From 70fab8b24ede0c57ea675453bf388b060ac14d5a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?avery=20=E2=9C=BF?= <avery@discord.com>
Date: Thu, 4 Apr 2024 22:49:25 -0700
Subject: [PATCH] add topViewabilityInset

---
 src/FlashListProps.ts                   |  1 +
 src/__tests__/ViewabilityHelper.test.ts |  3 +++
 src/viewability/ViewabilityHelper.ts    | 22 ++++++++++++++--------
 src/viewability/ViewabilityManager.ts   |  3 +++
 4 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/src/FlashListProps.ts b/src/FlashListProps.ts
index 9f03ee7dc..ce8b00701 100644
--- a/src/FlashListProps.ts
+++ b/src/FlashListProps.ts
@@ -344,4 +344,5 @@ export interface FlashListProps<TItem> extends ScrollViewProps {
    * can consider the visible area of the bottom sheet in its calculations.
    */
   bottomViewabilityInsetRef?: React.MutableRefObject<number>;
+  topViewabilityInsetRef?: React.MutableRefObject<number>;
 }
diff --git a/src/__tests__/ViewabilityHelper.test.ts b/src/__tests__/ViewabilityHelper.test.ts
index 52ac04a29..64c1da0ff 100644
--- a/src/__tests__/ViewabilityHelper.test.ts
+++ b/src/__tests__/ViewabilityHelper.test.ts
@@ -257,6 +257,7 @@ describe("ViewabilityHelper", () => {
     horizontal,
     scrollOffset,
     bottomViewabilityInset,
+    topViewabilityInset,
     listSize,
     getLayout,
     runAllTimers,
@@ -265,6 +266,7 @@ describe("ViewabilityHelper", () => {
     horizontal?: boolean;
     scrollOffset?: number;
     bottomViewabilityInset?: number;
+    topViewabilityInset?: number;
     listSize?: Dimension;
     getLayout?: (index: number) => Layout | undefined;
     runAllTimers?: boolean;
@@ -273,6 +275,7 @@ describe("ViewabilityHelper", () => {
       horizontal ?? false,
       scrollOffset ?? 0,
       bottomViewabilityInset ?? 0,
+      topViewabilityInset ?? 0,
       listSize ?? { height: 300, width: 300 },
       getLayout ??
         ((index) => {
diff --git a/src/viewability/ViewabilityHelper.ts b/src/viewability/ViewabilityHelper.ts
index d432b7900..6e704da1e 100644
--- a/src/viewability/ViewabilityHelper.ts
+++ b/src/viewability/ViewabilityHelper.ts
@@ -49,6 +49,7 @@ class ViewabilityHelper {
     horizontal: boolean,
     scrollOffset: number,
     bottomViewabilityInset: number,
+    topViewabilityInset: number,
     listSize: Dimension,
     getLayout: (index: number) => Layout | undefined,
     viewableIndices?: number[]
@@ -78,6 +79,7 @@ class ViewabilityHelper {
         horizontal,
         scrollOffset,
         bottomViewabilityInset,
+        topViewabilityInset,
         listSize,
         this.viewabilityConfig?.viewAreaCoveragePercentThreshold,
         this.viewabilityConfig?.itemVisiblePercentThreshold,
@@ -127,6 +129,7 @@ class ViewabilityHelper {
     horizontal: boolean,
     scrollOffset: number,
     bottomViewabilityInset: number,
+    topViewabilityInset: number,
     listSize: Dimension,
     viewAreaCoveragePercentThreshold: number | null | undefined,
     itemVisiblePercentThreshold: number | null | undefined,
@@ -137,27 +140,30 @@ class ViewabilityHelper {
       return false;
     }
     const itemTop = (horizontal ? itemLayout.x : itemLayout.y) - scrollOffset;
-    const itemSize = horizontal ? itemLayout.width : itemLayout.height;
-    const listMainSize = horizontal
-      ? listSize.width
-      : listSize.height - bottomViewabilityInset;
-    const pixelsVisible =
-      Math.min(itemTop + itemSize, listMainSize) - Math.max(itemTop, 0);
+    const itemEnd =
+      itemTop + (horizontal ? itemLayout.width : itemLayout.height);
+    const listStart = topViewabilityInset;
+    const listEnd =
+      (horizontal ? listSize.width : listSize.height) - bottomViewabilityInset;
+    const visibleStart = Math.max(itemTop, listStart);
+    const visibleEnd = Math.min(itemEnd, listEnd);
+    const pixelsVisible = Math.max(0, visibleEnd - visibleStart);
 
     // Always consider item fully viewable if it is fully visible, regardless of the `viewAreaCoveragePercentThreshold`
     // Account for floating point imprecision.
+    const itemSize = horizontal ? itemLayout.width : itemLayout.height;
     if (Math.abs(pixelsVisible - itemSize) <= 0.001) {
       return true;
     }
     // Skip checking item if it's not visible at all
-    if (pixelsVisible === 0) {
+    if (pixelsVisible <= 0) {
       return false;
     }
     const viewAreaMode =
       viewAreaCoveragePercentThreshold !== null &&
       viewAreaCoveragePercentThreshold !== undefined;
     const percent = viewAreaMode
-      ? pixelsVisible / listMainSize
+      ? pixelsVisible / (listEnd - listStart)
       : pixelsVisible / itemSize;
     const viewableAreaPercentThreshold = viewAreaMode
       ? viewAreaCoveragePercentThreshold * 0.01
diff --git a/src/viewability/ViewabilityManager.ts b/src/viewability/ViewabilityManager.ts
index cc23bfe9b..b03a0e9b0 100644
--- a/src/viewability/ViewabilityManager.ts
+++ b/src/viewability/ViewabilityManager.ts
@@ -79,12 +79,15 @@ export default class ViewabilityManager<T> {
 
     const bottomViewabilityInset =
       this.flashListRef.props.bottomViewabilityInsetRef?.current ?? 0;
+    const topViewabilityInset =
+      this.flashListRef.props.topViewabilityInsetRef?.current ?? 0;
 
     this.viewabilityHelpers.forEach((viewabilityHelper) => {
       viewabilityHelper.updateViewableItems(
         this.flashListRef.props.horizontal ?? false,
         scrollOffset,
         bottomViewabilityInset,
+        topViewabilityInset,
         listSize,
         (index: number) =>
           this.flashListRef.recyclerlistview_unsafe?.getLayout(index),