add Server side event support

[WolfgangFahl]

see

• https://html.spec.whatwg.org/multipage/server-sent-events.html
• https://stackoverflow.com/a/51969441/1497139

Code snippet:

 def getEvent(self):
        '''this could be any function that blocks until data is ready'''
        time.sleep(1.0)
        s=datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S')
        return s    

    def eventFeed(self):
        return Response(self.genEventStream(), mimetype="text/event-stream")

    def genEventStream(self):
        while True:
            # wait for source data to be available, then push it
            yield 'data: {}\n\n'.format(self.getEvent())

[WolfgangFahl]

see

• https://github.com/singingwolfboy/flask-sse
• https://maxhalford.github.io/blog/flask-sse-no-deps/
• https://github.com/MaxHalford/flask-sse-no-deps

[WolfgangFahl]

• https://datatables.net/forums/discussion/26983/how-to-do-live-table-updates-with-html5-sse

[WolfgangFahl]

http://wiki.bitplan.com/index.php/Server-Sent-Events

[WolfgangFahl]

What pubsub module to use? See

• https://stackoverflow.com/questions/1092531/event-system-in-python
• https://stackoverflow.com/questions/115844/recommended-python-publish-subscribe-dispatch-module
• http://pydispatcher.sourceforge.net/
• https://pypi.org/project/PyPubSub/
• https://www.blog.pythonlibrary.org/2013/09/06/wxpython-using-pydispatcher-instead-of-pubsub/
• https://pypi.org/project/PyDispatcher/

[WolfgangFahl]

see

• https://github.com/WolfgangFahl/pyFlaskBootstrap4/blob/main/fb4_example/bootstrap_flask/templates/event.html for the javascript code