Kotlin context parameters Complete Guide — Implicit Context/DSL Design

#android #kotlin #jetpackcompose #programming

What You'll Learn

context parameters (implicit context passing, DSL design, logging/transaction/auth contexts) explained.

What are context parameters?

Introduced experimentally in Kotlin 2.1. Allows passing context to functions implicitly.

// Enable in build.gradle.kts
kotlin {
    compilerOptions {
        freeCompilerArgs.add("-Xcontext-parameters")
    }
}

Basics

interface Logger {
    fun log(message: String)
}

// context(Logger) = callable only within Logger scope
context(logger: Logger)
fun processData(data: String) {
    logger.log("Processing: $data")
    // processing logic
    logger.log("Done: $data")
}

// Call site
class ConsoleLogger : Logger {
    override fun log(message: String) = println("[LOG] $message")
}

fun main() {
    with(ConsoleLogger()) {
        processData("sample")  // Logger context passed implicitly
    }
}

Multiple Contexts

interface TransactionScope {
    suspend fun <T> execute(query: String): T
}

interface AuthContext {
    val currentUserId: String
    val isAdmin: Boolean
}

context(auth: AuthContext, tx: TransactionScope)
suspend fun deleteUser(userId: String) {
    require(auth.isAdmin) { "Admin required" }
    tx.execute<Unit>("DELETE FROM users WHERE id = '$userId'")
}

Compose DSL Combination

interface AnalyticsScope {
    fun trackEvent(name: String, params: Map<String, String> = emptyMap())
}

context(analytics: AnalyticsScope)
@Composable
fun TrackedButton(
    text: String,
    eventName: String,
    onClick: () -> Unit
) {
    Button(onClick = {
        analytics.trackEvent(eventName, mapOf("button" to text))
        onClick()
    }) {
        Text(text)
    }
}

Providing Context with with/run

class AppAnalytics : AnalyticsScope {
    override fun trackEvent(name: String, params: Map<String, String>) {
        Firebase.analytics.logEvent(name) {
            params.forEach { (k, v) -> param(k, v) }
        }
    }
}

@Composable
fun MyScreen() {
    val analytics = remember { AppAnalytics() }
    with(analytics) {
        TrackedButton(
            text = "Purchase",
            eventName = "purchase_click",
            onClick = { /* purchase logic */ }
        )
    }
}

Summary

Concept	Description
`context(T)`	Implicit context declaration
`with(instance)`	Context provision
Multiple contexts	`context(A, B)`
DSL design	Scoped functions

context parameters reduce boilerplate
Perfect for cross-cutting concerns: Logger/Auth/Transaction
Provide context with with/run
Experimental feature in Kotlin 2.1+

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