components/paper-dropdown-menu/paper-dropdown-menu.html

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<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="../iron-a11y-keys-behavior/iron-a11y-keys-behavior.html">
<link rel="import" href="../iron-behaviors/iron-button-state.html">
<link rel="import" href="../iron-behaviors/iron-control-state.html">
<link rel="import" href="../iron-form-element-behavior/iron-form-element-behavior.html">
<link rel="import" href="../iron-icon/iron-icon.html">
<link rel="import" href="../iron-validatable-behavior/iron-validatable-behavior.html">
<link rel="import" href="../paper-input/paper-input.html">
<link rel="import" href="../paper-menu-button/paper-menu-button.html">
<link rel="import" href="../paper-ripple/paper-ripple.html">
<link rel="import" href="../paper-styles/default-theme.html">

<link rel="import" href="paper-dropdown-menu-icons.html">
<link rel="import" href="paper-dropdown-menu-shared-styles.html">

<!--
Material design: [Dropdown menus](https://www.google.com/design/spec/components/buttons.html#buttons-dropdown-buttons)

`paper-dropdown-menu` is similar to a native browser select element.
`paper-dropdown-menu` works with selectable content. The currently selected
item is displayed in the control. If no item is selected, the `label` is
displayed instead.

Example:

    <paper-dropdown-menu label="Your favourite pastry">
      <paper-listbox class="dropdown-content">
        <paper-item>Croissant</paper-item>
        <paper-item>Donut</paper-item>
        <paper-item>Financier</paper-item>
        <paper-item>Madeleine</paper-item>
      </paper-listbox>
    </paper-dropdown-menu>

This example renders a dropdown menu with 4 options.

The child element with the class `dropdown-content` is used as the dropdown
menu. This can be a [`paper-listbox`](paper-listbox), or any other or
element that acts like an [`iron-selector`](iron-selector).

Specifically, the menu child must fire an
[`iron-select`](iron-selector#event-iron-select) event when one of its
children is selected, and an [`iron-deselect`](iron-selector#event-iron-deselect)
event when a child is deselected. The selected or deselected item must
be passed as the event's `detail.item` property.

Applications can listen for the `iron-select` and `iron-deselect` events
to react when options are selected and deselected.

### Styling

The following custom properties and mixins are also available for styling:

Custom property | Description | Default
----------------|-------------|----------
`--paper-dropdown-menu` | A mixin that is applied to the element host | `{}`
`--paper-dropdown-menu-disabled` | A mixin that is applied to the element host when disabled | `{}`
`--paper-dropdown-menu-ripple` | A mixin that is applied to the internal ripple | `{}`
`--paper-dropdown-menu-button` | A mixin that is applied to the internal menu button | `{}`
`--paper-dropdown-menu-input` | A mixin that is applied to the internal paper input | `{}`
`--paper-dropdown-menu-icon` | A mixin that is applied to the internal icon | `{}`

You can also use any of the `paper-input-container` and `paper-menu-button`
style mixins and custom properties to style the internal input and menu button
respectively.

@group Paper Elements
@element paper-dropdown-menu
@hero hero.svg
@demo demo/index.html
-->

<dom-module id="paper-dropdown-menu">
  <template>
    <style include="paper-dropdown-menu-shared-styles"></style>

    <!-- this div fulfills an a11y requirement for combobox, do not remove -->
    <span role="button"></span>
    <paper-menu-button
      id="menuButton"
      vertical-align="[[verticalAlign]]"
      horizontal-align="[[horizontalAlign]]"
      dynamic-align="[[dynamicAlign]]"
      vertical-offset="[[_computeMenuVerticalOffset(noLabelFloat)]]"
      disabled="[[disabled]]"
      no-animations="[[noAnimations]]"
      on-iron-select="_onIronSelect"
      on-iron-deselect="_onIronDeselect"
      opened="{{opened}}"
      close-on-activate
      allow-outside-scroll="[[allowOutsideScroll]]"
      restore-focus-on-close="[[restoreFocusOnClose]]">
      <div class="dropdown-trigger">
        <paper-ripple></paper-ripple>
        <!-- paper-input has type="text" for a11y, do not remove -->
        <paper-input
          type="text"
          invalid="[[invalid]]"
          readonly
          disabled="[[disabled]]"
          value="[[selectedItemLabel]]"
          placeholder="[[placeholder]]"
          error-message="[[errorMessage]]"
          always-float-label="[[alwaysFloatLabel]]"
          no-label-float="[[noLabelFloat]]"
          label="[[label]]">
          <iron-icon icon="paper-dropdown-menu:arrow-drop-down" suffix></iron-icon>
        </paper-input>
      </div>
      <content id="content" select=".dropdown-content"></content>
    </paper-menu-button>
  </template>

  <script>
    (function() {
      'use strict';

      Polymer({
        is: 'paper-dropdown-menu',

        behaviors: [
          Polymer.IronButtonState,
          Polymer.IronControlState,
          Polymer.IronFormElementBehavior,
          Polymer.IronValidatableBehavior
        ],

        properties: {
          /**
           * The derived "label" of the currently selected item. This value
           * is the `label` property on the selected item if set, or else the
           * trimmed text content of the selected item.
           */
          selectedItemLabel: {
            type: String,
            notify: true,
            readOnly: true
          },

          /**
           * The last selected item. An item is selected if the dropdown menu has
           * a child with class `dropdown-content`, and that child triggers an
           * `iron-select` event with the selected `item` in the `detail`.
           *
           * @type {?Object}
           */
          selectedItem: {
            type: Object,
            notify: true,
            readOnly: true
          },

          /**
           * The value for this element that will be used when submitting in
           * a form. It is read only, and will always have the same value
           * as `selectedItemLabel`.
           */
          value: {
            type: String,
            notify: true,
            readOnly: true
          },

          /**
           * The label for the dropdown.
           */
          label: {
            type: String
          },

          /**
           * The placeholder for the dropdown.
           */
          placeholder: {
            type: String
          },

          /**
           * The error message to display when invalid.
           */
          errorMessage: {
              type: String
          },

          /**
           * True if the dropdown is open. Otherwise, false.
           */
          opened: {
            type: Boolean,
            notify: true,
            value: false,
            observer: '_openedChanged'
          },

          /**
           * By default, the dropdown will constrain scrolling on the page
           * to itself when opened.
           * Set to true in order to prevent scroll from being constrained
           * to the dropdown when it opens.
           */
          allowOutsideScroll: {
            type: Boolean,
            value: false
          },

          /**
           * Set to true to disable the floating label. Bind this to the
           * `<paper-input-container>`'s `noLabelFloat` property.
           */
          noLabelFloat: {
              type: Boolean,
              value: false,
              reflectToAttribute: true
          },

          /**
           * Set to true to always float the label. Bind this to the
           * `<paper-input-container>`'s `alwaysFloatLabel` property.
           */
          alwaysFloatLabel: {
            type: Boolean,
            value: false
          },

          /**
           * Set to true to disable animations when opening and closing the
           * dropdown.
           */
          noAnimations: {
            type: Boolean,
            value: false
          },

          /**
           * The orientation against which to align the menu dropdown
           * horizontally relative to the dropdown trigger.
           */
          horizontalAlign: {
            type: String,
            value: 'right'
          },

          /**
           * The orientation against which to align the menu dropdown
           * vertically relative to the dropdown trigger.
           */
          verticalAlign: {
            type: String,
            value: 'top'
          },

          /**
           * If true, the `horizontalAlign` and `verticalAlign` properties will
           * be considered preferences instead of strict requirements when
           * positioning the dropdown and may be changed if doing so reduces
           * the area of the dropdown falling outside of `fitInto`.
           */
          dynamicAlign: {
            type: Boolean
          },

          /**
           * Whether focus should be restored to the dropdown when the menu closes.
           */
          restoreFocusOnClose: {
            type: Boolean,
            value: true
          },
        },

        listeners: {
          'tap': '_onTap'
        },

        keyBindings: {
          'up down': 'open',
          'esc': 'close'
        },

        hostAttributes: {
          role: 'combobox',
          'aria-autocomplete': 'none',
          'aria-haspopup': 'true'
        },

        observers: [
          '_selectedItemChanged(selectedItem)'
        ],

        attached: function() {
          // NOTE(cdata): Due to timing, a preselected value in a `IronSelectable`
          // child will cause an `iron-select` event to fire while the element is
          // still in a `DocumentFragment`. This has the effect of causing
          // handlers not to fire. So, we double check this value on attached:
          var contentElement = this.contentElement;
          if (contentElement && contentElement.selectedItem) {
            this._setSelectedItem(contentElement.selectedItem);
          }
        },

        /**
         * The content element that is contained by the dropdown menu, if any.
         */
        get contentElement() {
          return Polymer.dom(this.$.content).getDistributedNodes()[0];
        },

        /**
         * Show the dropdown content.
         */
        open: function() {
          this.$.menuButton.open();
        },

        /**
         * Hide the dropdown content.
         */
        close: function() {
          this.$.menuButton.close();
        },

        /**
         * A handler that is called when `iron-select` is fired.
         *
         * @param {CustomEvent} event An `iron-select` event.
         */
        _onIronSelect: function(event) {
          this._setSelectedItem(event.detail.item);
        },

        /**
         * A handler that is called when `iron-deselect` is fired.
         *
         * @param {CustomEvent} event An `iron-deselect` event.
         */
        _onIronDeselect: function(event) {
          this._setSelectedItem(null);
        },

        /**
         * A handler that is called when the dropdown is tapped.
         *
         * @param {CustomEvent} event A tap event.
         */
        _onTap: function(event) {
          if (Polymer.Gestures.findOriginalTarget(event) === this) {
            this.open();
          }
        },

        /**
         * Compute the label for the dropdown given a selected item.
         *
         * @param {Element} selectedItem A selected Element item, with an
         * optional `label` property.
         */
        _selectedItemChanged: function(selectedItem) {
          var value = '';
          if (!selectedItem) {
            value = '';
          } else {
            value = selectedItem.label || selectedItem.getAttribute('label') || selectedItem.textContent.trim();
          }

          this._setValue(value);
          this._setSelectedItemLabel(value);
        },

        /**
         * Compute the vertical offset of the menu based on the value of
         * `noLabelFloat`.
         *
         * @param {boolean} noLabelFloat True if the label should not float
         * above the input, otherwise false.
         */
        _computeMenuVerticalOffset: function(noLabelFloat) {
          // NOTE(cdata): These numbers are somewhat magical because they are
          // derived from the metrics of elements internal to `paper-input`'s
          // template. The metrics will change depending on whether or not the
          // input has a floating label.
          return noLabelFloat ? -4 : 8;
        },

        /**
         * Returns false if the element is required and does not have a selection,
         * and true otherwise.
         * @param {*=} _value Ignored.
         * @return {boolean} true if `required` is false, or if `required` is true
         * and the element has a valid selection.
         */
        _getValidity: function(_value) {
          return this.disabled || !this.required || (this.required && !!this.value);
        },

        _openedChanged: function() {
          var openState = this.opened ? 'true' : 'false';
          var e = this.contentElement;
          if (e) {
            e.setAttribute('aria-expanded', openState);
          }
        }
      });
    })();
  </script>
</dom-module>