Reponsive images are now loaded on demand

Signed-off-by: Jim Martens <github@2martens.de>

_assets/js/echo.js

@@ -0,0 +1,135 @@
+/*! echo.js v1.7.0 | (c) 2015 @toddmotto | https://github.com/toddmotto/echo */
+(function (root, factory) {
+  if (typeof define === 'function' && define.amd) {
+    define(function() {
+      return factory(root);
+    });
+  } else if (typeof exports === 'object') {
+    module.exports = factory;
+  } else {
+    root.echo = factory(root);
+  }
+})(this, function (root) {
+
+  'use strict';
+
+  let echo = {};
+
+  let callback = function () {};
+
+  let offset, poll, delay, useDebounce, unload;
+
+  const isHidden = function (element) {
+    return (element.offsetParent === null);
+  };
+  
+  const inView = function (element, view) {
+    if (isHidden(element)) {
+      return false;
+    }
+
+    let box = element.getBoundingClientRect();
+    return (box.right >= view.l && box.bottom >= view.t && box.left <= view.r && box.top <= view.b);
+  };
+
+  const debounceOrThrottle = function () {
+    if(!useDebounce && !!poll) {
+      return;
+    }
+    clearTimeout(poll);
+    poll = setTimeout(function(){
+      echo.render();
+      poll = null;
+    }, delay);
+  };
+
+  echo.init = function (opts) {
+    opts = opts || {};
+    let offsetAll = opts.offset || 0;
+    let offsetVertical = opts.offsetVertical || offsetAll;
+    let offsetHorizontal = opts.offsetHorizontal || offsetAll;
+    const optionToInt = function (opt, fallback) {
+      return parseInt(opt || fallback, 10);
+    };
+    offset = {
+      t: optionToInt(opts.offsetTop, offsetVertical),
+      b: optionToInt(opts.offsetBottom, offsetVertical),
+      l: optionToInt(opts.offsetLeft, offsetHorizontal),
+      r: optionToInt(opts.offsetRight, offsetHorizontal)
+    };
+    delay = optionToInt(opts.throttle, 250);
+    useDebounce = opts.debounce !== false;
+    unload = !!opts.unload;
+    callback = opts.callback || callback;
+    echo.render();
+    if (document.addEventListener) {
+      root.addEventListener('scroll', debounceOrThrottle, false);
+      root.addEventListener('load', debounceOrThrottle, false);
+    } else {
+      root.attachEvent('onscroll', debounceOrThrottle);
+      root.attachEvent('onload', debounceOrThrottle);
+    }
+  };
+
+  echo.render = function () {
+    let nodes = document.querySelectorAll('img[data-echo], [data-echo-background]');
+    let length = nodes.length;
+    let src, elem;
+    let view = {
+      l: 0 - offset.l,
+      t: 0 - offset.t,
+      b: (root.innerHeight || document.documentElement.clientHeight) + offset.b,
+      r: (root.innerWidth || document.documentElement.clientWidth) + offset.r
+    };
+    for (let i = 0; i < length; i++) {
+      elem = nodes[i];
+      if (inView(elem, view)) {
+
+        if (unload) {
+          elem.setAttribute('data-echo-placeholder', elem.src);
+        }
+
+        if (elem.getAttribute('data-echo-background') !== null) {
+          elem.style.backgroundImage = "url(" + elem.getAttribute('data-echo-background') + ")";
+        }
+        else {
+          elem.src = elem.getAttribute('data-echo');
+        }
+
+        if (!unload) {
+          elem.removeAttribute('data-echo');
+          elem.removeAttribute('data-echo-background');
+        }
+
+        callback(elem, 'load');
+      }
+      else if (unload && !!(src = elem.getAttribute('data-echo-placeholder'))) {
+
+        if (elem.getAttribute('data-echo-background') !== null) {
+          elem.style.backgroundImage = "url(" + src + ")";
+        }
+        else {
+          elem.src = src;
+        }
+
+        elem.removeAttribute('data-echo-placeholder');
+        callback(elem, 'unload');
+      }
+    }
+    if (!length) {
+      echo.detach();
+    }
+  };
+
+  echo.detach = function () {
+    if (document.removeEventListener) {
+      root.removeEventListener('scroll', debounceOrThrottle);
+    } else {
+      root.detachEvent('onscroll', debounceOrThrottle);
+    }
+    clearTimeout(poll);
+  };
+
+  return echo;
+
+});

_includes/head.html

@@ -19,5 +19,13 @@
             integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN"
             crossorigin="anonymous"></script>
     {% asset main.js %}
+    {% asset echo.js %}
+    <script defer>
+        echo.init({
+            offset: 500,
+            throttle: 250,
+            unload: false
+        });
+    </script>
     {% asset fontawesome-all.js defer %}
 </head>

_includes/image.html

@@ -1,3 +1,4 @@
 <img alt="{{ include.alt }}"
-     src="{% if site.data.cdn.use %}{{ site.data.cdn.url }}{% else %}{{ site.baseurl }}{% endif %}{{ include.link }}"
+     src=""
+     data-echo="{% if site.data.cdn.use %}{{ site.data.cdn.url }}{% else %}{{ site.baseurl }}{% endif %}{{ include.link }}"
      class="img-fluid" />