Add scrollableArea snippet to GesturesSnippets

compose/snippets/src/main/java/com/example/compose/snippets/touchinput/gestures/GesturesSnippets.kt

@@ -20,17 +20,21 @@ package com.example.compose.snippets.touchinput.gestures
 
 import android.os.Bundle
 import androidx.activity.ComponentActivity
+import androidx.compose.foundation.MutatePriority
 import androidx.compose.foundation.background
 import androidx.compose.foundation.border
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.gestures.Orientation
+import androidx.compose.foundation.gestures.ScrollScope
+import androidx.compose.foundation.gestures.ScrollableState
 import androidx.compose.foundation.gestures.detectDragGestures
 import androidx.compose.foundation.gestures.detectTapGestures
 import androidx.compose.foundation.gestures.draggable
 import androidx.compose.foundation.gestures.rememberDraggableState
 import androidx.compose.foundation.gestures.rememberScrollableState
 import androidx.compose.foundation.gestures.rememberTransformableState
 import androidx.compose.foundation.gestures.scrollable
+import androidx.compose.foundation.gestures.scrollableArea
 import androidx.compose.foundation.gestures.transformable
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -51,12 +55,17 @@ import androidx.compose.material.swipeable
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.annotation.FrequentlyChangingValue
+import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.saveable.Saver
+import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.snapshots.Snapshot
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.geometry.Offset
@@ -67,12 +76,18 @@ import androidx.compose.ui.input.nestedscroll.NestedScrollConnection
 import androidx.compose.ui.input.nestedscroll.NestedScrollSource
 import androidx.compose.ui.input.nestedscroll.nestedScroll
 import androidx.compose.ui.input.pointer.pointerInput
+import androidx.compose.ui.layout.Layout
 import androidx.compose.ui.platform.ComposeView
 import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.platform.rememberNestedScrollInteropConnection
+import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.IntOffset
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import androidx.compose.ui.util.fastCoerceIn
+import androidx.compose.ui.util.fastRoundToInt
+import kotlin.math.abs
 import kotlin.math.roundToInt
 
 @Preview
@@ -174,6 +189,177 @@ private fun ScrollableSample() {
 }
 // [END android_compose_touchinput_gestures_scrollable]
 
+@Preview
+// [START android_compose_touchinput_gestures_scrollableArea]
+@Composable
+private fun ScrollableAreaSample() {
+    // [START_EXCLUDE]
+    // rememberScrollableAreaSampleScrollState() holds the scroll position and other relevant
+    // information. It implements the ScrollableState interface, making it compatible with the
+    // scrollableArea modifier
+    val scrollState = rememberScrollableAreaSampleScrollState()
+    // [END_EXCLUDE]
+    Layout(
+        modifier =
+            Modifier
+                .size(150.dp)
+                .scrollableArea(scrollState, Orientation.Vertical)
+                .background(Color.LightGray),
+        // [START_EXCLUDE]
+        content = {
+            repeat(40) {
+                Text(
+                    modifier = Modifier.padding(vertical = 2.dp),
+                    text = "Item $it",
+                    fontSize = 24.sp,
+                    textAlign = TextAlign.Center,
+                )
+            }
+        },
+        // [END_EXCLUDE]
+    ) { measurables, constraints ->
+        // [START_EXCLUDE]
+        var totalHeight = 0
+
+        val childConstraints = constraints.copy(minWidth = 0, minHeight = 0)
+        val placeables =
+            measurables.map { measurable ->
+                val placeable = measurable.measure(childConstraints)
+                totalHeight += placeable.height
+                placeable
+            }
+
+        val viewportHeight = constraints.maxHeight
+        // [END_EXCLUDE]
+        // Update the maximum scroll value to not scroll beyond limits and stop when scroll
+        // reaches the end.
+        scrollState.maxValue = (totalHeight - viewportHeight).coerceAtLeast(0)
+
+        // Position the children within the layout.
+        layout(constraints.maxWidth, viewportHeight) {
+            // The current vertical scroll position, in pixels.
+            val scrollY = scrollState.value
+            val viewportCenterY = scrollY + viewportHeight / 2
+
+            var placeableLayoutPositionY = 0
+            placeables.forEach { placeable ->
+                // This sample applies a scaling effect to items based on their distance
+                // from the center, creating a wheel-like effect.
+                // [START_EXCLUDE]
+                val itemCenterY = placeableLayoutPositionY + placeable.height / 2
+                val distanceFromCenter = abs(itemCenterY - viewportCenterY)
+                val normalizedDistance =
+                    (distanceFromCenter / (viewportHeight / 2f)).fastCoerceIn(0f, 1f)
+
+                // Items scale between 0.4 at the edges of the viewport and 1 at the center.
+                val scaleFactor = 1f - (normalizedDistance * 0.6f)
+                // [END_EXCLUDE]
+                // Place the item horizontally centered with a layer transformation for
+                // scaling to achieve wheel-like effect.
+                placeable.placeRelativeWithLayer(
+                    x = constraints.maxWidth / 2 - placeable.width / 2,
+                    // Offset y by the scroll position to make placeable visible in the viewport.
+                    y = placeableLayoutPositionY - scrollY,
+                ) {
+                    scaleX = scaleFactor
+                    scaleY = scaleFactor
+                }
+                // Move to the next item's vertical position.
+                placeableLayoutPositionY += placeable.height
+            }
+        }
+    }
+}
+// [START_EXCLUDE]
+/*
+ * A custom implementation of ScrollableState that manages a scroll position and its maximum allowed
+ * value.
+ *
+ * This is a simplified version of the `ScrollState` used by `Modifier.verticalScroll` and
+ * `Modifier.horizontalScroll`, demonstrating how to implement a custom state for custom scrollable
+ * containers.
+ */
+private class ScrollableAreaSampleScrollState(initial: Int) : ScrollableState {
+
+    // The current integer scroll position in pixels.
+    // This is backed by a mutableStateOf to trigger recomposition when it changes.
+    @get:FrequentlyChangingValue
+    var value by mutableIntStateOf(initial)
+        private set
+
+    // The maximum scroll position allowed. This is typically derived from the content size minus
+    // viewport size.
+    var maxValue: Int
+        get() = _maxValueState.intValue
+        set(newMax) {
+            _maxValueState.intValue = newMax
+            Snapshot.withoutReadObservation {
+                if (value > newMax) {
+                    value = newMax
+                }
+            }
+        }
+
+    private var _maxValueState = mutableIntStateOf(Int.MAX_VALUE)
+
+    // Accumulates sub-pixel scroll deltas. This ensures that even small, fractional scroll
+    // movements are accounted for and contribute to the total scroll position over time, preventing
+    // loss of precision.
+    private var accumulator: Float = 0f
+
+    // The underlying ScrollableState that handles the actual scroll consumption logic. This lambda
+    // is invoked when a scroll delta is received.
+    private val scrollableState = ScrollableState {
+        val absolute = (value + it + accumulator)
+        val newValue = absolute.coerceIn(0f, maxValue.toFloat())
+        val changed = absolute != newValue
+        val consumed = newValue - value
+        val consumedInt = consumed.fastRoundToInt()
+        value += consumedInt
+        accumulator = consumed - consumedInt
+
+        if (changed) consumed else it
+    }
+
+    override suspend fun scroll(
+        scrollPriority: MutatePriority,
+        block: suspend ScrollScope.() -> Unit,
+    ): Unit = scrollableState.scroll(scrollPriority, block)
+
+    override fun dispatchRawDelta(delta: Float): Float = scrollableState.dispatchRawDelta(delta)
+
+    override val isScrollInProgress: Boolean
+        get() = scrollableState.isScrollInProgress
+
+    override val canScrollForward: Boolean by derivedStateOf { value < maxValue }
+
+    override val canScrollBackward: Boolean by derivedStateOf { value > 0 }
+
+    override val lastScrolledForward: Boolean
+        get() = scrollableState.lastScrolledForward
+
+    override val lastScrolledBackward: Boolean
+        get() = scrollableState.lastScrolledBackward
+
+    companion object {
+        // Saver for CustomSampleScrollState, allowing it to be saved and restored across
+        // process death or configuration changes. Only the current scroll 'value' is saved.
+        val Saver: Saver<ScrollableAreaSampleScrollState, *> =
+            Saver(save = { it.value }, restore = { ScrollableAreaSampleScrollState(it) })
+    }
+}
+
+@Composable
+private fun rememberScrollableAreaSampleScrollState(
+    initial: Int = 0
+): ScrollableAreaSampleScrollState {
+    return rememberSaveable(saver = ScrollableAreaSampleScrollState.Saver) {
+        ScrollableAreaSampleScrollState(initial = initial)
+    }
+}
+// [END_EXCLUDE]
+// [END android_compose_touchinput_gestures_scrollableArea]
+
 // [START android_compose_touchinput_gestures_nested_scroll]
 @Composable
 private fun AutomaticNestedScroll() {