In this article, you’ll learn how to build a reusable upload service that:

1. Uploads single and multiple files to Firebase Storage

2. Returns file download URLs

3. Uses Dependency Injection (DI) with injectable to keep the code modular, testable, and easy to maintain

By the end, you will have a production-ready upload service for your Flutter Web project.

Table of Contents:

1. Why File Uploads Matter in Flutter Web

2. Upload Flow Overview

3. Prerequisites

4. How to Define the Upload Data Model and Service Interface

5. How to Implement the Upload Service

6. How to Handle Errors

7. Dependency Injection with injectable

8. How to Use the Upload Service

9. Best Practices

10. Conclusion

11. References

Why File Uploads Matter in Flutter Web

When building for the web, users often expect features like uploading a profile picture, submitting documents, or sharing media. Unlike mobile, the web environment requires handling files via browser APIs, which then need to be integrated with backend services like Firebase for persistence.

Upload Flow Overview

Here’s a high-level look at how the upload process works:

1. The user selects a file or image using a browser file picker.

2. Flutter reads the file as a Uint8List.

3. The file is uploaded to Firebase Storage.

4. A download URL is generated and stored in Firestore (or used directly).

Prerequisites

Before starting, ensure you have the following:

1. A Flutter Web project

    flutter config --enable-web
    flutter create my_web_project
    cd my_web_project
   

2. Firebase set up in your Flutter app: Follow Add Firebase to your Flutter app (Web) and include the Firebase SDK snippet in index.html.

3. Firebase Storage enabled in the Firebase Console: Go to Build > Storage > Get Started and allow read/write access for testing. Example rules:

    service firebase.storage {
      match /b/{bucket}/o {
        match /{allPaths=**} {
          allow read, write: if true;
        }
      }
    }
   

   Don’t use these rules in production.

4. Required dependencies in your pubspec.yaml:

    dependencies:
      firebase_core: ^3.13.0
      firebase_storage: ^12.4.2
      injectable: ^2.3.2
      get_it: ^8.0.3
   
    dev_dependencies:
      build_runner: ^2.4.13
      injectable_generator: ^2.4.1
   

   Run flutter pub get to install.

How to Define the Upload Data Model and Service Interface

We begin with a data model to represent the file and a service interface to define the upload contract.

import 'dart:typed_data';

class UploadData {
  final Uint8List fileData;   // File in binary format
  final String folderName;    // Folder path in Firebase Storage
  final String fileName;      // File name

  const UploadData({
    required this.fileData,
    required this.fileName,
    required this.folderName,
  });
}

Next, create an abstract service that defines what the upload logic should do.

abstract class IUploadService {
  Future<String> uploadDoc({
    required UploadData file,
  });

  Future<List<String>> uploadMultipleDoc({
    required List<UploadData> files,
  });
}

Here’s what’s happening in this code:

• uploadDoc: Uploads one file and returns its download URL

• uploadMultipleDoc: Uploads multiple files in parallel and returns a list of URLs

How to Implement the Upload Service

Now let’s implement the upload logic with Firebase Storage.

import 'package:firebase_storage/firebase_storage.dart';
import 'package:flutter/foundation.dart';
import 'package:injectable/injectable.dart';
import 'i_upload_service.dart';
import 'custom_error.dart';

@LazySingleton(as: IUploadService)
class UploadService extends IUploadService {
  final FirebaseStorage firebaseStorage;

  UploadService({required this.firebaseStorage});

  @override
  Future<String> uploadDoc({required UploadData file}) async {
    try {
      var storageRef = firebaseStorage.ref('${file.folderName}/${file.fileName}');
      var uploadTask = storageRef.putData(file.fileData);
      TaskSnapshot snapshot = await uploadTask;
      return await snapshot.ref.getDownloadURL();
    } on FirebaseException catch (e) {
      throw CustomError(
        errorMsg: "Firebase upload failed: ${e.message}",
        code: e.code,
        plugin: e.plugin,
      );
    } catch (e) {
      if (kDebugMode) print("Unexpected error: $e");
      rethrow;
    }
  }

  @override
  Future<List<String>> uploadMultipleDoc({required List<UploadData> files}) async {
    return await Future.wait(
      files.map((file) => uploadDoc(file: file)),
    );
  }
}

This code defines a service class for uploading documents to Firebase Storage in a Flutter app. Let’s break it down step by step so you see exactly what’s happening:

1. Imports

1. firebase_storage: provides Firebase Storage SDK for uploading and managing files.

2. flutter/foundation.dart: gives access to constants like kDebugMode for debug logging.

3. injectable.dart: enables dependency injection using the injectable + getIt package.

4. i_upload_service.dart: defines the abstract contract/interface for the upload service.

5. custom_error.dart: defines a custom error class to standardize error handling.

2. Dependency Injection Setup

@LazySingleton(as: IUploadService)
class UploadService extends IUploadService {

1. @LazySingleton(as: IUploadService) registers UploadService as the implementation of IUploadService.

2. This means anywhere in the app where IUploadService is requested, getIt will provide an instance of UploadService.

3. It’s a singleton, so only one instance is created and reused across the app.

3. Constructor

final FirebaseStorage firebaseStorage;

UploadService({required this.firebaseStorage});

1. The class requires a FirebaseStorage instance, which will also be injected automatically.

2. This makes the service easier to test and replace.

4. Upload a Single File

@override
Future<String> uploadDoc({required UploadData file}) async {
  try {
    var storageRef = firebaseStorage.ref('${file.folderName}/${file.fileName}');
    var uploadTask = storageRef.putData(file.fileData);
    TaskSnapshot snapshot = await uploadTask;
    return await snapshot.ref.getDownloadURL();
  } on FirebaseException catch (e) {
    throw CustomError(
      errorMsg: "Firebase upload failed: ${e.message}",
      code: e.code,
      plugin: e.plugin,
    );
  } catch (e) {
    if (kDebugMode) print("Unexpected error: $e");
    rethrow;
  }
}

What this code does:

1. Creates a reference in Firebase Storage at the path folderName/fileName

2. Uploads the raw file bytes (file.fileData) using putData.

3. Waits for the upload to complete and retrieves a TaskSnapshot.

4. From the snapshot, gets the download URL of the uploaded file and returns it.

5. If a FirebaseException occurs, it wraps the error inside a custom CustomError.

6. Any other unexpected error is logged (only in debug mode) and rethrown.

5. Upload Multiple Files

@override
Future<List<String>> uploadMultipleDoc({required List<UploadData> files}) async {
  return await Future.wait(
    files.map((file) => uploadDoc(file: file)),
  );
}

What the code does:

1. Accepts a list of UploadData objects.

2. For each file, it calls uploadDoc (the single upload function).

3. Future.wait runs all uploads in parallel, waits for them to complete, and returns a list of download URLs.

This class is a Firebase Storage upload service. It can upload single or multiple documents. It follows dependency injection principles for testability and scalability. It uses error handling with CustomError to provide cleaner error messages. Multiple uploads are executed in parallel for efficiency.

How to Handle Errors

Instead of relying on raw print statements, it’s better to use a structured error class. A structured error class organizes all error information, like the message, code, and source, into a single object. This makes error handling consistent, reusable, and easy to manage. You can inspect, log, or display errors programmatically, which is much more maintainable than scattered prints.

import 'package:equatable/equatable.dart';

class CustomError extends Equatable {
  final String errorMsg;
  final String code;
  final String plugin;

  const CustomError({
    required this.errorMsg,
    required this.code,
    required this.plugin,
  });

  @override
  List<Object?> get props => [errorMsg, code, plugin];

  @override
  String toString() {
    return 'CustomError(errorMsg: $errorMsg, code: $code, plugin: $plugin)';
  }
}

Why you should use this approach:

• Ensures consistency across the project.

• Makes errors reusable anywhere in the app.

• Allows programmatic handling (for example, act differently based on the error code).

• Provides clear debugging information through toString().

• Scales well as your app grows.

Dependency Injection with injectable

In a typical app, you might manually create service instances like UploadService or FirebaseStorage wherever you need them. But as your app grows, manually creating and passing dependencies becomes messy, error-prone, and hard to test.

This is where Dependency Injection (DI) comes in. DI allows you to declare dependencies once, and let a framework handle creating and providing them wherever they’re needed. The injectable package in Flutter works with getIt to automate this process.

Instead of creating UploadService manually, you configure it with injectable so that your app automatically gets the correct instance when needed, following the singleton or lazy-loading patterns.

Step 1: Annotate your service

@LazySingleton(as: IUploadService)
class UploadService implements IUploadService {
  // your upload logic here
}

@LazySingleton(as: IUploadService) tells injectable:

• Lazy: Only create the instance when it’s first used.

• Singleton: Reuse the same instance throughout the app.

• as: IUploadService: Expose the service via its interface, making testing and swapping implementations easier.

Step 2: Run the generator

flutter pub run build_runner build

This command generates code that wiring all your injectable dependencies together, so you don’t have to manually instantiate them.

Step 3: Create an injectable module

import 'package:firebase_storage/firebase_storage.dart';
import 'package:injectable/injectable.dart';

@module
abstract class InjectableModule {
  @lazySingleton
  FirebaseStorage get firebaseStorage => FirebaseStorage.instance;
}

This code is setting up dependency injection for FirebaseStorage using the injectable package. Let me break it down:

1. @module: The @module annotation tells injectable that this class will act as a provider of external dependencies (things you don’t create manually but get from libraries, SDKs, or APIs).

   In this case, FirebaseStorage is coming from the Firebase SDK, so you don’t construct it yourself. You just get an instance from the SDK.

2. abstract class InjectableModule: This is a special module class that contains dependency definitions. Since it’s abstract, it won’t be instantiated directly. Instead, injectable generates code to handle the injection.

3. @lazySingleton: This annotation tells injectable that the dependency should be created only once and reused throughout the app (singleton pattern).

   • Lazy means it won’t be created until it’s actually needed.

   • Singleton means the same instance will be reused everywhere after the first creation.

4. FirebaseStorage get firebaseStorage => FirebaseStorage.instance;: This line defines what dependency to provide. Here it’s saying:

   • Whenever something in the app needs a FirebaseStorage instance, inject FirebaseStorage.instance.

   • This way, you don’t manually create or pass around FirebaseStorage yourself – injectable plus getIt handle that automatically.

In practice, this ensures that everywhere in your app where you need FirebaseStorage, you can simply inject it via constructor injection (for example, in your UploadService) without manually instantiating it.

Step 4: Resolve the service anywhere

final uploadService = getIt<IUploadService>();

Why we do this

By using injectable:

1. You stop manually instantiating dependencies everywhere.

2. Your services are easier to test, because you can swap implementations via interfaces.

3. You ensure singleton patterns and lazy loading without extra boilerplate.

4. Your app becomes more maintainable, especially as it grows.

In practice:
Anywhere in your app where UploadService needs FirebaseStorage, you just inject it via the constructor:

class UploadService implements IUploadService {
  final FirebaseStorage _firebaseStorage;

  UploadService(this._firebaseStorage);

  // Use _firebaseStorage here
}

Injectable + getIt takes care of providing the correct _firebaseStorage instance automatically.

How to Use the Upload Service

The Upload Service is a modular, reusable service in your app that handles uploading files to Firebase Storage. By using this service, you abstract away direct Firebase interactions, keep your code clean, and leverage dependency injection to access the service anywhere in your app.

The Upload Service provides several options:

• Single file upload – Upload one file at a time and get its download URL.

• Multiple file upload – Upload a batch of files in one go and receive a list of download URLs.

• Error handling – Any issues during upload (like network errors or permission problems) are caught and can be handled gracefully.

Below, we’ll go step by step through how to use these options in practice.

Example: Upload a single file.

Future<void> uploadFile(Uint8List fileData) async {
  final file = UploadData(
    fileData: fileData,
    fileName: 'example.txt',
    folderName: 'documents',
  );

  try {
    final uploadService = getIt<IUploadService>();
    final downloadUrl = await uploadService.uploadDoc(file: file);
    print('Uploaded successfully: $downloadUrl');
  } catch (e) {
    print('Upload failed: $e');
  }
}

This function uploadFile is a wrapper that prepares a file for upload and delegates the actual uploading to your UploadService via dependency injection. Let me break it down step by step:

Future<void> uploadFile(Uint8List fileData) async {
  final file = UploadData(
    fileData: fileData,
    fileName: 'example.txt',
    folderName: 'documents',
  );

1. First, it takes a file as raw bytes (Uint8List fileData).

2. Then it wraps this data in an UploadData object, giving it a fileName (example.txt) and a folderName (documents). This essentially creates metadata about the file, so your upload service knows what to call it and where to store it in Firebase Storage.

  try {
    final uploadService = getIt<IUploadService>();
    final downloadUrl = await uploadService.uploadDoc(file: file);
    print('Uploaded successfully: $downloadUrl');
  } catch (e) {
    print('Upload failed: $e');
  }
}

3. Next, it retrieves the IUploadService instance using getIt (your dependency injection container). Thanks to the binding you defined earlier (UploadService registered as IUploadService), getIt knows to give you the correct implementation.

4. It calls uploadService.uploadDoc(file: file) which triggers the actual upload to Firebase Storage. If successful, Firebase returns a download URL of the uploaded file.

5. The function then prints out:

   • "Uploaded successfully: <downloadUrl>" if the upload worked.

   • "Upload failed: <error>" if an error occurred (for example, no internet or Firebase permission issues).

In simple terms:

1. Input: Raw file data (bytes).

2. Process: Wraps it in an UploadData object → sends it to Firebase via UploadService.

3. Output: Prints the public download URL if upload succeeds, or prints an error message if it fails.

Example: Upload multiple files.

Future<void> uploadMultiple(List<Uint8List> filesData) async {
  final uploadService = getIt<IUploadService>();

  final files = filesData.map((data) => UploadData(
    fileData: data,
    fileName: '${DateTime.now().millisecondsSinceEpoch}.txt',
    folderName: 'batch_docs',
  )).toList();

  try {
    final urls = await uploadService.uploadMultipleDoc(files: files);
    print('All files uploaded: $urls');
  } catch (e) {
    print('Batch upload failed: $e');
  }
}

This function handles batch uploading of multiple files to Firebase Storage using the IUploadService. Let’s break it down step by step:

1. Access the upload service

final uploadService = getIt<IUploadService>();

Here, getIt retrieves the registered IUploadService instance through dependency injection. This service abstracts all the logic of uploading files, so you don’t deal with Firebase APIs directly in this method.

2. Prepare the list of files

final files = filesData.map((data) => UploadData(
  fileData: data,
  fileName: '${DateTime.now().millisecondsSinceEpoch}.txt',
  folderName: 'batch_docs',
)).toList();

filesData is a list of raw file contents (Uint8List). For each file in the list, it creates an UploadData object.

The filename is generated dynamically using the current timestamp (DateTime.now().millisecondsSinceEpoch), ensuring each file has a unique name.

All files are placed in the "batch_docs" folder in Firebase Storage. This way, you have a structured list of files ready for uploading.

3. Upload Multiple File Mechanism

final urls = await uploadService.uploadMultipleDoc(files: files);

The uploadService is asked to upload all files in one go using its uploadMultipleDoc method. It uploads each file to Firebase Storage. Once done, it returns a list of download URLs, one for each uploaded file.

4. Handle success or failure

print('All files uploaded: $urls');

On success, it prints out the URLs of all uploaded files (so you can later use them, for example, to display or share the documents).

print('Batch upload failed: $e');

If something goes wrong, it catches the exception and logs the error message.

In short, this function takes multiple raw files, wraps them into UploadData objects, uploads them all to Firebase Storage using the service layer, and prints the resulting download URLs.

Best Practices

1. Validate file size before uploading to avoid oversized files.

2. Restrict file types (for example, only image/*) to improve security.

3. Store metadata (like user ID, timestamp) in Firestore along with the download URL.

4. Use unique paths (uploads/userId/filename) to prevent collisions.

Conclusion

You now have a reusable and modular upload service for Flutter Web that supports single and multiple file uploads, handles errors in a structured way, and uses Dependency Injection for clean architecture.

This foundation makes it easy to extend the service further, for example by adding file deletion, upload progress tracking, or authenticated uploads.

References

1. Firebase Storage for Flutter

2. Firebase Storage API Reference

3. injectable Package

4. get_it Package