return top logic

@@ -343,7 +343,7 @@
             </div>
         </div>
     </div>
-    <div class="return-top">
+    <div class="return-top hide">
         <span class="iconfont">&#xe610;</span>
     </div>
 </div>
\ No newline at end of file

@@ -10,8 +10,6 @@ require('./footer');
 require('./simple-header');
 require('./header');
 
-
-
 function cookie(name) {
     var cookies = document.cookie,
         cookieVal,

@@ -6,6 +6,8 @@
 
 var $ = require('yoho.jquery');
 
+var $returnTop = $('.return-top');
+
 var apiDomain;
 
 /**
@@ -126,6 +128,39 @@ function actionhomeFootChange() {
     });
 }
 
+function rePosReturn() {
+    if ($(window).height() < $(document).height()) {
+        $returnTop.removeClass('hide');
+    } else {
+        $returnTop.addClass('hide');
+    }
+}
+
+//返回顶部
+$returnTop.click(function() {
+    $('html,body').animate({
+        scrollTop: 0
+    }, 500);
+});
+
+$(window).scroll(function() {
+    if ($(window).scrollTop() === 0) {
+        $returnTop.addClass('hide');
+    } else {
+        $returnTop.removeClass('hide');
+    }
+});
+
+rePosReturn();
+
+//如果初始是隐藏的，监听图片加载，重新确定return-top的高度
+if ($returnTop.hasClass('hide')) {
+    $('img').load(rePosReturn);
+}
+
 //初始化
 actionSubscription();
 actionhomeFootChange();
+
+//暴露给有可能通AJAX改变内容的页面去用
+window.rePosReturnTop = rePosReturn;