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.
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
<template is="x-list">
<style>
:host {
display: block;
height: 100vh;
display: flex;
flex-direction: column;
}
iron-list {
flex: 1 1 auto;
}
</style>
<app-toolbar>App name</app-toolbar>
<iron-list items="[[items]]">
<template>
<div>
...
</div>
</template>
</iron-list>
</template>
Explicit size - jsbin
<template is="x-list">
<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>
</template>
Main document scrolling - jsbin
<head>
<style>
body {
height: 100vh;
margin: 0;
display: flex;
flex-direction: column;
}
app-toolbar {
position: fixed;
top: 0;
left: 0;
right: 0;
}
iron-list {
/* add padding since the app-toolbar is fixed at the top */
padding-top: 64px;
}
</style>
</head>
<body>
<app-toolbar>App name</app-toolbar>
<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).
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:
[
{"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>
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.
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>
You can use the --iron-list-items-container
mixin to style the container of items:
iron-list {
--iron-list-items-container: {
margin: auto;
};
}
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');
- In v1,
selectItem
accepted an item or the index to the item in thelist.items
array. In v2,selectItem
only accepts an item in thelist.items
array. As a result,selectIndex
anddeselectIndex
has been introduced to allow for fast selection of items by index. If you are implementing aselectAll
method, you should useselectIndex
instead ofselectItem
because it's faster (O(1) run time). - Mutating a selected item in v2 isn't supported anymore due to the removal of
Polymer.Collection
in 2.0, for example:
list.items = [ item1, item2, ... ,itemN];
list.selectionEnabled = true;
list.selectIndex(0);
list.set('items.0', aDifferentItem);
// list.selectedItem == item1
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.