Nativescript Show element while scrolling up in ListView - parallax effect

Question

I would like to get an advice on how to go around implementing a feature seen in many apps like Whatsapp or Facebook, where you have a list and a header which is not visible always, but is gradually shown when the user begins to scroll upwards from any place within a list.

In whatsapp and facebook the upward scrolling gesture causes the Search bar to slowly appear at the top of the screen, while the list itself is not scrolling until the search bar appears at full (at least this is the android implementation).

I need an advice on how to implement this using Nativescript angular with Telerik RadListView (android + ios). As far as I know, putting a ListView inside a ScrollView generally is not advised by telerik.

Thanks!

Edit: I learned it is called a parallax effect, and found examples of it in native android, however, not in nativescript with ListView (did find an example with ScrollView and regular StackLayout, not with a ListView inside).

Answer 1

Here is an example of a scrollable parallax effect RadListView implemented using Angular (no ScrollView needed). It also provides an example of sticking the list header at the top.

Please me know if it works for you.

Component template:

<GridLayout class="page">
<RadListView (scrolled)="onScroll($event)" [items]="dataItems" itemReorder="true"
    (itemReordered)="onItemReordered($event)">
  <ListViewGridLayout tkListViewLayout scrollDirection="Vertical" spanCount="1" ios:itemHeight="150"
      dynamicItemSize="false"></ListViewGridLayout>

  <ng-template tkListItemTemplate let-item="item">
    <StackLayout orientation="vertical">
      <!-- list item content goes here -->
    </StackLayout>
  </ng-template>

  <ng-template tkListViewHeader>
    <StackLayout>
      <GridLayout #fixedHeaderContainer class="fixed-header-container">
        <label text="Fixed Content" verticalAlignment="center"></label>
      </GridLayout>

      <StackLayout class="list-header-container">
        <StackLayout #listHeaderContainer>
          <label text="List Title"></label>
        </StackLayout>
      </StackLayout>
    </StackLayout>
  </ng-template>
</RadListView>

<GridLayout verticalAlignment="top" [height]="dockContainerHeight" [opacity]="dockContainerOpacity">
  <FlexboxLayout justifyContent="flex-start" alignItems="center" class="docked-label-wrapper">
    <button class="fas" text="&#xf053;"></button>

    <StackLayout flexGrow="1" height="100%" [opacity]="dockContentOpacity" orientation="horizontal">
      <label text="List Title"></label>
    </StackLayout>
  </FlexboxLayout>
</GridLayout>

Component scss:

.fixed-header-container {
  height: 200;
  padding: 0 16;
  background-color: green;

  label {
    font-size: 30;
    font-weight: 700;
    color: $white;
  }
}

.list-header-container {
  margin-top: -12;
  border-radius: 12 12 0 0;
  background-color: #ffffff;

  label {
    margin: 16 0;
    font-size: 22;
    color: black;
    vertical-align: center;
  }

  .smaller-label {
    font-size: 12;
    color: #909090;
  }
}

RadListView {
  height: 100%;
  background-color: #ffffff;
}

.docked-label-wrapper {
  margin: 0 0 10;
  background-color: #ffffff;

  .fas {
    margin: 0;
    font-size: 18;
  }

  label {
    font-size: 18;
    color: black;
    vertical-align: center;
  }
}

Component ts:

  import { Component, ElementRef, OnInit, ViewChild } from '@angular/core';
  import { ListViewEventData, ListViewScrollEventData } from 'nativescript-ui-listview';
  import { DataItem, DataItemService } from './data-items.service';

  export const DOCK_HEADER_HEIGHT = 58;

  @Component({
    moduleId: module.id,
    selector: 'app-comp-name',
    templateUrl: './comp-name.component.html',
    styleUrls: ['./comp-name.component.scss']
  })
  export class CompNameComponent implements OnInit {
    dataItems: DataItem[];
    dockContainerHeight = DOCK_HEADER_HEIGHT;
    dockContainerOpacity = 0;
    dockContentOpacity = 0;

    @ViewChild('fixedHeaderContainer', { static: false })
    fixedHeaderContainerRef: ElementRef;

    @ViewChild('listHeaderContainer')
    listHeaderContainerRef: ElementRef;

    constructor(private _dataItemService: DataItemService) {}

    ngOnInit(): void {
        this.dataItems = this._dataItemService.getDataItems();
    }

    onItemReordered(args: ListViewEventData) {
        console.log('Item reordered. Old index: ' + args.index + ' ' + 'new index: ' + args.data.targetIndex);
    }

    onScroll(args: ListViewScrollEventData) {
        if (!this.fixedHeaderContainerRef) {
        return;
        }

        const offset = args.scrollOffset < 0 ? 0 : args.scrollOffset;
        const fixedHeaderHeight = this.fixedHeaderContainerRef.nativeElement.getActualSize().height;

        this.applyFixedHeaderTransition(offset);
        this.applyTitleTransition(offset, fixedHeaderHeight);
        this.applyDockHeaderTransition(offset, fixedHeaderHeight);
    }

    private applyFixedHeaderTransition(scrollOffset: number) {
        this.fixedHeaderContainerRef.nativeElement.translateY = scrollOffset;
    }

    private applyTitleTransition(scrollOffset: number, fixedHeaderHeight: number) {
        const maxHeightChange = fixedHeaderHeight - DOCK_HEADER_HEIGHT;
        const titleElement = this.listHeaderContainerRef.nativeElement;

        if (maxHeightChange < scrollOffset) {
        titleElement.translateX = -(scrollOffset - maxHeightChange) / 1.2;
        titleElement.translateY = -(scrollOffset - maxHeightChange) * 2;
        titleElement.scaleX = 1 - (scrollOffset - maxHeightChange) / fixedHeaderHeight;
        titleElement.scaleY = 1 - (scrollOffset - maxHeightChange) / fixedHeaderHeight;
        } else {
        titleElement.translateX = 0;
        titleElement.translateY = 0;
        titleElement.scaleX = 1;
        titleElement.scaleY = 1;
        }
    }

    private applyDockHeaderTransition(scrollOffset: number, fixedHeaderHeight: number) {
        const maxHeightChange = fixedHeaderHeight - DOCK_HEADER_HEIGHT;
        const containerOpacity = 1 - scrollOffset / maxHeightChange;

        this.dockContainerOpacity = containerOpacity <= 0 ? 1 : 0;
        this.dockContentOpacity = (scrollOffset - (fixedHeaderHeight - DOCK_HEADER_HEIGHT)) / DOCK_HEADER_HEIGHT - 0.2;
    }
  }

Answer 2

You can check the available "Implementing a Parallax Scroll Effect" sample in the 'Samples' section of the official NativeScript marketplace website that shows how to implement just that effect. Just go to Market.nativescript.org and search for 'parallax'. Also there is a plugin that provides such functionality but I am not sure of its quality.