Extract an interface for LoadingStrategy

CHANGELOG.md

@@ -5,6 +5,7 @@
 
 New:
 - Source-based schema parser is now the default. Can be disabled in your schema module with `redwood { useFir = false }`.
+- Introduce a `LoadingStrategy` interface to manage `LazyList` preloading.
 
 Changed:
 - Nothing yet!

redwood-lazylayout-compose/api/redwood-lazylayout-compose.api

@@ -24,28 +24,41 @@ public abstract interface class app/cash/redwood/lazylayout/compose/LazyListScop
 public class app/cash/redwood/lazylayout/compose/LazyListState {
 	public static final field $stable I
 	public fun <init> ()V
-	public final fun getDefaultPreloadItemCount ()I
-	public final fun getFirstIndex ()I
-	public final fun getLastIndex ()I
-	public final fun getPrimaryPreloadItemCount ()I
+	public fun <init> (Lapp/cash/redwood/lazylayout/compose/LoadingStrategy;)V
+	public synthetic fun <init> (Lapp/cash/redwood/lazylayout/compose/LoadingStrategy;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
 	public final fun getProgrammaticScrollIndex ()Lapp/cash/redwood/lazylayout/api/ScrollItemIndex;
-	public final fun getScrollInProgressPreloadItemCount ()I
-	public final fun getSecondaryPreloadItemCount ()I
+	public final fun getStrategy ()Lapp/cash/redwood/lazylayout/compose/LoadingStrategy;
 	public final fun loadRange (I)Lkotlin/ranges/IntRange;
 	public final fun onUserScroll (II)V
 	public final fun programmaticScroll (IZZ)V
 	public static synthetic fun programmaticScroll$default (Lapp/cash/redwood/lazylayout/compose/LazyListState;IZZILjava/lang/Object;)V
-	public final fun setDefaultPreloadItemCount (I)V
-	public final fun setPrimaryPreloadItemCount (I)V
-	public final fun setScrollInProgressPreloadItemCount (I)V
-	public final fun setSecondaryPreloadItemCount (I)V
 }
 
 public final class app/cash/redwood/lazylayout/compose/LazyListStateKt {
-	public static final fun rememberLazyListState (Landroidx/compose/runtime/Composer;I)Lapp/cash/redwood/lazylayout/compose/LazyListState;
+	public static final fun rememberLazyListState (Lapp/cash/redwood/lazylayout/compose/LoadingStrategy;Landroidx/compose/runtime/Composer;II)Lapp/cash/redwood/lazylayout/compose/LazyListState;
+}
+
+public abstract interface class app/cash/redwood/lazylayout/compose/LoadingStrategy {
+	public abstract fun getFirstIndex ()I
+	public abstract fun loadRange (I)Lkotlin/ranges/IntRange;
+	public abstract fun onUserScroll (II)V
+	public abstract fun scrollTo (I)V
 }
 
 public final class app/cash/redwood/lazylayout/compose/RefreshableLazyListKt {
 	public static final fun RefreshableLazyList-usczW4I (ZLkotlin/jvm/functions/Function2;IIZLkotlin/jvm/functions/Function0;IILapp/cash/redwood/ui/Margin;ILapp/cash/redwood/lazylayout/api/ScrollItemIndex;ILapp/cash/redwood/Modifier;Lkotlin/jvm/functions/Function2;Lkotlin/jvm/functions/Function2;Landroidx/compose/runtime/Composer;III)V
 }
 
+public final class app/cash/redwood/lazylayout/compose/ScrollOptimizedLoadingStrategy : app/cash/redwood/lazylayout/compose/LoadingStrategy {
+	public static final field $stable I
+	public fun <init> ()V
+	public fun <init> (IIIIZ)V
+	public synthetic fun <init> (IIIIZILkotlin/jvm/internal/DefaultConstructorMarker;)V
+	public fun getFirstIndex ()I
+	public final fun getLastIndex ()I
+	public final fun getProgrammaticScrollIndex ()Lapp/cash/redwood/lazylayout/api/ScrollItemIndex;
+	public fun loadRange (I)Lkotlin/ranges/IntRange;
+	public fun onUserScroll (II)V
+	public fun scrollTo (I)V
+}
+

redwood-lazylayout-compose/src/commonMain/kotlin/app/cash/redwood/lazylayout/compose/LazyListState.kt

@@ -17,33 +17,27 @@ package app.cash.redwood.lazylayout.compose
 
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
-import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.saveable.Saver
 import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import app.cash.redwood.lazylayout.api.ScrollItemIndex
 
-private const val DEFAULT_PRELOAD_ITEM_COUNT = 15
-private const val SCROLL_IN_PROGRESS_PRELOAD_ITEM_COUNT = 5
-private const val PRIMARY_PRELOAD_ITEM_COUNT = 20
-private const val SECONDARY_PRELOAD_ITEM_COUNT = 10
-
-private const val DEFAULT_SCROLL_INDEX = -1
-
 /**
  * Creates a [LazyListState] that is remembered across compositions.
  */
 @Composable
-public fun rememberLazyListState(): LazyListState {
+public fun rememberLazyListState(
+  strategy: LoadingStrategy = ScrollOptimizedLoadingStrategy(),
+): LazyListState {
   return rememberSaveable(saver = saver) {
-    LazyListState()
+    LazyListState(strategy)
   }
 }
 
 /** The default [Saver] implementation for [LazyListState]. */
 private val saver: Saver<LazyListState, *> = Saver(
-  save = { it.firstIndex },
+  save = { it.strategy.firstIndex },
   restore = {
     LazyListState().apply {
       programmaticScroll(firstIndex = it, animated = false, clobberUserScroll = false)
@@ -56,7 +50,9 @@ private val saver: Saver<LazyListState, *> = Saver(
  *
  * In most cases, this will be created via [rememberLazyListState].
  */
-public open class LazyListState {
+public open class LazyListState(
+  public val strategy: LoadingStrategy = ScrollOptimizedLoadingStrategy(),
+) {
   /**
    * Update this to trigger a programmatic scroll. This may be updated multiple times, including
    * when the previous scroll state is restored.
@@ -69,26 +65,6 @@ public open class LazyListState {
   /** Once we receive a user scroll, we limit which programmatic scrolls we apply. */
   private var userScrolled = false
 
-  /** Bounds of what the user is looking at. Everything else is placeholders! */
-  public var firstIndex: Int by mutableIntStateOf(0)
-    private set
-  public var lastIndex: Int by mutableIntStateOf(0)
-    private set
-
-  internal var preloadItems: Boolean = true
-
-  public var defaultPreloadItemCount: Int = DEFAULT_PRELOAD_ITEM_COUNT
-  public var scrollInProgressPreloadItemCount: Int = SCROLL_IN_PROGRESS_PRELOAD_ITEM_COUNT
-  public var primaryPreloadItemCount: Int = PRIMARY_PRELOAD_ITEM_COUNT
-  public var secondaryPreloadItemCount: Int = SECONDARY_PRELOAD_ITEM_COUNT
-
-  private var firstIndexFromPrevious1: Int by mutableIntStateOf(DEFAULT_SCROLL_INDEX)
-  private var firstIndexFromPrevious2: Int by mutableIntStateOf(DEFAULT_SCROLL_INDEX)
-  private var lastIndexFromPrevious1: Int by mutableIntStateOf(DEFAULT_SCROLL_INDEX)
-
-  private var beginFromPrevious1: Int by mutableIntStateOf(DEFAULT_SCROLL_INDEX)
-  private var endFromPrevious1: Int by mutableStateOf(DEFAULT_SCROLL_INDEX)
-
   /** Perform a programmatic scroll. */
   public fun programmaticScroll(
     firstIndex: Int,
@@ -98,16 +74,14 @@ public open class LazyListState {
     require(firstIndex >= 0)
     if (!clobberUserScroll && userScrolled) return
 
+    strategy.scrollTo(firstIndex)
+
     val previous = programmaticScrollIndex
     this.programmaticScrollIndex = ScrollItemIndex(
       id = previous.id + 1,
       index = firstIndex,
       animated = animated,
     )
-
-    val delta = (lastIndex - this.firstIndex)
-    this.firstIndex = firstIndex
-    this.lastIndex = firstIndex + delta
   }
 
   /** React to a user-initiated scroll. */
@@ -116,85 +90,10 @@ public open class LazyListState {
       userScrolled = true
     }
 
-    this.firstIndex = firstIndex
-    this.lastIndex = lastIndex
+    strategy.onUserScroll(firstIndex, lastIndex)
   }
 
   public fun loadRange(itemCount: Int): IntRange {
-    // Ensure that the range includes `firstIndex` through `lastIndex`.
-    var begin = firstIndex
-    var end = lastIndex
-
-    val isScrollingDown = firstIndexFromPrevious1 != DEFAULT_SCROLL_INDEX && firstIndexFromPrevious1 < firstIndex
-    val isScrollingUp = firstIndexFromPrevious1 != DEFAULT_SCROLL_INDEX && firstIndexFromPrevious1 > firstIndex
-    val hasStoppedScrolling = firstIndexFromPrevious2 != DEFAULT_SCROLL_INDEX && firstIndex == firstIndexFromPrevious1
-    val wasScrollingDown = firstIndexFromPrevious1 > firstIndexFromPrevious2
-    val wasScrollingUp = firstIndexFromPrevious1 < firstIndexFromPrevious2
-
-    // Expand the range depending on scroll direction.
-    when {
-      // Ignore preloads.
-      !preloadItems -> {
-        // No-op
-      }
-
-      isScrollingDown -> {
-        begin -= scrollInProgressPreloadItemCount
-        end += primaryPreloadItemCount
-      }
-
-      isScrollingUp -> {
-        begin -= primaryPreloadItemCount
-        end += scrollInProgressPreloadItemCount
-      }
-
-      hasStoppedScrolling && wasScrollingDown -> {
-        begin -= secondaryPreloadItemCount
-        end += primaryPreloadItemCount
-      }
-
-      hasStoppedScrolling && wasScrollingUp -> {
-        begin -= primaryPreloadItemCount
-        end += secondaryPreloadItemCount
-      }
-
-      // New.
-      else -> {
-        end += defaultPreloadItemCount
-      }
-    }
-
-    // On initial load, set lastIndex to the end of the loaded window.
-    if (lastIndex == 0) {
-      lastIndex = end
-    }
-
-    // If we're contiguous with the previous visible window,
-    // don't rush to remove things from the previous range.
-    if (beginFromPrevious1 != DEFAULT_SCROLL_INDEX &&
-      endFromPrevious1 != DEFAULT_SCROLL_INDEX
-    ) {
-      // Case one: Contiguous scroll down
-      if (begin in firstIndexFromPrevious1..lastIndexFromPrevious1) {
-        begin = beginFromPrevious1
-      }
-
-      // Case two: Contiguous scroll up
-      if (end in firstIndexFromPrevious1..lastIndexFromPrevious1) {
-        end = endFromPrevious1
-      }
-    }
-
-    begin = begin.coerceIn(0, itemCount)
-    end = end.coerceIn(0, itemCount)
-
-    this.firstIndexFromPrevious2 = firstIndexFromPrevious1
-    this.firstIndexFromPrevious1 = firstIndex
-    this.lastIndexFromPrevious1 = lastIndex
-
-    this.beginFromPrevious1 = begin
-    this.endFromPrevious1 = end
-
-    return begin until end
+    return strategy.loadRange(itemCount)
   }
 }

redwood-lazylayout-compose/src/commonMain/kotlin/app/cash/redwood/lazylayout/compose/LoadingStrategy.kt

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2024 Square, Inc.
+ *
+ * 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
+ *
+ *      http://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.
+ */
+package app.cash.redwood.lazylayout.compose
+
+public interface LoadingStrategy {
+  /**
+   * Returns the most recent first index scrolled to. This is used to save the scroll position when
+   * the view is unloaded.
+   */
+  public val firstIndex: Int
+
+  /** Perform a programmatic scroll to [firstIndex]. */
+  public fun scrollTo(firstIndex: Int)
+
+  /** React to a user-initiated scroll to the target range. */
+  public fun onUserScroll(firstIndex: Int, lastIndex: Int)
+
+  /**
+   * Returns the range of items to render into the view tree. This should be a slice of
+   * `0..(itemCount - 1)`. It should cover the most-recently scrolled to `firstIndex..lastIndex`
+   * range, plus any adjacent indexes to preload.
+   */
+  public fun loadRange(itemCount: Int): IntRange
+}