ng-vs-for

This is a port of https://github.com/kamilkp/angular-vs-repeat for Angular 4+

The name stands for Virtual Scroll For. It manipulates the collection you want to ngFor over in a way that only elements that are actually visible for the user are rendered in the DOM. So if you repeat over a thousand items only a few of them are rendered in the DOM, because the user can't see the rest anyway. And it takes time for the browser to render so many elements, which also might have some event listeners/bindings etc. So you should see a considerable boost in performance.

Installation

npm install ng-vs-for

Examples

Basic usage: all items shall have the same height

<div *vsFor="items; let _items = vsCollection">
    <div *ngFor="let item of _items">
        <!-- item html here -->
    </div>
</div>

Items have various sizes but they are known up front (calculatable based on their properties)

import { Component } from '@angular/core';
 
@Component({
  selector: 'some-component',
  template: `
        <div *vsFor="items; size:getSize; let _items = vsCollection">
            <div *ngFor="let item of _items">
                <!-- item html here -->
            </div>
        </div>
 
    `,
  inputs: ['items']
})
export class SomeComponent {
  items: any;
  getSize(item, index) {
    let size;
    // ... do some calculations here
    return size;
  }
}

The getSize could either be a number (or string castable to number) or a function on your component. If it's a function it will be called for each item in the original collection with two arguments: item (the item in the collection), and index (the index in the original collection). This function shall return a number - the height in pixels of the item.

Local variables

The vsFor directive is a structural directive and it exposes two local variables:

• vsCollection - the sliced collection that should be assigned to a local variable and be used in ngFor
• vsStartIndex - the index of the first element that is actually rendered (see last example at the bottom of the readme)

Parameters

Other parameters that you can pass to the vsFor directive:

• offsetBefore (defaults to 0)
• offsetAfter (defaults to 0)
• excess (defaults to 2)
• autoresize (set to true recalculates on window resize)
• horizontal (hooks to scrolling horizontally and the optional size parameter calculates widths instead of heights)
• tagName (defaults to div) - should be the same type as the tag name of the element you put the ngFor directive on
• scrollParent (defaults to direct parent element) - a selector of the closest element that is the scrollable container for the repeated items. You can set window as a scroll parent in case the main window scrollbar should be used.

Example with some more parameters:

<table>
    <tbody *vsFor="items; size:getSize; tagName:'tr'; autoresize:true; scrollParent:'window'; excess:3; #_items = vsCollection; #_startIndex = vsStartIndex">
        <tr *ngFor="#item of _items; #i = index">
            {{ i + _startIndex }} <!-- the actual index in the original collection  -->
        </tr>
    </tbody>
</table>