You also need to update NetworkStateHandler to propagate the correct event. It currently calls the DeviceListChanged() handler. Theoretically, this should only be called if devices are added/removed from the list, and DevicePropertiesUpdated() should be called when a property of a device changes.

However, looking at the other instances of devices changing, it seems that several other places call DeviceListChanged() when only properties of a single device are being changed.

stevenjb@: Can you clarify when one should be called and when the other should be called? For example, [1] shows it being called in UpdateDeviceProperty(), which seems incorrect to me.

[1] https://cs.chromium.org/chromium/src/chromeos/network/network_state_handler.cc?l=1177

Both of those observers have evolved over time.

At this point DeviceListChanged is a superset; as the header comment says "The list of devices changed, or a property changed". It is intended to be used by observers that want to know when a device property may have changed. This shouldn't be all that frequent. Could you elaborate on what "almost anything" is in your observations?

DevicePropertiesUpdated should be used when updates for properties on a specific device are desired, but changes to the list of devices (e.g. adding or removing a device) are not interesting.

Thanks for the explanation, Steven!

So, looking into this more, I realize that the DeviceState class doesn't actually contain the technology state (enabled, unavailable, etc). That is stored within NetworkStateHandler. Looks like the correct thing to do is what we're already doing.

Thus, the only real issue here is that we log too much. I'll spin up a quick CL to fix this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9aa44ab0d44d18475363d33873f8160d9022cab1

commit 9aa44ab0d44d18475363d33873f8160d9022cab1
Author: khorimoto <khorimoto@chromium.org>
Date: Wed Jun 28 23:33:39 2017

[CrOS Tether] Remove spammy log from Initializer.

Previously, we printed a warning log each time Init() was called when the feature had already been initialized. This resulted in spammy logs since this is expected to happen under some circumstances.

This CL also adds some extra tests in TetherService which ensure that Init() is called correctly.

BUG=672263, 736443 

Review-Url: https://codereview.chromium.org/2961733002
Cr-Commit-Position: refs/heads/master@{#483195}

[modify] https://crrev.com/9aa44ab0d44d18475363d33873f8160d9022cab1/chrome/browser/chromeos/tether/tether_service.cc
[modify] https://crrev.com/9aa44ab0d44d18475363d33873f8160d9022cab1/chrome/browser/chromeos/tether/tether_service.h
[modify] https://crrev.com/9aa44ab0d44d18475363d33873f8160d9022cab1/chrome/browser/chromeos/tether/tether_service_unittest.cc
[modify] https://crrev.com/9aa44ab0d44d18475363d33873f8160d9022cab1/chromeos/components/tether/initializer.cc
[modify] https://crrev.com/9aa44ab0d44d18475363d33873f8160d9022cab1/chromeos/components/tether/initializer.h