Add $let widget (#6148)

* $let widget added and tested

* Documentation for $let, doc improvements for $vars

* let properly avoids refreshing when possible

* $let Changes as recommended by others

* Removed superfluous super method call

Also improved $let test

core/modules/widgets/let.js

@@ -0,0 +1,96 @@
+/*\
+title: $:/core/modules/widgets/let.js
+type: application/javascript
+module-type: widget
+
+This widget allows defining multiple variables at once, while allowing
+the later variables to depend upon the earlier ones.
+
+```
+\define helloworld() Hello world!
+<$let currentTiddler="target" value={{!!value}} currentTiddler="different">
+  {{!!value}} will be different from <<value>>
+</$let>
+```
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var Widget = require("$:/core/modules/widgets/widget.js").widget;
+
+var LetWidget = function(parseTreeNode,options) {
+	// Initialise
+	this.initialise(parseTreeNode,options);
+};
+
+/*
+Inherit from the base widget class
+*/
+LetWidget.prototype = new Widget();
+
+/*
+Render this widget into the DOM
+*/
+LetWidget.prototype.render = function(parent,nextSibling) {
+	this.parentDomNode = parent;
+	this.computeAttributes();
+	this.execute();
+	this.renderChildren(parent,nextSibling);
+};
+
+LetWidget.prototype.computeAttributes = function() {
+	// Before computing attributes, we must make clear that none of the
+	// existing attributes are staged for lookup, even on a refresh
+	var changedAttributes = {},
+		self = this;
+	this.currentValueFor = Object.create(null);
+	$tw.utils.each(this.parseTreeNode.orderedAttributes,function(attribute,index) {
+		var value = self.computeAttribute(attribute),
+			name = attribute.name;
+		if(name.charAt(0) !== "$") {
+			// Now that it's prepped, we're allowed to look this variable up
+			// when defining later variables
+			self.currentValueFor[name] = value;
+		}
+	});
+	// Run through again, setting variables and looking for differences
+	$tw.utils.each(this.currentValueFor,function(value,name) {
+		if (self.attributes[name] !== value) {
+			self.attributes[name] = value;
+			self.setVariable(name,value);
+			changedAttributes[name] = true;
+		}
+	});
+	return changedAttributes;
+};
+
+LetWidget.prototype.getVariableInfo = function(name,options) {
+	// Special handling: If this variable exists in this very $let, we can
+	// use it, but only if it's been staged.
+	if ($tw.utils.hop(this.currentValueFor,name)) {
+		return {
+			text: this.currentValueFor[name]
+		};
+	}
+	return Widget.prototype.getVariableInfo.call(this,name,options);
+};
+
+/*
+Refresh the widget by ensuring our attributes are up to date
+*/
+LetWidget.prototype.refresh = function(changedTiddlers) {
+	var changedAttributes = this.computeAttributes();
+	if($tw.utils.count(changedAttributes) > 0) {
+		this.refreshSelf();
+		return true;
+	}
+	return this.refreshChildren(changedTiddlers);
+};
+
+exports["let"] = LetWidget;
+
+})();