JarCasting
<iron-list>
iron-list displays a virtual, 'infinite' list. The template inside the iron-list element represents the DOM to create for each list item. The items property specifies an array of list item data.
For performance reasons, not every item in the list is rendered at once; instead a small subset of actual template elements (enough to fill the viewport) are rendered and reused as the user scrolls. As such, it is important that all state of the list template be bound to the model driving it, since the view may be reused with a new model at any time. Particularly, any state that may change as the result of a user interaction with the list item must be bound to the model to avoid view state inconsistency.
Sizing iron-list
iron-list must either be explicitly sized, or delegate scrolling to an explicitly sized parent. By "explicitly sized", we mean it either has an explicit CSS height property set via a class or inline style, or else is sized by other layout means (e.g. the flex or fit classes).
Flexbox - jsbin
<style>
:host {
height: 100vh;
display: flex;
flex-direction: column;
}
iron-list {
flex: 1;
}
</style>
<app-toolbar>App name</app-toolbar>
<iron-list items="[[items]]">
<template>
<div>
...
</div>
</template>
</iron-list>
Explicit size - jsbin
<style>
:host {
display: block;
}
iron-list {
height: 100vh; /* don't use % values unless the parent element is sized. */
}
</style>
<iron-list items="[[items]]">
<template>
<div>
...
</div>
</template>
</iron-list>
Main document scrolling - jsbin
<head>
<style>
body {
margin: 0;
}
</style>
</head>
<body>
<iron-list scroll-target="document">
<template>
<div>
...
</div>
</template>
</iron-list>
</body>
iron-list must be given a <template> which contains exactly one element. In the examples above we used a <div>, but you can provide any element (including custom elements).
Template model
List item templates should bind to template models of the following structure:
{
index: 0, // index in the item array
selected: false, // true if the current item is selected
tabIndex: -1, // a dynamically generated tabIndex for focus management
item: {} // user data corresponding to items[index]
}
Alternatively, you can change the property name used as data index by changing the indexAs property. The as property defines the name of the variable to add to the binding scope for the array.
For example, given the following data array:
data.json
[
{"name": "Bob"},
{"name": "Tim"},
{"name": "Mike"}
]
The following code would render the list (note the name property is bound from the model object provided to the template scope):
<iron-ajax url="data.json" last-response="{{data}}" auto></iron-ajax>
<iron-list items="[[data]]" as="item">
<template>
<div>
Name: [[item.name]]
</div>
</template>
</iron-list>
Grid layout
iron-list supports a grid layout in addition to linear layout by setting the grid attribute. In this case, the list template item must have both fixed width and height (e.g. via CSS). Based on this, the number of items per row are determined automatically based on the size of the list viewport.
Accessibility
iron-list automatically manages the focus state for the items. It also provides a tabIndex property within the template scope that can be used for keyboard navigation. For example, users can press the up and down keys, as well as the left and right keys (the grid attribute is present), to move to focus between items in the list:
<iron-list items="[[data]]" as="item">
<template>
<div tabindex$="[[tabIndex]]">
Name: [[item.name]]
</div>
</template>
</iron-list>
Styling
You can use the --iron-list-items-container mixin to style the container of items:
iron-list {
--iron-list-items-container: {
margin: auto;
};
}
Resizing
iron-list lays out the items when it receives a notification via the iron-resize event. This event is fired by any element that implements IronResizableBehavior.
By default, elements such as iron-pages, paper-tabs or paper-dialog will trigger this event automatically. If you hide the list manually (e.g. you use display: none) you might want to implement IronResizableBehavior or fire this event manually right after the list became visible again. For example:
document.querySelector('iron-list').fire('iron-resize');
When should <iron-list> be used?
iron-list should be used when a page has significantly more DOM nodes than the ones visible on the screen. e.g. the page has 500 nodes, but only 20 are visible at a time. This is why we refer to it as a virtual list. In this case, a dom-repeat will still create 500 nodes which could slow down the web app, but iron-list will only create 20.
However, having an iron-list does not mean that you should load all the data at once. For example, if you have a million records in the database, it is better split the data into pages so you can bring in a page at a time. The page could contain 500 items, and iron-list might only render 20.
See: Documentation, Demo.
Usage
Installation
npm install --save @polymer/iron-list
In a Polymer 3 element
import {PolymerElement, html} from '@polymer/polymer';
import '@polymer/iron-list/iron-list.js';
class SampleElement extends PolymerElement {
static get template() {
return html`
<style>
:host {
display: block;
}
iron-list {
height: 100vh; /* don't use % values unless the parent element is sized. */
}
</style>
<iron-list items="[[items]]">
<template>
<div>
...
</div>
</template>
</iron-list>
`;
}
}
customElements.define('sample-element', SampleElement);
Contributing
If you want to send a PR to this element, here are the instructions for running the tests and demo locally:
Installation
git clone https://github.com/PolymerElements/iron-list
cd iron-list
npm install
npm install -g polymer-cli
Running the demo locally
polymer serve --npm
open http://127.0.0.1:<port>/demo/
Running the tests
polymer test --npm
