Firebase Auth + Calling — KMP

A Kotlin Multiplatform app with Firebase Authentication, Firestore, Realtime Database signaling, and Agora video/audio calling — all from a shared codebase using Compose Multiplatform and the experimental SwiftPM integration for iOS.

Experimental Notice This project uses several cutting-edge features:

• Kotlin SwiftPM Import — Consuming Swift Package Manager dependencies directly from Kotlin/Native (experimental)
• Kotlin 2.3.x (Titan) — Pre-release Kotlin build (2.3.20-titan-222)

This is a proof-of-concept and learning resource — not a production template.

Features

• Email/password sign up & sign in
• Google Sign-In (Credential Manager on Android, Google Sign-In SDK on iOS)
• Password reset via email
• Auth state persistence across app launches
• User listing via Cloud Firestore
• Video & audio calling via Agora RTC
• In-app call signaling via Firebase Realtime Database (no push notifications needed)
• Incoming call overlay with accept/reject
• Auto-timeout on unanswered calls (30s)
• Shared ViewModel & UI across platforms
• Dark-themed UI with animations

Architecture

How Calling Works

1. Caller taps call button
   └─► Writes invite to /calls/{receiverUid} in Realtime DB
   └─► Joins Agora channel

2. Receiver's app has real-time listener on /calls/{myUid}
   └─► Shows full-screen IncomingCallOverlay (accept / reject)

3. Accept → Joins same Agora channel → Both connected
   Reject → Clears invite from Realtime DB
   Timeout → Auto-clears after 30 seconds

4. Channel name = sorted(callerUid, receiverUid).joinToString("_")
   └─► Deterministic — same channel regardless of who calls whom

expect/actual Pattern

All platform-specific code uses Kotlin's expect/actual mechanism:

Common (expect)	Android (actual)	iOS (actual)
FirebaseAuth	Firebase SDK + Credential Manager	FIRAuth + GIDSignIn via SwiftPM
FirebaseFirestoree	FirebaseFirestore SDK	FIRFirestore via SwiftPM
CallSignaling	Firebase Realtime Database SDK	FIRDatabase via SwiftPM
CallService	CallActivity with Agora RTC	UIViewController with Agora RTC

Tech Stack

Layer	Technology
Shared UI	Compose Multiplatform 1.10.0 + Material3
Shared Logic	Kotlin 2.3.x, Coroutines, ViewModel, StateFlow
Auth	Firebase Auth (Android SDK / iOS SwiftPM)
Database	Cloud Firestore (user data) + Realtime DB (call signaling)
Calling	Agora RTC SDK 4.6.2 (video + audio)
iOS Deps	SwiftPM (Firebase, GoogleSignIn, Agora)
Architecture	expect/actual, callbackFlow, StateFlow

Project Structure

composeApp/src/
├── commonMain/                    # Shared code
│   ├── App.kt                    # Navigation + global incoming call overlay
│   ├── AuthViewModel.kt          # Auth state management (StateFlow)
│   ├── AuthUser.kt               # User data class
│   ├── FirebaseAuth.kt           # expect — auth interface
│   ├── FirebaseFirestoree.kt     # expect — Firestore interface
│   ├── CallService.kt            # expect — start video/audio calls
│   ├── CallSignaling.kt          # expect — Realtime DB signaling + CallInvite model
│   └── ui/
│       ├── LoginScreen.kt
│       ├── SignUpScreen.kt
│       ├── HomeScreen.kt         # User list with call buttons
│       ├── IncomingCallOverlay.kt # Full-screen accept/reject UI
│       └── AuthComponent.kt
├── androidMain/                   # Android implementations
│   ├── FirebaseAuth.kt           # Credential Manager + Firebase Auth
│   ├── FirebaseFirestoree.kt     # FirebaseFirestore SDK
│   ├── CallSignaling.kt          # Firebase Realtime Database
│   ├── CallService.kt            # Launches CallActivity
│   ├── CallActivity.kt           # Agora RTC + programmatic UI
│   └── MainActivity.kt
└── iosMain/                       # iOS implementations
    ├── FirebaseAuth.kt           # GIDSignIn + FIRAuth via SwiftPM
    ├── FirebaseFireStoree.kt     # FIRFirestore via SwiftPM
    ├── CallSignaling.kt          # FIRDatabase via SwiftPM
    ├── CallService.kt            # Agora RTC + AgoraRtcEngineDelegate
    └── MainViewController.kt

iosApp/                            # Xcode project entry point
├── iosApp/
│   ├── iOSApp.swift              # FirebaseApp.configure() + Google Sign-In URL handler
│   ├── ContentView.swift         # Compose-to-UIViewController bridge
│   └── Info.plist                # Permissions (camera, mic) + Google client ID
└── _internal_linkage_SwiftPMImport/  # Auto-generated SwiftPM linkage

Prerequisites

• Android Studio (with KMP plugin)
• Xcode 16+
• A Firebase project with:
  • Android app registered (package: dev.jasmeetsingh.firebaseauth)
  • iOS app registered
  • Authentication > Email/Password enabled
  • Authentication > Google enabled
  • Cloud Firestore database created
  • Realtime Database created (for call signaling)
• An Agora account with an App ID

Setup (Fresh Clone)

Step 1 — Clone & open

git clone <repo-url>

Open the project in Android Studio (with KMP plugin installed).

Step 2 — Create a Firebase project

1. Go to Firebase Console → Create a project
2. Register an Android app with package name dev.jasmeetsingh.firebaseauth
   • Download google-services.json → place in composeApp/
3. Register an iOS app with bundle ID dev.jasmeetsingh.firebaseauth.FirebaseAuthApp
   • Download GoogleService-Info.plist → place in iosApp/iosApp/
4. Enable these in Firebase Console:
   • Authentication → Sign-in method → Email/Password ✅
   • Authentication → Sign-in method → Google ✅
   • Cloud Firestore → Create database (test mode is fine to start)
   • Realtime Database → Create database → use these rules:

{
  "rules": {
    "calls": {
      "$uid": {
        ".read": "auth != null && auth.uid == $uid",
        ".write": "auth != null"
      }
    }
  }
}

Step 3 — Create an Agora account

1. Go to Agora Console → Create a project
2. Choose "Testing mode: App ID" (no certificate)
3. Copy the App ID and update it in:

composeApp/src/commonMain/kotlin/.../CallService.kt

const val AGORA_APP_ID = "your-agora-app-id-here"

Step 4 — iOS configuration

4a. Update Info.plist (iosApp/iosApp/Info.plist):

Open your downloaded GoogleService-Info.plist and copy these values:

Info.plist key	Value from GoogleService-Info.plist
GIDClientID	CLIENT_ID
CFBundleURLSchemes	REVERSED_CLIENT_ID

Camera & mic permissions are already in the template.

4b. Set your Team ID in iosApp/Configuration/Config.xcconfig:

TEAM_ID=YOUR_APPLE_TEAM_ID

(Find it in Xcode → Settings → Accounts → your team)

4c. SwiftPM linkage (one-time, after first Gradle sync):

XCODEPROJ_PATH='iosApp/iosApp.xcodeproj' ./gradlew :composeApp:integrateLinkagePackage

Then resolve packages:

xcodebuild -resolvePackageDependencies -project iosApp/iosApp.xcodeproj -scheme iosApp

Step 5 — Build & Run

Android:

Select the Android run configuration in Android Studio → Run, or:

./gradlew :composeApp:assembleDebug

iOS:

Open iosApp/iosApp.xcodeproj in Xcode → select your real device → ⌘R

Or use the iOS run configuration in Android Studio.

Note: iOS simulator on Intel Macs is not supported with SwiftPM dependencies. Use a real device.

Gotchas

Problem	Fix
PRODUCT_NAME collision with Firebase module names	Rename in Config.xcconfig to FirebaseAuthApp (already done)
Missing package product '_internal_linkage_SwiftPMImport'	Run xcodebuild -resolvePackageDependencies
integrateLinkagePackage needed	Run after adding/changing SwiftPM deps in build.gradle.kts
Intel Mac — iosX64() fails with SwiftPM	Don't add iosX64(). Use a real iOS device instead
Agora "no token" mode	Fine for testing. For production, enable token auth in Agora Console
xcrun: error: missing DEVELOPER_DIR	Run sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
iOS buttons not working after a while	Kotlin/Native GC — ensure strong references to handlers (already handled)

Demo

Auth Demo

screen-recording.mp4

Video-Calling/Voice-Calling Demo

screenRecord.mp4