WebCompDev / COMPONENTS

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  [![Build status](https://travis-ci.org/PolymerElements/iron-list.svg?branch=master)](https://travis-ci.org/PolymerElements/iron-list)
  
  _[Demo and API docs](https://elements.polymer-project.org/elements/iron-list)_
  
  
  ##&lt;iron-list&gt;
  
  `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 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).
  
  ### Template model
  
  List item templates should bind to template models of the following structure:
  

  ```js
  {

    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
  
  ```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: [[item.name]]

        </div>
      </template>
    </iron-list>
  </template>
  ```

  ### 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 to move to previous and next
  items in the list:

  ```html
  <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:

  ```css
  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:
  
  ```js
  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 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.