Pacemaker (iOS & Android) [KMP]

Run together! A running companion monitoring the heart rate of a group of people doing sports together.

Supports

• External Bluetooth (LE) heart rate monitors (tested with Polar H10)
• iPhone <-> iPhone connections (No internet necessary, BLE)
• Android <-> Android connections (No internet necessary, BLE)
• iPhone <-> Android connections (No internet necessary, BLE)

Planned

WatchOS support (via UWB chip and Internet)

Screenshots

Install

• Google Play: https://play.google.com/store/apps/details?id=io.sellmair.pacemaker
• Apple App Store: https://apps.apple.com/us/app/pacemaker-heart-rate-monitor/id6446760560

Technical Details

Kotlin Multiplatform

This application is built as a Test/Dogfooding project for Kotlin/Multiplatform, Compose and JetBrains Fleet.

Architecture

No! ViewModels!: This project uses 'State Actors' instead.

The State Actor pattern used in 'Pacemaker' can be defined by two high level concepts:

Events

Every component in the application can emit any kind of event, including intents. Example: Some UI button that emits an event to the application

@Composable
fun MyButton() {
    Button(
        onClick = Launching { LoginIntent.emit() }
    ) {
        // ...
    }
}

State Producers

States can be produced and observed. Lets look at the producing site first:
Lets take the classic login example:

data class LoginState(val email: String, val password: String, val isLoggedIn: Boolean) : State {
    companion object Key : State.Key<LoginState> {
        val default get() = LoginState(email = "", password = "", isLoggedIn = false)
    }
}

fun CoroutineScope.launchLoginStateActor() = launchState(LoginState) {
    var state = LoginState.default

    collectEventsAsync<EmailChangedEvent> {
        state = state.copy(email = it.email)
        state.emit()
    }

    collectEventsAsync<PasswordChangedEvent> {
        state = state.copy(password = it.password)
        state.emit()
    }

    collectEventsAsync<LoginIntent> {
        val isLoggedIn = attemptLogin(state.email, state.password)
        state = state.copy(isLoggedIn = isLoggedIn)
        state.emit()
    }
}

Such states can then be used in the Application UI/Frontend easily

@Composable
fun MyLoginScreen() {
    val loginState by LoginState.collectAsState()
    MyLoginScreen(
        email = loginState.email,
        password = loginState.password
    )
}

@Composable
fun MyLoginScreen(
    email: String,
    password: String
) {
    Text(email)
    Text(password)
    Button(
        onClick = Launching { LoginIntent.emit() }
    ) {
        Text("Login")
    }
}

Libraries used

• kotlinx.coroutines
• kotlinx.datetime
• SQLDelight
• Multiplatform Settings
• Okio