From 1e780a7961607f60d0fd10d0d665dafbb9d94d45 Mon Sep 17 00:00:00 2001
From: carson <carson@leaddyno.com>
Date: Sun, 3 Jan 2021 15:59:01 -0700
Subject: [PATCH] include `preload` in the website

---
 scripts/www.js           |   1 +
 www/_includes/layout.njk |   1 +
 www/js/preload.js        | 130 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 132 insertions(+)
 create mode 100644 www/js/preload.js
diff --git a/scripts/www.js b/scripts/www.js
index 44a6671b..f5a3a3b7 100644
--- a/scripts/www.js
+++ b/scripts/www.js
@@ -14,6 +14,7 @@ fs.copySync("test/", currentReleaseRoot + "/test");
 fs.copySync("src/", currentReleaseRoot + "/src");
 fs.copySync("src/htmx.js", "www/js/htmx.js");
 fs.copySync("src/ext/class-tools.js", "www/js/class-tools.js");
+fs.copySync("src/ext/preload.js", "www/js/preload.js");
 
 var testHTML = "<html><body style='font-family: sans-serif'><h1>HTMX TESTS</h1><ul>\n"
 fs.readdirSync(testRoot).reverse().forEach(function(file){
diff --git a/www/_includes/layout.njk b/www/_includes/layout.njk
index 8fb1e2dc..405f2a6d 100644
--- a/www/_includes/layout.njk
+++ b/www/_includes/layout.njk
@@ -8,6 +8,7 @@
     <script src="https://unpkg.com/prismjs@1.20.0/plugins/autoloader/prism-autoloader.min.js"></script>
     <script src="/js/htmx.js"></script>
     <script src="/js/class-tools.js"></script>
+    <script src="/js/preload.js"></script>
     <script>
         htmx.onLoad(function(){
             Prism.highlightAll();
diff --git a/www/js/preload.js b/www/js/preload.js
new file mode 100644
index 00000000..46355d2f
--- /dev/null
+++ b/www/js/preload.js
@@ -0,0 +1,130 @@
+// This adds the "preload" extension to htmx.  By default, this will 
+// preload the targets of any tags with `href` or `hx-get` attributes 
+// if they also have a `preload` attribute as well.  See documentation
+// for more detauls
+htmx.defineExtension("preload", {
+
+	onEvent: function(name, event) {
+
+		// Only take actions on "htmx:afterProcessNode"
+		if (name !== "htmx:afterProcessNode") {
+			return;
+		}
+
+		// SOME HELPER FUNCTIONS WE'LL NEED ALONG THE WAY
+
+		// config gets the closest non-empty value from the preload="" attribute. (default = "mousedown")
+		var config = function(node) {
+			if (node == undefined) {return undefined;}
+			return node.getAttribute("preload") || node.getAttribute("data-preload") || config(node.parentElement) || "mousedown"
+		}
+		
+		// load handles the actual HTTP fetch, and uses htmx.ajax in cases where we're 
+		// preloading an htmx resource (this sends the same HTTP headers as a regular htmx request)
+		var load = function(node) {
+
+			return function() {
+
+				// If this value has already been loaded, then do not try again.
+				if (node.preloadState !== "READY") {
+					return;
+				}
+
+				// This is used after a successful AJAX request, to mark the
+				// content as loaded (and prevent additional AJAX calls.)
+				var done = function() {
+					node.preloadState = "DONE"
+				}
+
+				// Special handling for HX-GET - use built-in htmx.ajax function
+				// so that headers match other htmx requests, then set 
+				// node.preloadState = TRUE so that requests are not duplicated
+				// in the future
+				if (node.getAttribute("hx-get")) {
+					htmx.ajax("GET", node.getAttribute("hx-get"), {handler:done});
+					return;
+				}
+
+				// Otherwise, perform a standard xhr request, then set 
+				// node.preloadState = TRUE so that requests are not duplicated 
+				// in the future.
+				if (node.getAttribute("href")) {
+					var r = new XMLHttpRequest();
+					r.open("GET", node.getAttribute("href"));
+					r.onload = done;
+					r.send();
+					return;
+				}
+			}
+		}
+
+		// This function processes a specific node and sets up event handlers.
+		// We'll search for nodes and use it below.
+		var init = function(node) {
+
+			// If this node DOES NOT include a "GET" transaction, then there's nothing to do here.
+			if (node.getAttribute("href") + node.getAttribute("hx-get") + node.getAttribute("data-hx-get") == "") {
+				return;
+			}
+
+			// Guarantee that we only initialize each node once.
+			if (node.preloadState !== undefined) {
+				return;
+			}
+			
+			// Get event name from config.
+			var on = config(node)
+						
+			// FALL THROUGH to here means we need to add an EventListener
+	
+			// Apply the listener to the node
+			node.addEventListener(on, function(evt) {
+				if (node.preloadState === "PAUSE") { // Only add one event listener
+					node.preloadState = "READY"; // Requred for the `load` function to trigger
+
+					// Special handling for "mouseover" events.  Wait 100ms before triggering load.
+					if (on === "mouseover") {
+						window.setTimeout(load(node), 100);
+					} else {
+						load(node)() // all other events trigger immediately.
+					}
+				}
+			})
+
+			// Special handling for certain built-in event handlers
+			switch (on) {
+
+				case "mouseover":
+					// Mirror `touchstart` events (fires immediately)
+					node.addEventListener("touchstart", load(node));
+
+					// WHhen the mouse leaves, immediately disable the preload
+					node.addEventListener("mouseout", function(evt) {
+						if ((evt.target === node) && (node.preloadState === "READY")) {
+							node.preloadState = "PAUSE";
+						}
+					})
+					break;
+
+				case "mousedown":
+					 // Mirror `touchstart` events (fires immediately)
+					node.addEventListener("touchstart", load(node));
+					break;
+			}
+
+			// Mark the node as ready to run.
+			node.preloadState = "PAUSE";
+			htmx.trigger(node, "preload:init") // This event can be used to load content immediately.
+		}
+
+		// Search for all child nodes that have a "preload" attribute
+		event.target.querySelectorAll("[preload]").forEach(function(node) {
+
+			// Initialize the node with the "preload" attribute
+			init(node)
+
+			// Initialize all child elements that are anchors or have `hx-get` (use with care)
+			node.querySelectorAll("a,[hx-get],[data-hx-get").forEach(init)
+		})
+	}
+})
