Alternate fix for inconsistent list template syntax (#7827)

* Alternate fix for inconsistent list template syntax

First attempt, which fails on the ListWidget/WithMissingTemplate test.

* Make WithMissingTemplate test pass, inefficiently

Unfortunately, this ends up being very inefficient, because the
clone-and-mutate logic is repeated for every list item. Not ideal.

* More efficient way to do it

This also makes the failing test pass, but far more efficiently.

* Improve performance of list template discovery

Since parse tree nodes never change after widget creation (whereas
attribute values *can* change), we can safely search for the explicit
list templtaes only once, at widget creation time. This saves time as
the search doesn't have to be done on each re-render, and also allows us
to safely do a clone-and-mutate step to extract the list widget's body
(if any) without any `$list-empty` or other items. That, in turn, allows
using the list widget's body as the template even if `$list-empty` is
specified inside the widget body.

core/modules/widgets/list.js

@@ -28,6 +28,18 @@ Inherit from the base widget class
 */
 ListWidget.prototype = new Widget();
 
+ListWidget.prototype.initialise = function(parseTreeNode,options) {
+	// Bail if parseTreeNode is undefined, meaning that the ListWidget constructor was called without any arguments so that it can be subclassed
+	if(parseTreeNode === undefined) {
+		return;
+	}
+	// First call parent constructor to set everything else up
+	Widget.prototype.initialise.call(this,parseTreeNode,options);
+	// Now look for <$list-template> and <$list-empty> widgets as immediate child widgets
+	// This is safe to do during initialization because parse trees never change after creation
+	this.findExplicitTemplates();
+}
+
 /*
 Render this widget into the DOM
 */
@@ -68,8 +80,6 @@ ListWidget.prototype.execute = function() {
 	this.counterName = this.getAttribute("counter");
 	this.storyViewName = this.getAttribute("storyview");
 	this.historyTitle = this.getAttribute("history");
-	// Look for <$list-template> and <$list-empty> widgets as immediate child widgets
-	this.findExplicitTemplates();
 	// Compose the list elements
 	this.list = this.getTiddlerList();
 	var members = [],
@@ -92,6 +102,7 @@ ListWidget.prototype.findExplicitTemplates = function() {
 	var self = this;
 	this.explicitListTemplate = null;
 	this.explicitEmptyTemplate = null;
+	this.hasTemplateInBody = false;
 	var searchChildren = function(childNodes) {
 		$tw.utils.each(childNodes,function(node) {
 			if(node.type === "list-template") {
@@ -100,6 +111,8 @@ ListWidget.prototype.findExplicitTemplates = function() {
 				self.explicitEmptyTemplate = node.children;
 			} else if(node.type === "element" && node.tag === "p") {
 				searchChildren(node.children);
+			} else {
+				self.hasTemplateInBody = true;
 			}
 		});
 	};
@@ -160,11 +173,11 @@ ListWidget.prototype.makeItemTemplate = function(title,index) {
 			// Check for a <$list-item> widget
 			if(this.explicitListTemplate) {
 				templateTree = this.explicitListTemplate;
-			} else if (!this.explicitEmptyTemplate) {
+			} else if(this.hasTemplateInBody) {
 				templateTree = this.parseTreeNode.children;
 			}
 		}
-		if(!templateTree) {
+		if(!templateTree || templateTree.length === 0) {
 			// Default template is a link to the title
 			templateTree = [{type: "element", tag: this.parseTreeNode.isBlock ? "div" : "span", children: [{type: "link", attributes: {to: {type: "string", value: title}}, children: [
 				{type: "text", text: title}
@@ -414,4 +427,27 @@ ListItemWidget.prototype.refresh = function(changedTiddlers) {
 
 exports.listitem = ListItemWidget;
 
+/*
+Make <$list-template> and <$list-empty> widgets that do nothing
+*/
+var ListTemplateWidget = function(parseTreeNode,options) {
+	// Main initialisation inherited from widget.js
+	this.initialise(parseTreeNode,options);
+};
+ListTemplateWidget.prototype = new Widget();
+ListTemplateWidget.prototype.render = function() {}
+ListTemplateWidget.prototype.refresh = function() { return false; }
+
+exports["list-template"] = ListTemplateWidget;
+
+var ListEmptyWidget = function(parseTreeNode,options) {
+	// Main initialisation inherited from widget.js
+	this.initialise(parseTreeNode,options);
+};
+ListEmptyWidget.prototype = new Widget();
+ListEmptyWidget.prototype.render = function() {}
+ListEmptyWidget.prototype.refresh = function() { return false; }
+
+exports["list-empty"] = ListEmptyWidget;
+
 })();