Add a "Back to top" button.

Resolves #17.

Signed-off-by: Tao He <sighingnow@gmail.com>

_includes/footer.html

@@ -12,4 +12,6 @@
 <script src="{{site.baseurl}}/gitbook/gitbook-plugin-search-pro/jquery.mark.min.js"></script>
 <script src="{{site.baseurl}}/gitbook/gitbook-plugin-search-pro/search.js"></script>
 
+<script src="{{site.baseurl}}/gitbook/gitbook-plugin-back-to-top-button/plugin.js"></script>
+
 <script type="text/javascript" id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-svg.js"></script>

_includes/head.html

@@ -23,6 +23,7 @@
 <link rel="stylesheet" href="{{site.baseurl}}/gitbook/style.css">
 <link rel="stylesheet" href="{{site.baseurl}}/gitbook/gitbook-plugin-fontsettings/website.css">
 <link rel="stylesheet" href="{{site.baseurl}}/gitbook/gitbook-plugin-search-pro/search.css">
+<link rel="stylesheet" href="{{site.baseurl}}/gitbook/gitbook-plugin-back-to-top-button/plugin.css">
 
 <link rel="stylesheet" href="{{site.baseurl}}/gitbook/rouge/{{ site.syntax_highlighter_style }}.css">
 

gitbook/gitbook-plugin-back-to-top-button/plugin.css

@@ -0,0 +1,55 @@
+.back-to-top {
+    position: fixed;
+    bottom: 25px;
+    right: calc((100% - 300px - min(100% - 300px, 800px)) / 2 + 25px);
+    background: rgba(0, 0, 0, 0.5);
+    width: 50px;
+    height: 50px;
+    display: block;
+    text-decoration: none;
+    -webkit-border-radius: 35px;
+    -moz-border-radius: 35px;
+    border-radius: 35px;
+    display: none;
+}
+.back-to-top i {
+    color: #fff;
+    margin: 0;
+    position: relative;
+    left: 15px;
+    top: 14px;
+    font-size: 22px;
+}
+.back-to-top:hover {
+    background: rgba(0, 0, 0, 0.9);
+    cursor: pointer;	
+}
+.book.color-theme-1 .back-to-top {
+    background: rgba(112, 66, 20, 0.5);
+}
+.book.color-theme-1 .back-to-top i {
+    color: #f3eacb;
+}
+.book.color-theme-1 .back-to-top:hover {
+    background: rgba(112, 66, 20, 0.9);
+}
+.book.color-theme-2 .back-to-top {
+    background: rgba(189, 202, 219, 0.5);
+}
+.book.color-theme-2 .back-to-top i {
+    color: #1C1F2B;
+}
+.book.color-theme-2 .back-to-top:hover {
+    background: rgba(189, 202, 219, 0.9);
+}
+
+@media only screen 
+  and (min-device-width: 320px) 
+  and (max-device-width: 480px)
+  and (-webkit-min-device-pixel-ratio: 2)
+  and (orientation: portrait) {
+	.back-to-top {
+		bottom: 10px;
+		right: 10px;
+	}
+}

gitbook/gitbook-plugin-back-to-top-button/plugin.js

@@ -0,0 +1,32 @@
+require([
+    'gitbook',
+    'jquery'
+], function(gitbook, $) {
+    gitbook.events.on('page.change', function() {
+        var back_to_top_button = ['<div class="back-to-top"><i class="fa fa-arrow-up"></i></div>'].join("");
+        $(".page-wrapper").append(back_to_top_button)
+
+        $(".back-to-top").hide();
+
+        $(".back-to-top").hover(function() {
+            $(this).css('cursor', 'pointer').attr('title', 'Back to top');
+        }, function() {
+            $(this).css('cursor', 'auto');
+        });
+
+        $('.book-body,.body-inner').on('scroll', function () {
+            if ($(this).scrollTop() > 100) { 
+                $('.back-to-top').fadeIn();
+            } else {
+                $('.back-to-top').fadeOut();
+            }
+        });
+
+        $('.back-to-top').on('click', function () { 
+            $('.book-body,.body-inner').animate({
+                scrollTop: 0
+            }, 800);
+            return false;
+        });
+    });
+});