What does GmsCoreServiceResolver do?

[jonsmirl]

I can't see anywhere in the code that calls GmsCoreServiceResolver. And if I put breakpoints in it, I can't hit them.
So what does this file do? It doesn't seem to be doing anything but overriding some internal GMS function and printing some debug.

How does this relate to NsdManager.DiscoveryListener?

[pierredelisle]

In the CHIP SDK, there is an Android interface binding for mDNS service resolution called ServiceResolver. A default implementation, NsdManagerServiceResolver, is provided as a convenience, using the Android platform NsdManager.

See how it is used in ChipClient: https://github.com/google-home/sample-app-for-matter-android/blob/main/app/src/main/java/com/google/homesampleapp/chip/ChipClient.kt#L56

Unfortunately, the use of NsdManagerServiceResolver is not spec compliant and is problematic when used with Matter. For example, it only returns 1 address and if there is an IPv4 address, it likely returns that one, which is not spec-defined/compliant.

The Google Home API provides “resolveService” on the local network for the Matter DNS-SD service. (see https://developers.home.google.com/reference/com/google/android/gms/home/matter/discovery/DiscoveryClient).

As its comments say, GmsCoreServiceResolver is “mDNS service resolver that leverages the GMSCore mDNS discovery API.”

I ran into issues when testing and put it aside. This issue should act as a reminder to get back to it.

[jonsmirl]

I used NsdManager directly like you did here:
https://github.com/google-home/sample-app-for-matter-android/blob/main/app/src/main/java/com/google/homesampleapp/screens/commissionable/mdns/MatterBeaconProducerMdns.kt#L61

Doing that causes an issue in NsdServiceFinder...
project-chip/connectedhomeip#26644

It is fairly hard to hit that null ref, I hit it about twice a week when running tests.