From fabc0a829435ac3dd782bb7c9e55ab5f8b66f1fa Mon Sep 17 00:00:00 2001
From: inokawa <48897392+inokawa@users.noreply.github.com>
Date: Sun, 23 Jun 2024 23:18:20 +0900
Subject: [PATCH] Support dynamic startMargin

---
 src/core/store.ts         |  4 ++--
 src/react/Virtualizer.tsx |  9 ++++++---
 src/vue/Virtualizer.tsx   | 12 +++++++++---
 3 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/src/core/store.ts b/src/core/store.ts
index af183c377..a51a586f6 100644
--- a/src/core/store.ts
+++ b/src/core/store.ts
@@ -140,12 +140,12 @@ export const createVirtualStore = (
   itemSize: number = 40,
   ssrCount: number = 0,
   cacheSnapshot?: CacheSnapshot | undefined,
-  shouldAutoEstimateItemSize: boolean = false,
-  startSpacerSize: number = 0
+  shouldAutoEstimateItemSize: boolean = false
 ): VirtualStore => {
   let isSSR = !!ssrCount;
   let stateVersion: StateVersion = [];
   let viewportSize = 0;
+  let startSpacerSize = 0;
   let scrollOffset = 0;
   let jumpCount = 0;
   let jump = 0;
diff --git a/src/react/Virtualizer.tsx b/src/react/Virtualizer.tsx
index 4ff19f6d2..5a638c0d9 100644
--- a/src/react/Virtualizer.tsx
+++ b/src/react/Virtualizer.tsx
@@ -16,6 +16,7 @@ import {
   SCROLL_IDLE,
   UPDATE_SCROLL_END_EVENT,
   getScrollSize,
+  ACTION_START_OFFSET_CHANGE,
 } from "../core/store";
 import { useIsomorphicLayoutEffect } from "./useIsomorphicLayoutEffect";
 import { createScroller } from "../core/scroller";
@@ -178,7 +179,7 @@ export const Virtualizer = forwardRef<VirtualizerHandle, VirtualizerProps>(
       shift,
       horizontal: horizontalProp,
       cache,
-      startMargin,
+      startMargin = 0,
       ssrCount,
       as: Element = "div",
       item: ItemElement = "div",
@@ -207,8 +208,7 @@ export const Virtualizer = forwardRef<VirtualizerHandle, VirtualizerProps>(
         itemSize,
         ssrCount,
         cache,
-        !itemSize,
-        startMargin
+        !itemSize
       );
       return [
         _store,
@@ -222,6 +222,9 @@ export const Virtualizer = forwardRef<VirtualizerHandle, VirtualizerProps>(
     if (count !== store._getItemsLength()) {
       store._update(ACTION_ITEMS_LENGTH_CHANGE, [count, shift]);
     }
+    if (startMargin !== store._getStartSpacerSize()) {
+      store._update(ACTION_START_OFFSET_CHANGE, startMargin);
+    }
 
     const rerender = useRerender(store);
 
diff --git a/src/vue/Virtualizer.tsx b/src/vue/Virtualizer.tsx
index 280490e39..0eeb288f2 100644
--- a/src/vue/Virtualizer.tsx
+++ b/src/vue/Virtualizer.tsx
@@ -21,6 +21,7 @@ import {
   createVirtualStore,
   ACTION_ITEMS_LENGTH_CHANGE,
   getScrollSize,
+  ACTION_START_OFFSET_CHANGE,
 } from "../core/store";
 import { createResizer } from "../core/resizer";
 import { createScroller } from "../core/scroller";
@@ -93,7 +94,7 @@ const props = {
   /**
    * If you put an element before virtualizer, you have to define its height with this prop.
    */
-  startMargin: Number,
+  startMargin: { type: Number, default: 0 },
   /**
    * A prop for SSR. If set, the specified amount of items will be mounted in the initial rendering regardless of the container size until hydrated.
    */
@@ -117,8 +118,7 @@ export const Virtualizer = /*#__PURE__*/ defineComponent({
       props.itemSize ?? 40,
       props.ssrCount,
       undefined,
-      !props.itemSize,
-      props.startMargin
+      !props.itemSize
     );
     const resizer = createResizer(store, isHorizontal);
     const scroller = createScroller(store, isHorizontal);
@@ -168,6 +168,12 @@ export const Virtualizer = /*#__PURE__*/ defineComponent({
         store._update(ACTION_ITEMS_LENGTH_CHANGE, [count, props.shift]);
       }
     );
+    watch(
+      () => props.startMargin,
+      (value) => {
+        store._update(ACTION_START_OFFSET_CHANGE, value);
+      }
+    );
 
     watch(
       [rerender, store._getJumpCount],