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In the world of modern mobile development, inter-app communication is evolving rapidly. Whether you're working on modular apps on a single device or syncing data between devices, a lightweight and scalable communication layer can make a big difference.
In this post, I'll walk you through an innovative concept - using Ktor to enable seamless data exchange between Android apps, both on the same device and across multiple devices.

💡 The Concept
What if:

  • App Bwants to send data (text, files, events) to App A?
  • App A is either:  - Installed on the same device  - Or running on another device in the same network (or even remotely)?

Ktor makes this not only possible but lightweight, flexible, and secure - all using pure Kotlin.

🔧 Why Ktor?

Ktor, developed by JetBrains, is an asynchronous framework for connected applications - from HTTP clients/servers to WebSockets. Its coroutine-based architecture makes it ideal for mobile-first scenarios.

Concept

Imagine a world where two Android apps - on the same or different devices - can communicate offline over a local network (like a hotspot) without relying on the internet. Sounds cool, right?
In this blog, I'll walk you through an open-source project I built that enables secure, offline data transfer between App A (Server) and App B (Client) using Ktor, QR codes, and Kotlin - all while maintaining a clean and modular architecture.
⚙️ Tech Stack:
 Ktor, Kotlin Coroutines, QR Code (ZXing), Jetpack Lifecycle, Content Negotiation, Android Networking (CIO Engine)

🌟 Use Case
Let's say App A wants to receive an image from App B. Instead of using Firebase or REST APIs over the internet, we:
Spin up a local Ktor server on App A.
Encode the server's IP address, port, and a secure token into a QR code.
App B scans this QR code, picks an image, and sends it directly to App A over a local Wi-Fi connection or hotspot.

💡 Great for offline apps, peer-to-peer sharing, or isolated environments like field operations, clinics, etc.

💪 App A: Ktor Server + QR Code Generator

This app:
Gets the device's local IP address.
Generates a QR code encoding IP|PORT|TOKEN.
Hosts a Ktor server listening on a specified port.
Accepts incoming POST requests and decodes image data sent in Base64.

🔐 Security

We use a hardcoded token (my_secure_token) for simple token-based validation.

val connectionInfo = "$ip|$port|$token"
val qrBitmap = generateQrCode(connectionInfo)
qrImageView.setImageBitmap(qrBitmap)
The Ktor server listens like so:
`post("/receive") {
    val auth = call.request.header("Authorization")?.removePrefix("Bearer ")
    if (auth == token) {
        val imageBase64 = call.receive()
        val bytes = Base64.decode(imageBase64, Base64.DEFAULT)
        val bitmap = BitmapFactory.decodeByteArray(bytes, 0, bytes.size)
        receivedImage.setImageBitmap(bitmap)
        call.respond(HttpStatusCode.OK, "Image received")
    } else {
        call.respond(HttpStatusCode.Unauthorized, "Invalid token")
    }
}

📲 App B: QR Scanner + Image Picker + Ktor Client
App B is the client. It:
Scans the QR code using ZXing (JourneyApps).
Extracts IP, port, and token.
Opens an image picker.
Converts the image to Base64 and sends it to App A.

val response = client.post("http://$ip:$port/receive") {
    header("Authorization", "Bearer $token")
    contentType(ContentType.Application.Json)
    setBody(base64Image)
}

💡 Why This is Unique
✅ Offline-First: No internet required
 ✅ Secure by Design: Token-based authorization
 ✅ Portable & Lightweight: Runs using Ktor CIO
 ✅ Modular Architecture: Client/server split encourages scalability
 ✅ Peer-to-Peer (P2P): Works in isolated networks (Wi-Fi Direct, hotspots)

📸 Demo Screenshot
App A generates QR code with IP, port, and token.

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App B scans, picks an image, and sends it.

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🧪 How to Test
1.Install App A and App B on two Android devices (same Wi-Fi or hotspot).
2.Open App A → Tap Start Server → QR Code data (IP|Port|Token) is displayed.
3.Open App B → Tap Scan QR → Scan QR code → Pick image from gallery.
4.App B sends image → App A receives and displays it.

🚀 Final Thoughts
With Ktor's power and a touch of Kotlin magic, we've built a fully offline, peer-to-peer communication system between two Android apps.
This opens up new possibilities for:
Medical & field data capture
Secure, local transfers without internet dependency
Portable apps that work anywhere

📂 Source Code
I published this project on GitHub . Stay tuned!

✨ Let's Connect
👨‍💻 Have questions, ideas, or want to collaborate on this concept? Let's connect on GitHub Mediumand LinkedInor drop a comment below!