Collapsing Toolbar with Smooth Scroll Animation in Jetpack Compose

CollapsingToolbar.kt

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.activity.enableEdgeToEdge
import androidx.compose.foundation.Image
import androidx.compose.foundation.background
import androidx.compose.foundation.interaction.MutableInteractionSource
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.Spacer
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.fillMaxWidth
import androidx.compose.foundation.layout.height
import androidx.compose.foundation.layout.padding
import androidx.compose.foundation.rememberScrollState
import androidx.compose.foundation.verticalScroll
import androidx.compose.material3.Divider
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.runtime.getValue
import androidx.compose.runtime.mutableStateOf
import androidx.compose.runtime.remember
import androidx.compose.runtime.setValue
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.graphicsLayer
import androidx.compose.ui.layout.ContentScale
import androidx.compose.ui.platform.LocalDensity
import androidx.compose.ui.res.painterResource
import androidx.compose.ui.text.font.FontWeight
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.lerp
import androidx.compose.ui.unit.sp

@Composable
fun CollapsingToolbar() {
    val scrollState = rememberScrollState()
    val imageHeight = 250.dp
    val toolbarHeight = 56.dp

    val imageHeightPx = with(LocalDensity.current) { imageHeight.toPx() }
    val toolbarHeightPx = with(LocalDensity.current) { toolbarHeight.toPx() }

    Box(modifier = Modifier.fillMaxSize()) {
        // Scrollable content
        Column(
            modifier = Modifier
                .fillMaxSize()
                .verticalScroll(scrollState)
        ) {
            // Spacer to push content below the image
            Spacer(modifier = Modifier.height(imageHeight))

            // Content
            repeat(20) { index ->
                Text(
                    text = "Item $index",
                    style = MaterialTheme.typography.bodySmall,
                    modifier = Modifier
                        .fillMaxWidth()
                        .padding(16.dp)
                )
                Divider()
            }
        }

        // Collapsing image
        val collapseRange = (imageHeightPx - toolbarHeightPx).coerceAtLeast(0f)
        val collapseFraction = (scrollState.value / collapseRange).coerceIn(0f, 1f)

        val imageSize = lerp(imageHeight, toolbarHeight, collapseFraction)
        val imageAlpha = 1f - (collapseFraction * 0.6f)
        val imageOffsetY = -scrollState.value * 0.6f * (1 - collapseFraction)

        Image(
            painter = painterResource(id = R.drawable.ic_launcher_foreground),
            contentDescription = "Header",
            contentScale = ContentScale.Crop,
            modifier = Modifier
                .fillMaxWidth()
                .height(imageSize)
                .graphicsLayer {
                    alpha = imageAlpha
                    translationY = imageOffsetY
                }
        )

        // Title
        val titleFontSize = lerp(24.sp, 18.sp, collapseFraction)
        val titleOffsetX = lerp(16.dp, 56.dp, collapseFraction)
        val titleTopPadding = lerp(imageHeight - 48.dp, 16.dp, collapseFraction)

        Text(
            text = "Collapsing Toolbar",
            fontSize = titleFontSize,
            color = Color.White,
            fontWeight = FontWeight.Bold,
            modifier = Modifier
                .padding(start = titleOffsetX, top = titleTopPadding)
                .graphicsLayer {
                    shadowElevation = 8.dp.toPx() * (1 - collapseFraction)
                }
        )
        
        val toolbarColor = MaterialTheme.colorScheme.primary.copy(
            alpha = collapseFraction
        )

        Box(
            modifier = Modifier
                .height(toolbarHeight)
                .fillMaxWidth()
                .background(toolbarColor)
        )
    }
}