Address PR feedback

Change-Id: I50b49d3e216535e384b64849f915b37f6e2acbbc

core/designsystem/src/main/kotlin/com/google/samples/apps/nowinandroid/core/designsystem/component/scrollbar/LazyScrollbarUtilities.kt

@@ -1,95 +1,23 @@
 /*
  * Copyright 2023 The Android Open Source Project
  *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
+ *   Licensed under the Apache License, Version 2.0 (the "License");
+ *   you may not use this file except in compliance with the License.
+ *   You may obtain a copy of the License at
  *
- *     https://www.apache.org/licenses/LICENSE-2.0
+ *       https://www.apache.org/licenses/LICENSE-2.0
  *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *   limitations under the License.
  */
 
 package com.google.samples.apps.nowinandroid.core.designsystem.component.scrollbar
 
 import androidx.compose.foundation.gestures.ScrollableState
-import androidx.compose.runtime.Composable
-import androidx.compose.runtime.LaunchedEffect
-import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableStateOf
-import androidx.compose.runtime.remember
-import androidx.compose.runtime.setValue
-import androidx.compose.runtime.snapshotFlow
-import kotlinx.coroutines.flow.distinctUntilChanged
-import kotlinx.coroutines.flow.filterNotNull
 import kotlin.math.abs
-import kotlin.math.min
-
-/**
- * Calculates the [ScrollbarState] for lazy layouts.
- * @param itemsAvailable the total amount of items available to scroll in the layout.
- * @param visibleItems a list of items currently visible in the layout.
- * @param firstVisibleItemIndex a function for interpolating the first visible index in the lazy layout
- * as scrolling progresses for smooth and linear scrollbar thumb progression.
- * [itemsAvailable].
- * @param reverseLayout if the items in the backing lazy layout are laid out in reverse order.
- * */
-@Composable
-internal inline fun <LazyState : ScrollableState, LazyStateItem> LazyState.scrollbarState(
-    itemsAvailable: Int,
-    crossinline visibleItems: LazyState.() -> List<LazyStateItem>,
-    crossinline firstVisibleItemIndex: LazyState.(List<LazyStateItem>) -> Float,
-    crossinline itemPercentVisible: LazyState.(LazyStateItem) -> Float,
-    crossinline reverseLayout: LazyState.() -> Boolean,
-): ScrollbarState {
-    var state by remember { mutableStateOf(ScrollbarState.FULL) }
-
-    LaunchedEffect(
-        key1 = this,
-        key2 = itemsAvailable,
-    ) {
-        snapshotFlow {
-            if (itemsAvailable == 0) return@snapshotFlow null
-
-            val visibleItemsInfo = visibleItems(this@scrollbarState)
-            if (visibleItemsInfo.isEmpty()) return@snapshotFlow null
-
-            val firstIndex = min(
-                a = firstVisibleItemIndex(visibleItemsInfo),
-                b = itemsAvailable.toFloat(),
-            )
-            if (firstIndex.isNaN()) return@snapshotFlow null
-
-            val itemsVisible = visibleItemsInfo.sumOf {
-                itemPercentVisible(it).toDouble()
-            }.toFloat()
-
-            val thumbTravelPercent = min(
-                a = firstIndex / itemsAvailable,
-                b = 1f,
-            )
-            val thumbSizePercent = min(
-                a = itemsVisible / itemsAvailable,
-                b = 1f,
-            )
-            ScrollbarState(
-                thumbSizePercent = thumbSizePercent,
-                thumbMovedPercent = when {
-                    reverseLayout() -> 1f - thumbTravelPercent
-                    else -> thumbTravelPercent
-                },
-            )
-        }
-            .filterNotNull()
-            .distinctUntilChanged()
-            .collect { state = it }
-    }
-    return state
-}
 
 /**
  * Linearly interpolates the index for the first item in [visibleItems] for smooth scrollbar

core/designsystem/src/main/kotlin/com/google/samples/apps/nowinandroid/core/designsystem/component/scrollbar/ScrollbarExt.kt

@@ -1,17 +1,17 @@
 /*
- * Copyright 2021 The Android Open Source Project
+ * Copyright 2023 The Android Open Source Project
  *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
+ *   Licensed under the Apache License, Version 2.0 (the "License");
+ *   you may not use this file except in compliance with the License.
+ *   You may obtain a copy of the License at
  *
- *     https://www.apache.org/licenses/LICENSE-2.0
+ *       https://www.apache.org/licenses/LICENSE-2.0
  *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ *   Unless required by applicable law or agreed to in writing, software
+ *   distributed under the License is distributed on an "AS IS" BASIS,
+ *   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *   See the License for the specific language governing permissions and
+ *   limitations under the License.
  */
 
 package com.google.samples.apps.nowinandroid.core.designsystem.component.scrollbar
@@ -24,6 +24,11 @@ import androidx.compose.foundation.lazy.grid.LazyGridState
 import androidx.compose.foundation.lazy.staggeredgrid.LazyStaggeredGridItemInfo
 import androidx.compose.foundation.lazy.staggeredgrid.LazyStaggeredGridState
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.produceState
+import androidx.compose.runtime.snapshotFlow
+import kotlinx.coroutines.flow.distinctUntilChanged
+import kotlinx.coroutines.flow.filterNotNull
+import kotlin.math.min
 
 /**
  * Calculates a [ScrollbarState] driven by the changes in a [LazyListState].
@@ -35,29 +40,58 @@ import androidx.compose.runtime.Composable
 fun LazyListState.scrollbarState(
     itemsAvailable: Int,
     itemIndex: (LazyListItemInfo) -> Int = LazyListItemInfo::index,
-): ScrollbarState =
-    scrollbarState(
-        itemsAvailable = itemsAvailable,
-        visibleItems = { layoutInfo.visibleItemsInfo },
-        firstVisibleItemIndex = { visibleItems ->
-            interpolateFirstItemIndex(
-                visibleItems = visibleItems,
+): ScrollbarState = produceState(
+    initialValue = ScrollbarState.FULL,
+    key1 = this,
+    key2 = itemsAvailable,
+) {
+    snapshotFlow {
+        if (itemsAvailable == 0) return@snapshotFlow null
+
+        val visibleItemsInfo = layoutInfo.visibleItemsInfo
+        if (visibleItemsInfo.isEmpty()) return@snapshotFlow null
+
+        val firstIndex = min(
+            a = interpolateFirstItemIndex(
+                visibleItems = visibleItemsInfo,
                 itemSize = { it.size },
                 offset = { it.offset },
-                nextItemOnMainAxis = { first -> visibleItems.find { it != first } },
+                nextItemOnMainAxis = { first -> visibleItemsInfo.find { it != first } },
                 itemIndex = itemIndex,
-            )
-        },
-        itemPercentVisible = itemPercentVisible@{ itemInfo ->
+            ),
+            b = itemsAvailable.toFloat(),
+        )
+        if (firstIndex.isNaN()) return@snapshotFlow null
+
+        val itemsVisible = visibleItemsInfo.floatSumOf { itemInfo ->
             itemVisibilityPercentage(
                 itemSize = itemInfo.size,
                 itemStartOffset = itemInfo.offset,
                 viewportStartOffset = layoutInfo.viewportStartOffset,
                 viewportEndOffset = layoutInfo.viewportEndOffset,
             )
-        },
-        reverseLayout = { layoutInfo.reverseLayout },
-    )
+        }
+
+        val thumbTravelPercent = min(
+            a = firstIndex / itemsAvailable,
+            b = 1f,
+        )
+        val thumbSizePercent = min(
+            a = itemsVisible / itemsAvailable,
+            b = 1f,
+        )
+        ScrollbarState(
+            thumbSizePercent = thumbSizePercent,
+            thumbMovedPercent = when {
+                layoutInfo.reverseLayout -> 1f - thumbTravelPercent
+                else -> thumbTravelPercent
+            },
+        )
+    }
+        .filterNotNull()
+        .distinctUntilChanged()
+        .collect { value = it }
+}.value
 
 /**
  * Calculates a [ScrollbarState] driven by the changes in a [LazyGridState]
@@ -69,41 +103,68 @@ fun LazyListState.scrollbarState(
 fun LazyGridState.scrollbarState(
     itemsAvailable: Int,
     itemIndex: (LazyGridItemInfo) -> Int = LazyGridItemInfo::index,
-): ScrollbarState =
-    scrollbarState(
-        itemsAvailable = itemsAvailable,
-        visibleItems = { layoutInfo.visibleItemsInfo },
-        firstVisibleItemIndex = { visibleItems ->
-            interpolateFirstItemIndex(
-                visibleItems = visibleItems,
-                itemSize = {
-                    layoutInfo.orientation.valueOf(it.size)
-                },
+): ScrollbarState = produceState(
+    initialValue = ScrollbarState.FULL,
+    key1 = this,
+    key2 = itemsAvailable,
+) {
+    snapshotFlow {
+        if (itemsAvailable == 0) return@snapshotFlow null
+
+        val visibleItemsInfo = layoutInfo.visibleItemsInfo
+        if (visibleItemsInfo.isEmpty()) return@snapshotFlow null
+
+        val firstIndex = min(
+            a = interpolateFirstItemIndex(
+                visibleItems = visibleItemsInfo,
+                itemSize = { layoutInfo.orientation.valueOf(it.size) },
                 offset = { layoutInfo.orientation.valueOf(it.offset) },
                 nextItemOnMainAxis = { first ->
                     when (layoutInfo.orientation) {
-                        Orientation.Vertical -> visibleItems.find {
+                        Orientation.Vertical -> visibleItemsInfo.find {
                             it != first && it.row != first.row
                         }
 
-                        Orientation.Horizontal -> visibleItems.find {
+                        Orientation.Horizontal -> visibleItemsInfo.find {
                             it != first && it.column != first.column
                         }
                     }
                 },
                 itemIndex = itemIndex,
-            )
-        },
-        itemPercentVisible = itemPercentVisible@{ itemInfo ->
+            ),
+            b = itemsAvailable.toFloat(),
+        )
+        if (firstIndex.isNaN()) return@snapshotFlow null
+
+        val itemsVisible = visibleItemsInfo.floatSumOf { itemInfo ->
             itemVisibilityPercentage(
                 itemSize = layoutInfo.orientation.valueOf(itemInfo.size),
                 itemStartOffset = layoutInfo.orientation.valueOf(itemInfo.offset),
                 viewportStartOffset = layoutInfo.viewportStartOffset,
                 viewportEndOffset = layoutInfo.viewportEndOffset,
             )
-        },
-        reverseLayout = { layoutInfo.reverseLayout },
-    )
+        }
+
+        val thumbTravelPercent = min(
+            a = firstIndex / itemsAvailable,
+            b = 1f,
+        )
+        val thumbSizePercent = min(
+            a = itemsVisible / itemsAvailable,
+            b = 1f,
+        )
+        ScrollbarState(
+            thumbSizePercent = thumbSizePercent,
+            thumbMovedPercent = when {
+                layoutInfo.reverseLayout -> 1f - thumbTravelPercent
+                else -> thumbTravelPercent
+            },
+        )
+    }
+        .filterNotNull()
+        .distinctUntilChanged()
+        .collect { value = it }
+}.value
 
 /**
  * Remembers a [ScrollbarState] driven by the changes in a [LazyStaggeredGridState]
@@ -116,28 +177,62 @@ fun LazyGridState.scrollbarState(
 fun LazyStaggeredGridState.scrollbarState(
     itemsAvailable: Int,
     itemIndex: (LazyStaggeredGridItemInfo) -> Int = LazyStaggeredGridItemInfo::index,
-): ScrollbarState =
-    scrollbarState(
-        itemsAvailable = itemsAvailable,
-        visibleItems = { layoutInfo.visibleItemsInfo },
-        firstVisibleItemIndex = { visibleItems ->
-            interpolateFirstItemIndex(
-                visibleItems = visibleItems,
+): ScrollbarState = produceState(
+    initialValue = ScrollbarState.FULL,
+    key1 = this,
+    key2 = itemsAvailable,
+) {
+    snapshotFlow {
+        if (itemsAvailable == 0) return@snapshotFlow null
+
+        val visibleItemsInfo = layoutInfo.visibleItemsInfo
+        if (visibleItemsInfo.isEmpty()) return@snapshotFlow null
+
+        val firstIndex = min(
+            a = interpolateFirstItemIndex(
+                visibleItems = visibleItemsInfo,
                 itemSize = { layoutInfo.orientation.valueOf(it.size) },
                 offset = { layoutInfo.orientation.valueOf(it.offset) },
                 nextItemOnMainAxis = { first ->
-                    visibleItems.find { it != first && it.lane == first.lane }
+                    visibleItemsInfo.find { it != first && it.lane == first.lane }
                 },
                 itemIndex = itemIndex,
-            )
-        },
-        itemPercentVisible = itemPercentVisible@{ itemInfo ->
+            ),
+            b = itemsAvailable.toFloat(),
+        )
+        if (firstIndex.isNaN()) return@snapshotFlow null
+
+        val itemsVisible = visibleItemsInfo.floatSumOf { itemInfo ->
             itemVisibilityPercentage(
                 itemSize = layoutInfo.orientation.valueOf(itemInfo.size),
                 itemStartOffset = layoutInfo.orientation.valueOf(itemInfo.offset),
                 viewportStartOffset = layoutInfo.viewportStartOffset,
                 viewportEndOffset = layoutInfo.viewportEndOffset,
             )
-        },
-        reverseLayout = { false },
-    )
+        }
+
+        val thumbTravelPercent = min(
+            a = firstIndex / itemsAvailable,
+            b = 1f,
+        )
+        val thumbSizePercent = min(
+            a = itemsVisible / itemsAvailable,
+            b = 1f,
+        )
+        ScrollbarState(
+            thumbSizePercent = thumbSizePercent,
+            thumbMovedPercent = thumbTravelPercent,
+        )
+    }
+        .filterNotNull()
+        .distinctUntilChanged()
+        .collect { value = it }
+}.value
+
+private inline fun <T> List<T>.floatSumOf(selector: (T) -> Float): Float {
+    var sum = 0f
+    for (element in this) {
+        sum += selector(element)
+    }
+    return sum
+}