somebody please explain it? The Future is definitely returning a <List>, but how do I get this list into the futurebuilder so that I can iterate over it and output the data into the gridview?

I am fairly new to flutter and come from a javascript web background, usually in javascript you can just loop over an array and output it that way. I'm tempted to do it that way here but I know that wouldn't be right.

When I looked up the Flutter documentation on how to fetch data from the internet it mentioned that I have to convert the response into a custom dart object, but nothing I try seems to work.

Appreciate your help!

Naveen Avidi · Accepted Answer · 2021-06-15 05:30:12Z

1

Try this !

return FutureBuilder<List<dynamic>>(
      future: fetchSectionData,
      builder: (context, snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          return GridView.builder(
            itemCount: 12,
            gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
              maxCrossAxisExtent: 300,
              crossAxisSpacing: 16.0,
              mainAxisSpacing: 18.0,
              childAspectRatio: MediaQuery.of(context).size.height / 930,
            ),
            itemBuilder: (BuildContext context, int index) => GestureDetector(
              onTap: () {
                Navigator.push(
                  context,
                  MaterialPageRoute(builder: (context) => OtherScreen()),
                );
              },
              child: Column(
                children: [
                  Container(
                    margin: const EdgeInsets.only(bottom: 12.0),
                    height: 144,
                  ),
                  Text(
                    '${snapshot.data[index]['Title']}',
                  ),
                  Text(
                    '${snapshot.data[index]['Duration'] + ' - ' + snapshot.data[index]['type']}',
                  ),
                ],
              ),
            ),
          );
        } else {
          return Center(
            child: CircularProgressIndicator(
              color: Colors.black,
            ),
          );
        }
      },
    );

answered Jun 15, 2021 at 5:30

Naveen Avidi

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1 Comment

danny471 Over a year ago

I had to add null checks to it and change the bottom to be like this: ${snapshot.data[index]['Duration'] + ' - ' + snapshot.data[index]['type']} but it worked. I also had to bear in mind about property names being case sensitive. Thanks!

Sajad Abdollahi · Accepted Answer · 2021-06-14 20:57:00Z

0

You have to pass the function like this

      future: fetchSectionData(),

in your GridView it's better to use

...
itemCount: snapshot.data.length
...

List<dynamic> equals to List it self

Future<List> fetchSectionData() async{
...

it's a good practice to determine type of FutureBuilder

FutureBuilder<List>(
...

edited Jun 14, 2021 at 20:57

answered Jun 14, 2021 at 20:51

Sajad Abdollahi

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6 Comments

danny471 Over a year ago

itemCount: snapshot.data.length I have added this and I getting this error in the debug console:

The property 'length' can't be unconditionally accessed because the receiver can be 'null'. Try making the access conditional (using '?.') or adding a null check to the target ('!').

danny471 Over a year ago

Under the line snapshot.data[index].title, I am also getting the error

The method '[]' can't be unconditionally invoked because the receiver can be 'null'. Try making the call conditional (using '?.') or adding a null check to the target ('!').

I assume this is a null safety thing

Sajad Abdollahi Over a year ago

@danny471 it's bc of dart null safety feature, add ! to them eg. snapshot.data![index] snapshot.data!.length

danny471 Over a year ago

I tried that but I get this error in the debug console now:

════════ Exception caught by widgets library ═══════════════════════════════════ Class '_InternalLinkedHashMap<String, dynamic>' has no instance getter 'title'. Receiver: _LinkedHashMap len:5 Tried calling: title

danny471 Over a year ago

itemCount: snapshot.data.length does seem to be working though as it's getting the right number of items in the json. So it's not an issue with the JSON not coming back, it doesn't seem to be able to get the properties.

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