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.

__Important:__ `iron-list` must ether 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).

### Template model

List item templates should bind to template models of the following structure:

```js
{
  index: 0,     // data index for this item
  item: {       // user data corresponding to items[index]
    /* user item data  */
  }
}
```

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

```js
[
  {"name": "Bob"},
  {"name": "Tim"},
  {"name": "Mike"}
]
```

The following code would render the list (note the name and checked properties are
bound from the model object provided to the template scope):

```html
<template is="dom-bind">
  <iron-ajax url="data.json" last-response="{{data}}" auto></iron-ajax>
  <iron-list items="[[data]]" as="item">
    <template>
      <div>
        Name: <span>[[item.name]]</span>
      </div>
    </template>
  </iron-list>
</template>
```

### Resizing

`iron-list` lays out the items when it recives a notification via the `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. e.g.

```js
document.querySelector('iron-list').fire('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 the 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 can load all the data at once. Say, you have a million records in the database, you want to split the data into pages so you can bring a page at the time. The page could contain 500 items, and iron-list will only render 20.
