From 0218c71e8ce556e004dac04576369eb404a03adc Mon Sep 17 00:00:00 2001
From: friedolin <friedolin@steuerbot.com>
Date: Wed, 24 Aug 2022 17:25:44 +0200
Subject: [PATCH] feat: support for min index

---
 .../scroll/ScrollView.java                    |  5 ++--
 src/FlatList.tsx                              |  3 +-
 src/config.ts                                 |  1 +
 src/hooks/usePrerenderedData.tsx              | 29 +++++++++++--------
 4 files changed, 23 insertions(+), 15 deletions(-)
 create mode 100644 src/config.ts

diff --git a/android/src/main/java/com/reactnativebidirectionalflatlist/scroll/ScrollView.java b/android/src/main/java/com/reactnativebidirectionalflatlist/scroll/ScrollView.java
index 3b9dc73..ed793ed 100644
--- a/android/src/main/java/com/reactnativebidirectionalflatlist/scroll/ScrollView.java
+++ b/android/src/main/java/com/reactnativebidirectionalflatlist/scroll/ScrollView.java
@@ -38,10 +38,11 @@ public void setShiftOffset(double shiftOffset) {
   @Override
   public void onLayoutChange(View v, int left, int top, int right, int bottom, int oldLeft, int oldTop, int oldRight, int oldBottom) {
     super.onLayoutChange(v, left, top, right, bottom, oldLeft, oldTop, oldRight, oldBottom);
+    int shiftHeight = Math.min(bottom - oldBottom, (int)mShiftHeight);
     int scrollWindowHeight = getHeight() - getPaddingBottom() - getPaddingTop();
-    if(mShiftHeight != 0 && mShiftOffset <= getScrollY() + scrollWindowHeight / 2) {
+    if(mShiftHeight != 0 && shiftHeight > 0 && mShiftOffset <= getScrollY() + scrollWindowHeight / 2) {
       // correct
-      scrollTo(0, getScrollY() + (int)mShiftHeight);
+      scrollTo(0, getScrollY() + shiftHeight);
       if(getOverScrollerFromParent() != null && !getOverScrollerFromParent().isFinished()) {
         // get current directed velocity from scroller
         int direction = getOverScrollerFromParent().getFinalY() - getOverScrollerFromParent().getStartY() > 0 ? 1 : -1;
diff --git a/src/FlatList.tsx b/src/FlatList.tsx
index f5cd894..d8b8480 100644
--- a/src/FlatList.tsx
+++ b/src/FlatList.tsx
@@ -3,8 +3,9 @@ import { FlatList as FlatListRN, LayoutChangeEvent, Platform, ScrollViewProps, S
 import { ScrollView } from './ScrollView';
 import { usePrerenderedData } from './hooks/usePrerenderedData';
 import type { BidirectionalFlatListProps, FlatListType } from './types';
+import { MIN_INDEX } from './config';
 
-const maintainVisibleContentPosition = { minIndexForVisible: 1 };
+const maintainVisibleContentPosition = { minIndexForVisible: MIN_INDEX };
 
 const FlatListImpl = forwardRef<FlatListType, BidirectionalFlatListProps>((props, ref) => {
   const renderScrollComponent = useCallback((props: ScrollViewProps) => {
diff --git a/src/config.ts b/src/config.ts
new file mode 100644
index 0000000..f2da17e
--- /dev/null
+++ b/src/config.ts
@@ -0,0 +1 @@
+export const MIN_INDEX = 1;
diff --git a/src/hooks/usePrerenderedData.tsx b/src/hooks/usePrerenderedData.tsx
index cbe28b8..2c54fcc 100644
--- a/src/hooks/usePrerenderedData.tsx
+++ b/src/hooks/usePrerenderedData.tsx
@@ -1,6 +1,7 @@
 import React, { MutableRefObject, ReactNode, useCallback, useEffect, useRef, useState } from 'react';
 import { FlatListProps, LayoutChangeEvent, View } from 'react-native';
 import type { FlatListType, OnUpdateData, RenderItem } from '../types';
+import { MIN_INDEX } from '../config';
 
 type OnLayout = (options: {id: string; height: number}) => void;
 
@@ -41,13 +42,15 @@ export const usePrerenderedData = ({data, keyExtractor, renderItem, scrollRef, o
         return p;
       }, {});
       const index = data.findIndex((d) => d === newD[0]);
-      const shift = {
-        height: newD.reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
-        offset: data.slice(0, index).reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
-      };
-      scrollRef.current?.shift(shift);
+      if(index > 0 && index <= MIN_INDEX) {
+        const shift = {
+          height: newD.reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
+          offset: data.slice(0, index).reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
+        };
+        scrollRef.current?.shift(shift);
+        onUpdateData?.({heights: heightsRef.current, ...shift});
+      }
       setFinalData(data);
-      onUpdateData?.({heights: heightsRef.current, ...shift});
       return;
     }
     setNewData(data.filter((d, i) => !heightsRef.current[keyExtractor(d, i)]));
@@ -67,14 +70,16 @@ export const usePrerenderedData = ({data, keyExtractor, renderItem, scrollRef, o
       return;
     }
     const index = data.findIndex((d) => d === newData[0]);
-    const shift = {
-      height: newData.reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
-      offset: data.slice(0, index).reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
-    };
-    scrollRef.current?.shift(shift);
+    if(index > 0 && index <= MIN_INDEX) {
+      const shift = {
+        height: newData.reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
+        offset: data.slice(0, index).reduce((p, c, i) => p + heightsRef.current[keyExtractor(c, i)], 0),
+      };
+      scrollRef.current?.shift(shift);
+      onUpdateData?.({heights: heightsRef.current, ...shift});
+    }
     setNewData([]);
     setFinalData(data);
-    onUpdateData?.({heights: heightsRef.current, ...shift});
   }, [data, keyExtractor, newData, onUpdateData, scrollRef]);
 
   return {