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Getting Started With Compose for Web

In this tutorial, we will create a simple web UI application using the Compose UI framework.

Prerequisites

You need to have the following software installed before you begin:

JDK 11 or later
IntelliJ IDEA Community Edition or Ultimate Edition 2020.2 or later (you can use other editors, but for this tutorial we assume you are using IntelliJ IDEA)

Creating a new project

If you don't want to create the project manually, you can download the template here

The project wizard doesn't support Compose for web projects yet, so we need to perform the following steps:

1. Create a Kotlin Multiplatform project:

Select Gradle on the left menu
Tick Kotlin DSL build script
Tick Kotlin/Multiplatform

2. Update `settings.gradle.kts`:

pluginManagement {
    repositories {
        gradlePluginPortal()
        maven("https://maven.pkg.jetbrains.space/public/p/compose/dev")
    }
}

3. Update `build.gradle.kts`:

// Add compose gradle plugin
plugins {
    kotlin("multiplatform") version "1.6.10"
    id("org.jetbrains.compose") version "1.1.0"
}

// Add maven repositories
repositories {
    mavenCentral()
    maven("https://maven.pkg.jetbrains.space/public/p/compose/dev")
    google()
}

// Enable JS(IR) target and add dependencies
kotlin {
    js(IR) {
        browser()
        binaries.executable()
    }
    sourceSets {
        val jsMain by getting {
            dependencies {
                implementation(compose.web.core)
                implementation(compose.runtime)
            }
        }
    }
}

5. Add the following directories to the project:

src/jsMain/kotlin
src/jsMain/resources

6. Add the `index.html` file to the `resources`:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Sample</title>
</head>
<body>
  <div id="root"></div>
  <script src="REPLACE_WITH_YOUR_MODULE_NAME.js"></script>
</body>
</html>

7. Add the `Main.kt` file to the `kotlin`:

import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.getValue
import androidx.compose.runtime.setValue
import androidx.compose.runtime.Composable
import org.jetbrains.compose.web.attributes.*
import org.jetbrains.compose.web.css.*
import org.jetbrains.compose.web.dom.*
import org.jetbrains.compose.web.renderComposable

fun main() {
    var count: Int by mutableStateOf(0)

    renderComposable(rootElementId = "root") {
        Div({ style { padding(25.px) } }) {
            Button(attrs = {
                onClick { count -= 1 }
            }) {
                Text("-")
            }

            Span({ style { padding(15.px) } }) {
                Text("$count")
            }

            Button(attrs = {
                onClick { count += 1 }
            }) {
                Text("+")
            }
        }
    }
}

Running the project

Use the command line to run:

./gradlew jsBrowserRun

Instead of manually compiling and executing a Kotlin/JS project every time you want to see the changes you made, you can use the continuous compilation mode:

./gradlew jsBrowserRun --continuous

Or run it from the IDE:

The browser will open localhost:8080:

Common issues when running the project

[webpack-cli] Unable to load '@webpack-cli/serve' command

https://youtrack.jetbrains.com/issue/KT-49124

[webpack-cli] Unable to load '@webpack-cli/serve' command
[webpack-cli] TypeError: options.forEach is not a function
...

There is a temporary workaround:

In build.gradle.kts:

// a temporary workaround for a bug in jsRun invocation - see https://youtrack.jetbrains.com/issue/KT-48273
afterEvaluate {
    rootProject.extensions.configure<org.jetbrains.kotlin.gradle.targets.js.nodejs.NodeJsRootExtension> {
        versions.webpackDevServer.version = "4.0.0"
        versions.webpackCli.version = "4.9.0"
    }
}

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