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
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Code distributed by Google as part of the polymer project is also
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-->

<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 &lt;img src="..."&gt; 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-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">

  <style>

    :host {
      display: inline-block;
      overflow: hidden;
      position: relative;
    }

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

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

    #placeholder {
      background-color: inherit;
      opacity: 1;
      @apply(--layout-fit);
      @apply(--iron-image-placeholder);
    }

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

  </style>

  <template>

    <img id="img" role="none" hidden$="[[_computeImageVisibility(sizing)]]">
    <div id="placeholder" hidden$="[[_computePlaceholderVisibility(fade,loaded,preload)]]" class$="[[_computePlaceholderClassName(fade,loaded,preload)]]"></div>
    <content></content>

  </template>

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

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

        /**
         * 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
        },

        /**
         * 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
        },

        /**
         * 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
        },

        /**
         * 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,
          type: Boolean,
          value: false
        },

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

        _placeholderBackgroundUrl: {
          type: String,
          computed: '_computePlaceholderBackgroundUrl(preload,placeholder)',
          observer: '_placeholderBackgroundUrlChanged'
        },

        requiresPreload: {
          type: Boolean,
          computed: '_computeRequiresPreload(preload,loaded)'
        },

        canLoad: {
          type: Boolean,
          computed: '_computeCanLoad(preventLoad, src)'
        }
      },

      observers: [
        '_transformChanged(sizing, position)',
        '_loadBehaviorChanged(canLoad, preload, loaded)',
        '_loadStateChanged(src, preload, loaded)',
      ],

      ready: function() {
        if (!this.hasAttribute('role')) {
          this.setAttribute('role', 'img');
        }
      },

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

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

      _computePlaceholderClassName: function() {
        if (!this.preload) {
          return '';
        }

        if (this.loaded && this.fade) {
          return 'faded-out';
        }

        return '';
      },

      _computePlaceholderBackgroundUrl: function() {
        if (this.preload && this.placeholder) {
          return 'url(' + this.placeholder + ')';
        }

        return null;
      },

      _computeRequiresPreload: function() {
        return this.preload && !this.loaded;
      },

      _computeCanLoad: function() {
        return Boolean(!this.preventLoad && this.src);
      },

      _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';
      },

      _srcChanged: function(newSrc, oldSrc) {
        if (newSrc !== oldSrc) {
          this.loaded = false;
        }
      },

      _placeholderBackgroundUrlChanged: function() {
        this.$.placeholder.style.backgroundImage =
          this._placeholderBackgroundUrl;
      },

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

        this.style.backgroundSize =
          placeholderStyle.backgroundSize = this.sizing;

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

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

      _loadBehaviorChanged: function() {
        var img;

        if (!this.canLoad) {
          return;
        }

        if (this.requiresPreload) {
          img = new Image();
          img.src = this.src;

          this.loading = true;

          img.onload = function() {
            this.loading = false;
            this.loaded = true;
          }.bind(this);
        } else {
          this.loaded = true;
        }
      },

      _loadStateChanged: function() {
        if (this.requiresPreload) {
          return;
        }

        if (this.sizing) {
          this.style.backgroundImage = this.src ? 'url(' + this.src + ')': '';
        } else {
          this.$.img.src = this.src || '';
        }
      }
    });
  </script>
</dom-module>