WebCompDev / COMPONENTS

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<link rel="import" href="../polymer/polymer.html">

<script>

/**
Polymer.IronFitBehavior fits an element in another element using `max-height` and `max-width`, and
optionally centers it in the window or another element.

The element will only be sized and/or positioned if it has not already been sized and/or positioned
by CSS.

CSS properties               | Action
-----------------------------|-------------------------------------------
`position` set               | Element is not centered horizontally or vertically
`top` or `bottom` set        | Element is not vertically centered
`left` or `right` set        | Element is not horizontally centered
`max-height` or `height` set | Element respects `max-height` or `height`
`max-width` or `width` set   | Element respects `max-width` or `width`

@demo demo/index.html
@polymerBehavior
*/

  Polymer.IronFitBehavior = {

    properties: {

      /**
       * The element that will receive a `max-height`/`width`. By default it is the same as `this`,
       * but it can be set to a child element. This is useful, for example, for implementing a
       * scrolling region inside the element.
       * @type {!Element}
       */
      sizingTarget: {
        type: Object,
        value: function() {
          return this;
        }
      },

      /**
       * The element to fit `this` into.
       */
      fitInto: {
        type: Object,
        value: window
      },

      /**
       * Set to true to auto-fit on attach.
       */
      autoFitOnAttach: {
        type: Boolean,
        value: false
      },

      /** @type {?Object} */
      _fitInfo: {
        type: Object
      }

    },

    get _fitWidth() {
      var fitWidth;
      if (this.fitInto === window) {
        fitWidth = this.fitInto.innerWidth;
      } else {
        fitWidth = this.fitInto.getBoundingClientRect().width;
      }
      return fitWidth;
    },

    get _fitHeight() {
      var fitHeight;
      if (this.fitInto === window) {
        fitHeight = this.fitInto.innerHeight;
      } else {
        fitHeight = this.fitInto.getBoundingClientRect().height;
      }
      return fitHeight;
    },

    get _fitLeft() {
      var fitLeft;
      if (this.fitInto === window) {
        fitLeft = 0;
      } else {
        fitLeft = this.fitInto.getBoundingClientRect().left;
      }
      return fitLeft;
    },

    get _fitTop() {
      var fitTop;
      if (this.fitInto === window) {
        fitTop = 0;
      } else {
        fitTop = this.fitInto.getBoundingClientRect().top;
      }
      return fitTop;
    },

    attached: function() {
      if (this.autoFitOnAttach) {
        if (window.getComputedStyle(this).display === 'none') {
          setTimeout(function() {
            this.fit();
          }.bind(this));
        } else {
          this.fit();
        }
      }
    },

    /**
     * Fits and optionally centers the element into the window, or `fitInfo` if specified.
     */
    fit: function() {
      this._discoverInfo();
      this.constrain();
      this.center();
    },

    /**
     * Memoize information needed to position and size the target element.
     */
    _discoverInfo: function() {
      if (this._fitInfo) {
        return;
      }
      var target = window.getComputedStyle(this);
      var sizer = window.getComputedStyle(this.sizingTarget);
      this._fitInfo = {
        inlineStyle: {
          top: this.style.top || '',
          left: this.style.left || ''
        },
        positionedBy: {
          vertically: target.top !== 'auto' ? 'top' : (target.bottom !== 'auto' ?
            'bottom' : null),
          horizontally: target.left !== 'auto' ? 'left' : (target.right !== 'auto' ?
            'right' : null),
          css: target.position
        },
        sizedBy: {
          height: sizer.maxHeight !== 'none',
          width: sizer.maxWidth !== 'none'
        },
        margin: {
          top: parseInt(target.marginTop, 10) || 0,
          right: parseInt(target.marginRight, 10) || 0,
          bottom: parseInt(target.marginBottom, 10) || 0,
          left: parseInt(target.marginLeft, 10) || 0
        }
      };
    },

    /**
     * Resets the target element's position and size constraints, and clear
     * the memoized data.
     */
    resetFit: function() {
      if (!this._fitInfo || !this._fitInfo.sizedBy.height) {
        this.sizingTarget.style.maxHeight = '';
        this.style.top = this._fitInfo ? this._fitInfo.inlineStyle.top : '';
      }
      if (!this._fitInfo || !this._fitInfo.sizedBy.width) {
        this.sizingTarget.style.maxWidth = '';
        this.style.left = this._fitInfo ? this._fitInfo.inlineStyle.left : '';
      }
      if (this._fitInfo) {
        this.style.position = this._fitInfo.positionedBy.css;
      }
      this._fitInfo = null;
    },

    /**
     * Equivalent to calling `resetFit()` and `fit()`. Useful to call this after the element,
     * the window, or the `fitInfo` element has been resized.
     */
    refit: function() {
      this.resetFit();
      this.fit();
    },

    /**
     * Constrains the size of the element to the window or `fitInfo` by setting `max-height`
     * and/or `max-width`.
     */
    constrain: function() {
      var info = this._fitInfo;
      // position at (0px, 0px) if not already positioned, so we can measure the natural size.
      if (!this._fitInfo.positionedBy.vertically) {
        this.style.top = '0px';
      }
      if (!this._fitInfo.positionedBy.horizontally) {
        this.style.left = '0px';
      }
      if (!this._fitInfo.positionedBy.vertically || !this._fitInfo.positionedBy.horizontally) {
        // need position:fixed to properly size the element
        this.style.position = 'fixed';
      }
      // need border-box for margin/padding
      this.sizingTarget.style.boxSizing = 'border-box';
      // constrain the width and height if not already set
      var rect = this.getBoundingClientRect();
      if (!info.sizedBy.height) {
        this._sizeDimension(rect, info.positionedBy.vertically, 'top', 'bottom', 'Height');
      }
      if (!info.sizedBy.width) {
        this._sizeDimension(rect, info.positionedBy.horizontally, 'left', 'right', 'Width');
      }
    },

    _sizeDimension: function(rect, positionedBy, start, end, extent) {
      var info = this._fitInfo;
      var max = extent === 'Width' ? this._fitWidth : this._fitHeight;
      var flip = (positionedBy === end);
      var offset = flip ? max - rect[end] : rect[start];
      var margin = info.margin[flip ? start : end];
      var offsetExtent = 'offset' + extent;
      var sizingOffset = this[offsetExtent] - this.sizingTarget[offsetExtent];
      this.sizingTarget.style['max' + extent] = (max - margin - offset - sizingOffset) + 'px';
    },

    /**
     * Centers horizontally and vertically if not already positioned. This also sets
     * `position:fixed`.
     */
    center: function() {
      if (!this._fitInfo.positionedBy.vertically || !this._fitInfo.positionedBy.horizontally) {
        // need position:fixed to center
        this.style.position = 'fixed';
      }
      if (!this._fitInfo.positionedBy.vertically) {
        var top = (this._fitHeight - this.offsetHeight) / 2 + this._fitTop;
        top -= this._fitInfo.margin.top;
        this.style.top = top + 'px';
      }
      if (!this._fitInfo.positionedBy.horizontally) {
        var left = (this._fitWidth - this.offsetWidth) / 2 + this._fitLeft;
        left -= this._fitInfo.margin.left;
        this.style.left = left + 'px';
      }
    }

  };

</script>