Android: Issues on hot-restart

[julianscheel]

If an app using flutter_nsd is hot-restarted while discovery is active, calling startDiscovery() will fail.
In case timber logging is active it can be seen that a warning is printed, because NSD is already active, but no error is sent back to the app.
This is due to the exception handler in private fun startDiscovery(serviceType: String), which is doing nothing but printing the warning.

To get working discovery after a hot-restart, currently a cycle of discoverServices()->stopDiscovery()->startDiscovery() is required. If the initial error would be notified back to the app it could detect that error condition and only then do an additional stop/start cycle.

Doing only stopDiscovery()->startDiscovery() after a restart is currently not working, because the onDiscoveryStopped handler is called after startDiscovery() only, as at the time of the initial stopDiscovery() call, no method handler is set in the plugin's channel.

[julianscheel]

Effectively it's even a bit worse. Due to stopDiscovery() not being synchronous and no callback being existant for onDiscoveryStopped, the simple start,stop,start sequence is unreliable and currently the only way out is doing some extra waiting.

[Nimrodda]

Thanks for reporting this issue. Have you tried what happens on iOS in this case?

[julianscheel]

It's actually pretty much the same on iOS. Just the plugin's native code is missing even more things to make it possible to detect this error condition on iOS. Working on this right now.

For Android I've got a branch which makes it possible to detect the error and work around it on the app level: https://github.com/jusst-engineering/flutter_nsd/tree/fix/notify-startdiscovery-failure

[Nimrodda]

Pull Request would be greatly appreciated 👍