fixed 'unload' param can't work

dist/echo.js

@@ -61,7 +61,10 @@
     useDebounce = opts.debounce !== false;
     unload = !!opts.unload;
     callback = opts.callback || callback;
-    echo.render();
+
+    //note by Alon Zhang | zhangjinlong3546@sina.com
+    //repeatedly render, debounceOrThrottle function has called render
+    // echo.render();
     if (document.addEventListener) {
       root.addEventListener('scroll', debounceOrThrottle, false);
       root.addEventListener('load', debounceOrThrottle, false);
@@ -86,7 +89,11 @@
       if (inView(elem, view)) {
 
         if (unload) {
-          elem.setAttribute('data-echo-placeholder', elem.src);
+          //add by Alon Zhang | zhangjinlong3546@sina.com
+          //Only the first time, storage placeholder picture
+          if(!elem.getAttribute('data-echo-placeholder')){
+            elem.setAttribute('data-echo-placeholder', elem.src);
+          }
         }
 
         if (elem.getAttribute('data-echo-background') !== null) {

dist/echo.min.js

@@ -1,2 +1,2 @@
 /*! echo-js v1.7.3 | (c) 2016 @toddmotto | https://github.com/toddmotto/echo */
-!function(t,e){"function"==typeof define&&define.amd?define(function(){return e(t)}):"object"==typeof exports?module.exports=e:t.echo=e(t)}(this,function(t){"use strict";var e,n,o,r,c,a={},u=function(){},d=function(t){return null===t.offsetParent},l=function(t,e){if(d(t))return!1;var n=t.getBoundingClientRect();return n.right>=e.l&&n.bottom>=e.t&&n.left<=e.r&&n.top<=e.b},i=function(){(r||!n)&&(clearTimeout(n),n=setTimeout(function(){a.render(),n=null},o))};return a.init=function(n){n=n||{};var d=n.offset||0,l=n.offsetVertical||d,f=n.offsetHorizontal||d,s=function(t,e){return parseInt(t||e,10)};e={t:s(n.offsetTop,l),b:s(n.offsetBottom,l),l:s(n.offsetLeft,f),r:s(n.offsetRight,f)},o=s(n.throttle,250),r=n.debounce!==!1,c=!!n.unload,u=n.callback||u,a.render(),document.addEventListener?(t.addEventListener("scroll",i,!1),t.addEventListener("load",i,!1)):(t.attachEvent("onscroll",i),t.attachEvent("onload",i))},a.render=function(n){for(var o,r,d=(n||document).querySelectorAll("[data-echo], [data-echo-background]"),i=d.length,f={l:0-e.l,t:0-e.t,b:(t.innerHeight||document.documentElement.clientHeight)+e.b,r:(t.innerWidth||document.documentElement.clientWidth)+e.r},s=0;i>s;s++)r=d[s],l(r,f)?(c&&r.setAttribute("data-echo-placeholder",r.src),null!==r.getAttribute("data-echo-background")?r.style.backgroundImage="url("+r.getAttribute("data-echo-background")+")":r.src!==(o=r.getAttribute("data-echo"))&&(r.src=o),c||(r.removeAttribute("data-echo"),r.removeAttribute("data-echo-background")),u(r,"load")):c&&(o=r.getAttribute("data-echo-placeholder"))&&(null!==r.getAttribute("data-echo-background")?r.style.backgroundImage="url("+o+")":r.src=o,r.removeAttribute("data-echo-placeholder"),u(r,"unload"));i||a.detach()},a.detach=function(){document.removeEventListener?t.removeEventListener("scroll",i):t.detachEvent("onscroll",i),clearTimeout(n)},a});
+!function(t,e){"function"==typeof define&&define.amd?define(function(){return e(t)}):"object"==typeof exports?module.exports=e:t.echo=e(t)}(this,function(t){"use strict";var e,o,n,r,c,a={},u=function(){},d=function(t){return null===t.offsetParent},l=function(t,e){if(d(t))return!1;var o=t.getBoundingClientRect();return o.right>=e.l&&o.bottom>=e.t&&o.left<=e.r&&o.top<=e.b},i=function(){(r||!o)&&(clearTimeout(o),o=setTimeout(function(){a.render(),o=null},n))};return a.init=function(o){o=o||{};var a=o.offset||0,d=o.offsetVertical||a,l=o.offsetHorizontal||a,f=function(t,e){return parseInt(t||e,10)};e={t:f(o.offsetTop,d),b:f(o.offsetBottom,d),l:f(o.offsetLeft,l),r:f(o.offsetRight,l)},n=f(o.throttle,250),r=o.debounce!==!1,c=!!o.unload,u=o.callback||u,document.addEventListener?(t.addEventListener("scroll",i,!1),t.addEventListener("load",i,!1)):(t.attachEvent("onscroll",i),t.attachEvent("onload",i))},a.render=function(o){for(var n,r,d=(o||document).querySelectorAll("[data-echo], [data-echo-background]"),i=d.length,f={l:0-e.l,t:0-e.t,b:(t.innerHeight||document.documentElement.clientHeight)+e.b,r:(t.innerWidth||document.documentElement.clientWidth)+e.r},h=0;i>h;h++)r=d[h],l(r,f)?(c&&(r.getAttribute("data-echo-placeholder")||r.setAttribute("data-echo-placeholder",r.src)),null!==r.getAttribute("data-echo-background")?r.style.backgroundImage="url("+r.getAttribute("data-echo-background")+")":r.src!==(n=r.getAttribute("data-echo"))&&(r.src=n),c||(r.removeAttribute("data-echo"),r.removeAttribute("data-echo-background")),u(r,"load")):c&&(n=r.getAttribute("data-echo-placeholder"))&&(null!==r.getAttribute("data-echo-background")?r.style.backgroundImage="url("+n+")":r.src=n,r.removeAttribute("data-echo-placeholder"),u(r,"unload"));i||a.detach()},a.detach=function(){document.removeEventListener?t.removeEventListener("scroll",i):t.detachEvent("onscroll",i),clearTimeout(o)},a});

src/echo.js

@@ -60,7 +60,10 @@
     useDebounce = opts.debounce !== false;
     unload = !!opts.unload;
     callback = opts.callback || callback;
-    echo.render();
+
+    //note by Alon Zhang | zhangjinlong3546@sina.com
+    //repeatedly render, debounceOrThrottle function has called render
+    // echo.render();
     if (document.addEventListener) {
       root.addEventListener('scroll', debounceOrThrottle, false);
       root.addEventListener('load', debounceOrThrottle, false);
@@ -85,7 +88,11 @@
       if (inView(elem, view)) {
 
         if (unload) {
-          elem.setAttribute('data-echo-placeholder', elem.src);
+          //add by Alon Zhang | zhangjinlong3546@sina.com
+          //Only the first time, storage placeholder picture
+          if(!elem.getAttribute('data-echo-placeholder')){
+            elem.setAttribute('data-echo-placeholder', elem.src);
+          }
         }
 
         if (elem.getAttribute('data-echo-background') !== null) {