💄 refactor infinite scroll (#325)

closes #321, closes #323
- use `requestAnimationFrame` and related techniques to minimise scroll processing
- add an `isLoading` guard to ensure we don't try to process anything until the next page has been loaded+inserted
- check for a 404 as an indication we've loaded as many pages as possible at which point the scroll behaviour is removed
- sanitize `window.location.pathname` to remove hash params and any double-slashes that may result
- add a 100px buffer from the bottom of the screen so that it's not necessary to exactly hit the bottom before infinite loading is triggered

assets/js/infinitescroll.js

@@ -1,29 +1,78 @@
 // Code snippet inspired by https://github.com/douglasrodrigues5/ghost-blog-infinite-scroll
 $(function ($) {
-    var currentPage = 1,
-        pathname = window.location.pathname,
-        $window = $(window),
-        $document = $(document),
-        $result = $('.post-feed');
+    var currentPage = 1;
+    var pathname = window.location.pathname;
+    var $document = $(document);
+    var $result = $('.post-feed');
+    var buffer = 100;
 
-    function handleScroll () {
-        // return if not scroll to the bottom
-        if ($window.scrollTop() + $window.height() !== $document.height()) {
+    var ticking = false;
+    var isLoading = false;
+
+    var lastScrollY = window.scrollY;
+    var lastWindowHeight = window.innerHeight;
+    var lastDocumentHeight = $document.height();
+
+    // remove hash params from pathname
+    pathname = pathname.replace(/#(.*)$/g, '').replace('/\//g', '/');
+
+    function onScroll() {
+        lastScrollY = window.scrollY;
+        requestTick();
+    }
+
+    function onResize() {
+        lastWindowHeight = window.innerHeight;
+        lastDocumentHeight = $document.height();
+        requestTick();
+    }
+
+    function requestTick() {
+        if (!ticking) {
+            requestAnimationFrame(infiniteScroll)
+        }
+        ticking = true;
+    }
+
+    function infiniteScroll () {
+        // return if already loading
+        if (isLoading) {
             return;
         }
 
-        if (currentPage >= maxPages) {
-            return $window.off('scroll', handleScroll);
+        // return if not scroll to the bottom
+        if (lastScrollY + lastWindowHeight <= lastDocumentHeight - buffer) {
+            ticking = false;
+            return;
         }
 
+        isLoading = true;
+
         // next page
         currentPage++;
 
         // Load more
-        $.get((pathname + 'page/' + currentPage + '/'), function (content) {
+        var nextPage = pathname + 'page/' + currentPage + '/';
+
+        $.get(nextPage, function (content) {
             $result.append($(content).find('.post').hide().fadeIn(100));
+
+        }).fail(function (xhr) {
+            // 404 indicates we've run out of pages
+            if (xhr.status === 404) {
+                window.removeEventListener('scroll', onScroll, {passive: true});
+                window.removeEventListener('resize', onResize);
+            }
+
+        }).always(function () {
+            lastDocumentHeight = $document.height();
+            isLoading = false;
+            ticking = false;
         });
     }
 
-    $window.on('scroll', handleScroll).trigger('scroll');
+    window.addEventListener('scroll', onScroll, {passive: true});
+    window.addEventListener('resize', onResize);
+
+    infiniteScroll();
 });