Add server sent events (#5279)

* Create server-sent-events.js

* Create sse-change-listener.js

* Implement server sent events

* Convert to ES5 and wrap in function

* Use the host string from tiddlyweb

* Improve comments in sse-server.js

* Can't use object reference as key

* Add retry timeout

* Fix a bug

* bug fix

* Fix formatting

* Fix ES5 compat

* capitalize comments

* more fixes

* Refactor tiddlywek/sse-server.js

* Extract helper functions for handling wikis and connections.
* Replace JSDoc comments.
* Fix formatting according to TW core.
* Simplify the logic for adding and removing connections.

* Fix formatting of tiddlyweb/sse-client.js

Fix formatting according to TW core.

* Fix formatting of server-sent-events.js

Fix formatting and comments following TW core guidelines.

* Extract a debounce function in sse-client.js

* Avoid using startsWith in server-sent-events.js

startsWith is part of ES2015, while TiddlyWiki uses the 5.1 dialect.

* New sse-enabled WebServer parameter

* If not set to "yes", disabled SSE request handling.
* Add documentation for the parameter in core/language/en-GB/Help/listen.tid
* Add new tiddler editions/tw5.com/tiddlers/webserver/WebServer Parameter_ sse-enabled.tid

* Disable polling for changes if SSE is enabled

* Add sse_enabled to /status JSON response
* Store syncer polling status in $:/config/SyncDisablePolling
* Handled disabling polling in core/modules/syncer.js

* Simply boolean logic in syncer.js

* Delete trailing whitespaces in syncer.js

Co-authored-by: Arlen22 <arlenbee@gmail.com>

plugins/tiddlywiki/tiddlyweb/sse-client.js

@@ -0,0 +1,53 @@
+/*\
+title: $:/plugins/tiddlywiki/tiddlyweb/sse-client.js
+type: application/javascript
+module-type: startup
+
+GET /recipes/default/tiddlers/:title
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+exports.name = "/events/plugins/tiddlywiki/tiddlyweb";
+exports.after = ["startup"];
+exports.synchronous = true;
+exports.platforms = ["browser"];
+exports.startup = function() {
+	// Make sure we're actually being used
+	if($tw.syncadaptor.name !== "tiddlyweb") {
+		return;
+	}
+	// Get the mount point in case a path prefix is used
+	var host = $tw.syncadaptor.getHost();
+	// Make sure it ends with a slash (it usually does)
+	if(host[host.length - 1] !== "/") {
+		host += "/";
+	}
+	// Setup the event listener
+	setupEvents(host);
+};
+
+function debounce(callback) {
+	var timeout = null;
+	return function() {
+		clearTimeout(timeout);
+		timeout = setTimeout(callback,$tw.syncer.throttleInterval);
+	};
+}
+
+function setupEvents(host) {
+	var events = new EventSource(host + "events/plugins/tiddlywiki/tiddlyweb");
+	var debouncedSync = debounce($tw.syncer.syncFromServer.bind($tw.syncer));
+	events.addEventListener("change",debouncedSync);
+	events.onerror = function() {
+		events.close();
+		setTimeout(function() {
+			setupEvents(host);
+		},$tw.syncer.errorRetryInterval);
+	};
+}
+})();

plugins/tiddlywiki/tiddlyweb/sse-server.js

@@ -0,0 +1,94 @@
+/*\
+title: $:/plugins/tiddlywiki/tiddlyweb/sse-server.js
+type: application/javascript
+module-type: route
+
+GET /events/plugins/tiddlywiki/tiddlyweb
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var wikis = [];
+var connections = [];
+
+/*
+Setup up the array for this wiki and add the change listener
+*/
+function setupWiki(wiki) {
+	var index = wikis.length;
+	// Add a new array for this wiki (object references work as keys)
+	wikis.push(wiki);
+	connections.push([]);
+	// Listen to change events for this wiki
+	wiki.addEventListener("change",function(changes) {
+		var jsonChanges = JSON.stringify(changes);
+		getWikiConnections(wiki).forEach(function(item) {
+			item.emit("change",jsonChanges);
+		});
+	});
+	return index;
+}
+
+/*
+Setup this particular wiki if we haven't seen it before
+*/
+function ensureWikiSetup(wiki) {
+	if(wikis.indexOf(wiki) === -1) {
+		setupWiki(wiki);
+	}
+}
+
+/*
+Return the array of connections for a particular wiki
+*/
+function getWikiConnections(wiki) {
+	return connections[wikis.indexOf(wiki)];
+}
+
+function addWikiConnection(wiki,connection) {
+	getWikiConnections(wiki).push(connection);
+}
+
+function removeWikiConnection(wiki,connection) {
+	var wikiConnections = getWikiConnections(wiki);
+	var index = wikiConnections.indexOf(connection);
+	if(index !== -1) {
+		wikiConnections.splice(index,1);
+	}
+}
+
+function handleConnection(request,state,emit,end) {
+	if(isDisabled(state)) {
+		return;
+	}
+
+	ensureWikiSetup(state.wiki);
+	// Add the connection to the list of connections for this wiki
+	var connection = {
+		request: request,
+		state: state,
+		emit: emit,
+		end: end
+	};
+	addWikiConnection(state.wiki,connection);
+	request.on("close",function() {
+		removeWikiConnection(state.wiki,connection);
+	});
+}
+
+function isDisabled(state) {
+	return state.server.get("sse-enabled") !== "yes";
+}
+
+// Import the ServerSentEvents class
+var ServerSentEvents = require("$:/core/modules/server/server-sent-events.js").ServerSentEvents;
+// Instantiate the class
+var events = new ServerSentEvents("plugins/tiddlywiki/tiddlyweb", handleConnection);
+// Export the route definition for this server sent events instance
+module.exports = events.getExports();
+
+})();

plugins/tiddlywiki/tiddlyweb/tiddlywebadaptor.js

@@ -91,10 +91,12 @@ TiddlyWebAdaptor.prototype.getStatus = function(callback) {
 				self.isLoggedIn = json.username !== "GUEST";
 				self.isReadOnly = !!json["read_only"];
 				self.isAnonymous = !!json.anonymous;
+
+				var isSseEnabled = !!json.sse_enabled;
 			}
 			// Invoke the callback if present
 			if(callback) {
-				callback(null,self.isLoggedIn,json.username,self.isReadOnly,self.isAnonymous);
+				callback(null,self.isLoggedIn,json.username,self.isReadOnly,self.isAnonymous,isSseEnabled);
 			}
 		}
 	});