aurelia-ui-virtualization

This library is part of the Aurelia platform and contains a plugin that provides a virtualized repeater and other virtualization services. This plugin enables "virtualization" of list through a new virtual-repeat.for. When used, the list "virtually" as tens or hundreds of thousands of rows, but the DOM only actually has rows for what is visible. It could be only tens of items. This allows rendering of massive lists of data with amazing performance. It works like repeat.for, it just creates a scrolling area and manages the list using UI virtualization techniques.

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Installation

Install via npm

npm install aurelia-ui-virtualization

Load the plugin

export function configure(aurelia) {
  aurelia.use
    .standardConfiguration()
    .developmentLogging()
    .plugin('aurelia-ui-virtualization'); // Add this line to load the plugin

  aurelia.start().then(a => a.setRoot());
}

Use the plugin

Simply bind an array to virtual-repeat like you would with the standard repeat. The repeated rows need to have equal height throughout the list, and one items per row.

div

<template>
  <div virtual-repeat.for="item of items">
    ${$index} ${item}
  </div>
</template>

unordered list

<template>
  <ul>
    <li virtual-repeat.for="item of items">
    ${$index} ${item}
    </li>
  </ul>
</template>

table

<template>
  <table>
    <tr virtual-repeat.for="item of items">
      <td>${$index}</td>
      <td>${item}</td>
    </tr>
  </table>
</template>

export class MyVirtualList {
    items = ['Foo', 'Bar', 'Baz'];
}

With a surrounding fixed height container with overflow scroll. Note that overflow: scroll styling is inlined on the elemenet. It can also be applied from CSS.

<template>
  <div style="overflow: scroll; height: 90vh">
    <div virtual-repeat.for="item of items">
      ${$index} ${item}
    </div>
  </div>
</template>

If you are running the plugin in the skeleton-naviagion project, make sure to remove overflow-x: hidden; and overflow-y: auto; from .page-host in styles.css.

infinite scroll

<template>
  <div virtual-repeat.for="item of items" infinite-scroll-next="getMore">
    ${$index} ${item}
  </div>
</template>

export class MyVirtualList {
  items = ['Foo', 'Bar', 'Baz'];
  getMore(topIndex, isAtBottom, isAtTop) {
    for(let i = 0; i < 100; ++i) {
      this.items.push('item' + i);
    }
  }
}

Or to use an expression, use .call as shown below.

<template>
  <div virtual-repeat.for="item of items" infinite-scroll-next.call="getMore($scrollContext)">
    ${$index} ${item}
  </div>
</template>

export class MyVirtualList {
  items = ['Foo', 'Bar', 'Baz'];
  getMore(scrollContext) {
    for(let i = 0; i < 100; ++i) {
      this.items.push('item' + i);
    }
  }
}

The infinite-scroll-next attribute can accept a function, a promise, or a function that returns a promise. The bound function will be called when the scroll container has reached a point where there are no more items to move into the DOM (i.e. when it reaches the end of a list, either from the top or the bottom).

There are three parameters that are passed to the function (getMore(topIndex, isAtBottom, isAtTop)) which helps determine the behavior or amount of items to get during scrolling.

1. topIndex - A integer value that represents the current item that exists at the top of the rendered items in the DOM.
2. isAtBottom - A boolean value that indicates whether the list has been scrolled to the bottom of the items list.
3. isAtTop - A boolean value that indicates whether the list has been scrolled to the top of the items list.

Caveats

1. <template/> is not supported as root element of a virtual repeat template. This is due to the requirement of aurelia ui virtualization technique: item height needs to be calculatable. With <tempate/>, there is no easy and performant way to acquire this value.
2. Similar to (1), other template controllers cannot be used in conjunction with virtual-repeat, unlike repeat. I.e: built-in template controllers: with, if, replaceable cannot be used with virtual-repeat. This can be workaround'd by nesting other template controllers inside the repeated element, with <template/> element, for example:

<template>
  <h1>${message}</h1>
  <div virtual-repeat.for="person of persons">
    <template with.bind="person">
      ${Name}
    </template>
  </div>
</template>

3. Beware of CSS selector :nth-child and similar selectors. Virtualization requires appropriate removing and inserting visible items, based on scroll position. This means DOM elements order will not stay the same, thus creating unexpected :nth-child CSS selector behavior. To work around this, you can use contextual properties $index, $odd, $even etc... to determine an item position, and apply CSS classes/styles against it, like the following example:

<template>
  <div virtual-repeat.for="person of persons" class="${$odd ? 'odd' : 'even'}-row">
    ${person.name}
  </div>
</template>

4. Similar to (3), virtualization requires appropriate removing and inserting visible items, so not all views will have their lifecycle invoked repeatedly. Rather, their binding contexts will be updated accordingly when the virtual repeat reuses the view and view model. To work around this, you can have your components work in a reactive way, which is natural in an Aurelia application. An example is to handle changes in change handler callback.

Demo

Online Playground

Platform Support

This library can be used in the browser only.

Building The Code

To build the code, follow these steps.

1. Ensure that NodeJS is installed. This provides the platform on which the build tooling runs.
2. From the project folder, execute the following command:

npm install

3. To build the code, you can now run:

npm run build

4. You will find the compiled code in the dist folder, available in module formats: es2015, es2017, AMD, CommonJS and UMD.

Running The Tests

To run the unit tests, first ensure that you have followed the steps above in order to install all dependencies and successfully build the library. Once you have done that, proceed with these additional steps:

1. Run the tests with this command:

npm run test