Check out example codes for "flutter ui upload multiple image". It will help you in understanding the concepts better.

Code Example 1

import 'package:flutter/material.dart';
import 'dart:async';

import 'package:multi_image_picker/multi_image_picker.dart';

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {
    List<Asset> images = List<Asset>();
  String _error = 'Selectionner une image';

  @override
  void initState() {
    super.initState();
  }

  Widget buildGridView() {
    return GridView.count(
      crossAxisCount: 3,
      children: List.generate(images.length, (index) {
        Asset asset = images[index];
        return AssetThumb(
          asset: asset,
          width: 300,
          height: 300,
        );
      }),
    );
  }

  Future<void> loadAssets() async {
    List<Asset> resultList = List<Asset>();
    String error = 'No Error Dectected';

    try {
      resultList = await MultiImagePicker.pickImages(
        maxImages: 4,
        enableCamera: true,
        selectedAssets: images,
        cupertinoOptions: CupertinoOptions(takePhotoIcon: "chat"),
        materialOptions: MaterialOptions(
          actionBarColor: "#abcdef",
          actionBarTitle: "Example App",
          allViewTitle: "All Photos",
          useDetailsView: false,
          selectCircleStrokeColor: "#000000",
        ),
      );
    } on Exception catch (e) {
      error = e.toString();
    }

    // If the widget was removed from the tree while the asynchronous platform
    // message was in flight, we want to discard the reply rather than calling
    // setState to update our non-existent appearance.
    if (!mounted) return;

    setState(() {
      images = resultList;
      _error = error;
    });
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        appBar: new AppBar(
          title: const Text('Image Upload'),
          centerTitle: true,
        ),
        body: Column(
          children: <Widget>[
            //Center(child: Text('Error: $_error')),
            Text('Ajouter image du produit'),
            RaisedButton(
              child: Text("Selectionner "),
              onPressed: loadAssets,
            ),
            Expanded(
              child: buildGridView(),
            )
          ],
        ),
      ),
    );
  }
}

Code Example 2

import 'package:flutter/material.dart';
import 'dart:async';

import 'package:multi_image_picker/multi_image_picker.dart';

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

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> {
  List<Asset> images = List<Asset>();
  String _error;

  @override
  void initState() {
    super.initState();
  }

  Widget buildGridView() {
    if (images != null)
      return GridView.count(
        crossAxisCount: 3,
        children: List.generate(images.length, (index) {
          Asset asset = images[index];
          return AssetThumb(
            asset: asset,
            width: 300,
            height: 300,
          );
        }),
      );
    else
      return Container(color: Colors.white);
  }

  Future<void> loadAssets() async {
    setState(() {
      images = List<Asset>();
    });

    List<Asset> resultList;
    String error;

    try {
      resultList = await MultiImagePicker.pickImages(
        maxImages: 300,
      );
    } on Exception catch (e) {
      error = e.toString();
    }

    // If the widget was removed from the tree while the asynchronous platform
    // message was in flight, we want to discard the reply rather than calling
    // setState to update our non-existent appearance.
    if (!mounted) return;

    setState(() {
      images = resultList;
      if (error == null) _error = 'No Error Dectected';
    });
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        appBar: new AppBar(
          title: const Text('Plugin example app'),
        ),
        body: Column(
          children: <Widget>[
            Center(child: Text('Error: $_error')),
            RaisedButton(
              child: Text("Pick images"),
              onPressed: loadAssets,
            ),
            Expanded(
              child: buildGridView(),
            )
          ],
        ),
      ),
    );
  }
}

Code Example 3

import 'dart:io';

//import 'package:blog_app/models/ImageUploadModel.dart';
import 'package:flutter/material.dart';
import 'package:image_picker/image_picker.dart';

class SingleImageUpload extends StatefulWidget {
  @override
  _SingleImageUploadState createState() {
    return _SingleImageUploadState();
  }
}

class _SingleImageUploadState extends State<SingleImageUpload> {
  List<Object> images = List<Object>();
  Future<File> _imageFile;
  @override
  void initState() {
    // TODO: implement initState
    super.initState();
    setState(() {
      images.add("Add Image");
      images.add("Add Image");
     // images.add("Add Image");
     // images.add("Add Image");
    });
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        appBar: new AppBar(
          centerTitle: true,
          title: const Text('Upload image'),
        ),
        body: Column(
          children: <Widget>[
            Expanded(
              child: buildGridView(),
            ),
          ],
        ),
      ),
    );
  }

  Widget buildGridView() {
    return GridView.count(
      shrinkWrap: true,
      crossAxisCount: 3,
      childAspectRatio: 1,
      children: List.generate(images.length, (index) {
        if (images[index] is ImageUploadModel) {
          ImageUploadModel uploadModel = images[index];
          return Card(
            clipBehavior: Clip.antiAlias,
            child: Stack(
              children: <Widget>[
                Image.file(
                  uploadModel.imageFile,
                  width: 300,
                  height: 300,
                ),
                Positioned(
                  right: 5,
                  top: 5,
                  child: InkWell(
                    child: Icon(
                      Icons.remove_circle,
                      size: 20,
                      color: Colors.red,
                    ),
                    onTap: () {
                      setState(() {
                        images.replaceRange(index, index + 1, ['Add Image']);
                      });
                    },
                  ),
                ),
              ],
            ),
          );
        } else {
          return Card(
            child: IconButton(
              icon: Icon(Icons.add),
              onPressed: () {
                _onAddImageClick(index);
              },
            ),
          );
        }
      }),
    );
  }

  Future _onAddImageClick(int index) async {
    setState(() {
      _imageFile = ImagePicker.pickImage(source: ImageSource.gallery);
      getFileImage(index);
    });
  }

  void getFileImage(int index) async {
//    var dir = await path_provider.getTemporaryDirectory();

    _imageFile.then((file) async {
      setState(() {
        ImageUploadModel imageUpload = new ImageUploadModel();
        imageUpload.isUploaded = false;
        imageUpload.uploading = false;
        imageUpload.imageFile = file;
        imageUpload.imageUrl = '';
        images.replaceRange(index, index + 1, [imageUpload]);
      });
    });
  }
}

class ImageUploadModel {
  bool isUploaded;
  bool uploading;
  File imageFile;
  String imageUrl;

  ImageUploadModel({
    this.isUploaded,
    this.uploading,
    this.imageFile,
    this.imageUrl,
  });
}

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