This is strategies of dynamic item size for @angular/cdk/scrolling. This allows you set specify each item size in the array to be used for Virtual Scroll.

This is a simple coding concept:

<cdk-virtual-scroll-viewport [itemDynamicSizes]="[{ itemSize: 100 } , { itemSize: 80} , { itemSize: 90 } , { itemSize: 100}]">
  <div *cdkVirtualFor="let item of [100, 80, 90, 100]; trackBy: trackByFn" [style.height.px]="item">
    itemSize: {{ item }}
  </div>
</cdk-virtual-scroll-viewport>

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

npm install @rdlabo/ngx-cdk-scroll-strategies

This is a simple example of how to use it.

• Demo: https://rdlabo-ionic-angular-library.netlify.app/main/scroll-strategies/simple
• Source: https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/demo/src/app/scroll-strategies/pages/scroll-simple

import { CdkDynamicSizeVirtualScroll, itemDynamicSize } from '@rdlabo/ngx-cdk-scroll-strategies';

@Component({
  ...
  imports: [
    CdkDynamicSizeVirtualScroll
  ],
})
export class ScrollStrategiesPage implements OnInit {
  readonly items = signal<itemDynamicSize[]>([]);
  readonly dynamicSize = computed<itemDynamicSize[]>(() => {
    return this.items().map((item) => ({ trackId: item.trackId, itemSize: item.itemSize }));
  });
}

<cdk-virtual-scroll-viewport [itemDynamicSizes]="dynamicSize()" minBufferPx="900" maxBufferPx="1350">
  <div *cdkVirtualFor="let item of items(); trackBy: trackByFn" class="dynamic-item" [style.height.px]="item.itemSize">
    itemSize: {{ item.itemSize }}
  </div>
</cdk-virtual-scroll-viewport>

Other than this, it works the same way as @angular/cdk/scrolling.

This is a practical demo. Make scroll items separate components and get a height for each component. It is difficult without basic knowledge of Angular.

• 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

This is a demo for reverse scrolling like WeChat.

• Demo: https://rdlabo-ionic-angular-library.netlify.app/main/scroll-strategies/reverse
• Source: https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/demo/src/app/scroll-strategies/pages/scroll-reverse

If reverse scroll, add isReverse directive to cdk-virtual-scroll-viewport tag.

<cdk-virtual-scroll-viewport [itemDynamicSizes]="dynamicSize()" [isReverse]="true" minBufferPx="900" maxBufferPx="1350">
  <div class="reverse-items">
    <div *cdkVirtualFor="let item of items(); trackBy: trackByFn" class="dynamic-item" [style.height.px]="item.itemSize">
      itemSize: {{ item.itemSize }}
    </div>
  </div>
</cdk-virtual-scroll-viewport>

Add css to cdk-virtual-scroll-viewport.reverse-scroll at global css file like styles.css.

cdk-virtual-scroll-viewport {
  width: 100%;
  height: 100%;

  // .reverse-scroll class is added from this directive.
  &.reverse-scroll {
    display: flex;
    flex-direction: column-reverse;

    .cdk-virtual-scroll-content-wrapper {
      top: auto;
      bottom: 0;
    }
  }
}

And add item wrapper. div.reverse-items class is example. You can decide this.

div.reverse-items {
  height: 100%;
  display: flex;
  flex-direction: column-reverse;

  position: relative;
  bottom: 0;
}

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

This package contains a Helper Service that simplifies development with Virtual Scroll.

import { DynamicSizeVirtualScrollService } from '@rdlabo/ngx-cdk-scroll-strategies';

Detail is here: https://github.com/rdlabo-team/ionic-angular-library/blob/main/projects/scroll-strategies/src/lib/dynamic-size-virtual-scroll.service.ts

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

https://github.com/angular/components/blob/main/src/cdk-experimental/scrolling/auto-size-virtual-scroll.ts#L49C3-L59

Dynamic size can be specified for more flexible application design.