compose-jb/tutorials/Web/Events_Handling

Events handling in Compose Web

The API is experimental, and breaking changes can be expected

You can add event listeners in the attrs block:

onClick

Button(
    attrs = {
        onClick { wrappedMouseEvent -> 
            // wrappedMouseEvent is of `WrappedMouseEvent` type    
            println("button clicked at ${wrappedMouseEvent.movementX}, ${wrappedMouseEvent.movementY}")
            
            val nativeEvent = wrappedMouseEvent.nativeEvent // [MouseEvent](https://developer.mozilla.org/en/docs/Web/API/MouseEvent)
        }
    }
) {
    Text("Button")
}

onInput

val text = remember { mutableStateOf("") }

TextArea(
    value = text.value,
    attrs = {
        onInput {
            text.value = it.value
        }
    }
)

Other event handlers

For events that don't have their own configuration functions in the attrs block, you can use addEventListener with the name of the event, options, and an pass an eventListener which receives a WrappedEvent. In this example, we're defining the behavior of a Form element when it triggers the submit event:

Form(attrs = {
    this.addEventListener("submit") {
        console.log("Hello, Submit!")
        it.nativeEvent.preventDefault()
    }
})

Your event handlers receive wrapped events that inherit from GenericWrappedEvent, which also provides access to the underlying nativeEvent – the actual event created by JS runtime - https://developer.mozilla.org/en-US/docs/Web/API/Event

There are more event listeners supported out of a box. We plan to add the documentation for them later on. In the meantime, you can find all supported event listeners in the source code.

Runnable example

import androidx.compose.runtime.*
import org.jetbrains.compose.web.css.*
import org.jetbrains.compose.web.dom.*
import org.jetbrains.compose.web.renderComposable

fun main() {
    renderComposable(rootElementId = "root") {
        Button(
            attrs = {
                onClick { wrappedMouseEvent ->
                    // wrappedMouseEvent is of `WrappedMouseEvent` type
                    println("button clicked at ${wrappedMouseEvent.movementX}, ${wrappedMouseEvent.movementY}")

                    val nativeEvent = wrappedMouseEvent.nativeEvent
                }
            }
        ) {
            Text("Button")
        }

        val text = remember { mutableStateOf("") }

        TextArea(
            value = text.value,
            attrs = {
                onInput {
                    text.value = it.value
                }
            }
        )

        Span {
            Text("Typed text = ${text.value}")
        }
    }
}