iron-image.html

<!--
@license
Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
-->

<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="../iron-flex-layout/iron-flex-layout.html">

<!--
`iron-image` is an element for displaying an image that provides useful sizing and
preloading options not found on the standard `<img>` tag.

The `sizing` option allows the image to be either cropped (`cover`) or
letterboxed (`contain`) to fill a fixed user-size placed on the element.

The `preload` option prevents the browser from rendering the image until the
image is fully loaded.  In the interim, either the element's CSS `background-color`
can be be used as the placeholder, or the `placeholder` property can be
set to a URL (preferably a data-URI, for instant rendering) for an
placeholder image.

The `fade` option (only valid when `preload` is set) will cause the placeholder
image/color to be faded out once the image is rendered.

Examples:

  Basically identical to `<img src="...">` tag:

    <iron-image src="http://lorempixel.com/400/400"></iron-image>

  Will letterbox the image to fit:

    <iron-image style="width:400px; height:400px;" sizing="contain"
      src="http://lorempixel.com/600/400"></iron-image>

  Will crop the image to fit:

    <iron-image style="width:400px; height:400px;" sizing="cover"
      src="http://lorempixel.com/600/400"></iron-image>

  Will show light-gray background until the image loads:

    <iron-image style="width:400px; height:400px; background-color: lightgray;"
      sizing="cover" preload src="http://lorempixel.com/600/400"></iron-image>

  Will show a base-64 encoded placeholder image until the image loads:

    <iron-image style="width:400px; height:400px;" placeholder="data:image/gif;base64,..."
      sizing="cover" preload src="http://lorempixel.com/600/400"></iron-image>

  Will fade the light-gray background out once the image is loaded:

    <iron-image style="width:400px; height:400px; background-color: lightgray;"
      sizing="cover" preload fade src="http://lorempixel.com/600/400"></iron-image>

Custom property | Description | Default
----------------|-------------|----------
`--iron-image-sized-img` | Mixin applied to #sizedImageDiv | `{}`
`--iron-image-placeholder` | Mixin applied to #placeholder | `{}`
`--iron-image-width` | Sets the width of the wrapped image | `auto`
`--iron-image-height` | Sets the height of the wrapped image | `auto`

@group Iron Elements
@element iron-image
@demo demo/index.html
-->

<dom-module id="iron-image">
  <template>
    <style>
      :host {
        display: inline-block;
        overflow: hidden;
        position: relative;
      }

      #sizedImgDiv {
        @apply(--layout-fit);

        display: none;

        @apply(--iron-image-sized-img);
      }

      #img {
        display: block;
        width: var(--iron-image-width, auto);
        height: var(--iron-image-height, auto);
      }

      :host([sizing]) #sizedImgDiv {
        display: block;
      }

      :host([sizing]) #img {
        display: none;
      }

      #placeholder {
        @apply(--layout-fit);

        background-color: inherit;
        opacity: 1;

        @apply(--iron-image-placeholder);
      }

      #placeholder.faded-out {
        transition: opacity 0.5s linear;
        opacity: 0;
      }
    </style>

    <div id="sizedImgDiv"
      role="img"
      hidden$="[[_computeImgDivHidden(sizing)]]"
      aria-hidden$="[[_computeImgDivARIAHidden(alt)]]"
      aria-label$="[[_computeImgDivARIALabel(alt, src)]]"></div>
    <img id="img" alt$="[[alt]]" hidden$="[[_computeImgHidden(sizing)]]">
    <div id="placeholder"
      hidden$="[[_computePlaceholderHidden(preload, fade, loading, loaded)]]"
      class$="[[_computePlaceholderClassName(preload, fade, loading, loaded)]]"></div>
  </template>

  <script>
    Polymer({
      is: 'iron-image',

      properties: {
        /**
         * The URL of an image.
         */
        src: {
          observer: '_srcChanged',
          type: String,
          value: ''
        },

        /**
         * A short text alternative for the image.
         */
        alt: {
          type: String,
          value: null
        },

        /**
         * When true, the image is prevented from loading and any placeholder is
         * shown.  This may be useful when a binding to the src property is known to
         * be invalid, to prevent 404 requests.
         */
        preventLoad: {
          type: Boolean,
          value: false,
          observer: '_preventLoadChanged'
        },

        /**
         * Sets a sizing option for the image.  Valid values are `contain` (full
         * aspect ratio of the image is contained within the element and
         * letterboxed) or `cover` (image is cropped in order to fully cover the
         * bounds of the element), or `null` (default: image takes natural size).
         */
        sizing: {
          type: String,
          value: null,
          reflectToAttribute: true
        },

        /**
         * When a sizing option is used (`cover` or `contain`), this determines
         * how the image is aligned within the element bounds.
         */
        position: {
          type: String,
          value: 'center'
        },

        /**
         * When `true`, any change to the `src` property will cause the `placeholder`
         * image to be shown until the new image has loaded.
         */
        preload: {
          type: Boolean,
          value: false
        },

        /**
         * This image will be used as a background/placeholder until the src image has
         * loaded.  Use of a data-URI for placeholder is encouraged for instant rendering.
         */
        placeholder: {
          type: String,
          value: null,
          observer: '_placeholderChanged'
        },

        /**
         * When `preload` is true, setting `fade` to true will cause the image to
         * fade into place.
         */
        fade: {
          type: Boolean,
          value: false
        },

        /**
         * Read-only value that is true when the image is loaded.
         */
        loaded: {
          notify: true,
          readOnly: true,
          type: Boolean,
          value: false
        },

        /**
         * Read-only value that tracks the loading state of the image when the `preload`
         * option is used.
         */
        loading: {
          notify: true,
          readOnly: true,
          type: Boolean,
          value: false
        },

        /**
         * Read-only value that indicates that the last set `src` failed to load.
         */
        error: {
          notify: true,
          readOnly: true,
          type: Boolean,
          value: false
        },

        /**
         * Can be used to set the width of image (e.g. via binding); size may also be
         * set via CSS.
         */
        width: {
          observer: '_widthChanged',
          type: Number,
          value: null
        },

        /**
         * Can be used to set the height of image (e.g. via binding); size may also be
         * set via CSS.
         *
         * @attribute height
         * @type number
         * @default null
         */
        height: {
          observer: '_heightChanged',
          type: Number,
          value: null
        },
      },

      observers: [
        '_transformChanged(sizing, position)'
      ],

      ready: function() {
        var img = this.$.img;

        img.onload = function() {
          if (this.$.img.src !== this._resolveSrc(this.src)) return;

          this._setLoading(false);
          this._setLoaded(true);
          this._setError(false);
        }.bind(this);

        img.onerror = function() {
          if (this.$.img.src !== this._resolveSrc(this.src)) return;

          this._reset();

          this._setLoading(false);
          this._setLoaded(false);
          this._setError(true);
        }.bind(this);

        this._resolvedSrc = '';
      },

      _load: function(src) {
        if (src) {
          this.$.img.src = src;
        } else {
          this.$.img.removeAttribute('src');
        }
        this.$.sizedImgDiv.style.backgroundImage = src ? 'url("' + src + '")' : '';

        this._setLoading(!!src);
        this._setLoaded(false);
        this._setError(false);
      },

      _reset: function() {
        this.$.img.removeAttribute('src');
        this.$.sizedImgDiv.style.backgroundImage = '';

        this._setLoading(false);
        this._setLoaded(false);
        this._setError(false);
      },

      _computePlaceholderHidden: function() {
        return !this.preload || (!this.fade && !this.loading && this.loaded);
      },

      _computePlaceholderClassName: function() {
        return (this.preload && this.fade && !this.loading && this.loaded) ? 'faded-out' : '';
      },

      _computeImgDivHidden: function() {
        return !this.sizing;
      },

      _computeImgDivARIAHidden: function() {
        return this.alt === '' ? 'true' : undefined;
      },

      _computeImgDivARIALabel: function() {
        if (this.alt !== null) {
          return this.alt;
        }

        // Polymer.ResolveUrl.resolveUrl will resolve '' relative to a URL x to
        // that URL x, but '' is the default for src.
        if (this.src === '') {
          return '';
        }

        var pathComponents = (new URL(this._resolveSrc(this.src))).pathname.split("/");
        return pathComponents[pathComponents.length - 1];
      },

      _computeImgHidden: function() {
        return !!this.sizing;
      },

      _widthChanged: function() {
        this.style.width = isNaN(this.width) ? this.width : this.width + 'px';
      },

      _heightChanged: function() {
        this.style.height = isNaN(this.height) ? this.height : this.height + 'px';
      },

      _preventLoadChanged: function() {
        if (this.preventLoad || this.loaded) return;

        this._reset();
        this._load(this.src);
      },

      _srcChanged: function(newSrc, oldSrc) {
        var newResolvedSrc = this._resolveSrc(newSrc);
        if (newResolvedSrc === this._resolvedSrc) return;
        this._resolvedSrc = newResolvedSrc;

        this._reset();
        if (!this.preventLoad) {
          this._load(newSrc);
        }
      },

      _placeholderChanged: function() {
        this.$.placeholder.style.backgroundImage =
          this.placeholder ? 'url("' + this.placeholder + '")' : '';
      },

      _transformChanged: function() {
        var sizedImgDivStyle = this.$.sizedImgDiv.style;
        var placeholderStyle = this.$.placeholder.style;

        sizedImgDivStyle.backgroundSize =
        placeholderStyle.backgroundSize =
          this.sizing;

        sizedImgDivStyle.backgroundPosition =
        placeholderStyle.backgroundPosition =
          this.sizing ? this.position : '';

        sizedImgDivStyle.backgroundRepeat =
        placeholderStyle.backgroundRepeat =
          this.sizing ? 'no-repeat' : '';
      },

      _resolveSrc: function(testSrc) {
        return Polymer.ResolveUrl.resolveUrl(testSrc, this.ownerDocument.baseURI);
      }
    });
  </script>
</dom-module>