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Fix dynamic virtual scroll geometry #21

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rdlabo merged 2 commits into from
Jul 17, 2026
Merged

Fix dynamic virtual scroll geometry #21

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d542677
fix dynamic virtual scroll geometry
rdlabo Jul 16, 2026
b831f71
refactor virtual scroll utilities
rdlabo Jul 16, 2026
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11 changes: 7 additions & 4 deletions projects/scroll-strategies/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -14,8 +14,9 @@ This is a simple coding concept:

Use `[itemDynamicSizes]` directive instead of `[itemSize]` or `[autosize]` directive. `[itemDynamicSizes]` value's type is `itemDynamicSize[]`.

This library is based largely on this blog: https://dev.to/georgii/virtual-scrolling-of-content-with-variable-height-with-angular-3a52
Every data item must have one corresponding `itemDynamicSizes` entry in the same order. Each `itemSize` must be a finite number greater than zero. If Angular updates the data and size signals in separate turns, the strategy keeps the last complete geometry until their lengths match; it never estimates unknown heights.

This library is based largely on this blog: https://dev.to/georgii/virtual-scrolling-of-content-with-variable-height-with-angular-3a52

## Installation

Expand Down Expand Up @@ -67,7 +68,6 @@ Other than this, it works the same way as `@angular/cdk/scrolling`.
- Demo: https://rdlabo-ionic-angular-library.netlify.app/main/scroll-strategies/advanced
- Source: https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/demo/src/app/scroll-strategies/pages/scroll-advanced


### Reverse Usage

> This is a demo for reverse scrolling like WeChat.
Expand Down Expand Up @@ -120,8 +120,10 @@ div.reverse-items {
}
```

__In Reverse Scroll, CdkVirtualScrollViewport's measureScrollOffset does not work. Please use the scrollOffset of this directive.__
https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/scroll-strategies/src/lib/dynamic-size-virtual-scroll-strategy.ts#L383-L386
**In Reverse Scroll, CdkVirtualScrollViewport's measureScrollOffset does not work. Please use the scrollOffset of this directive.**
https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/scroll-strategies/src/lib/dynamic-size-virtual-scroll-strategy.ts

The reverse layout uses negative native `scrollTop` values. `scrollToIndex()` accepts a logical item index as usual and converts its cumulative offset to that native coordinate internally.

### Optional

Expand All @@ -134,6 +136,7 @@ import { DynamicSizeVirtualScrollService } from '@rdlabo/ngx-cdk-scroll-strategi
Detail is here: https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/scroll-strategies/src/lib/dynamic-size-virtual-scroll.service.ts

## FQA

### Why don't use `autosize` directive?

`autosize` directive use average item size. This is not support "item size is changed" "item is removed". Because don't have item size cache.
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