ESP8266mDNS - Add support for overwriting/removing TXT records

[WeeJeWel]

I advertise a device's name on mDNS using MDNS.addServiceTxt("myservice", "tcp", "name", "Foo");, but this name can change over time by user input. When calling the method again, another TXT entry is added, but I expected the previous one to be replaced.

[igrr]

@swarren and @ccooper21 have contributed to mDNS library recently. Maybe they would be willing to implement this?

Alternatively, you can get MDNS object into a clean state like this:

MDNS = MDNSResponder();

which should delete the old one, freeing all resources, and create a new one. Now if this doesn't work for some reason, feel free to open a new issue :)

[mkillewald]

I realize this is probably low priority, but I would like to see support for being able to change or remove TXT records as well. I could not get the suggestion of MDNS = MDNSResponder() to work. It clears the MDNS object but then I can't seem to re-initialize the object with either MDNS.begin(), MDNS.setInstanceName(), or MDNS.addService().

UPDATE: I have added a method to clear the TXT record as a whole. This will clear all key, value pairs and allow the entire TXT record to be rebuilt as needed. I will submit a pull request.

@mkillewald
Thanks for the UPDATE. But when I keep monitoring the updates on my Android app, the updates are not realtime / immediate.

  MDNS.clearServiceTxt("http", "tcp");
  MDNS.addServiceTxt("http", "tcp", "name",myname); 

Please suggest if I am wrong or let me know how can I instantly update my client mobile app.

[mkillewald]

@romuye123
Due to the nature of mDNS the updates are not going to occur immediately. As a work around for this in my mobile clients, I've made a refresh action that drops all discovered nodes and re-discovers them causing the changes to be shown. Hope that helps!

[devyte]

Closing in view of #5442 with a full rewrite of mdns.