On openSUSE Tumbleweed, If I run "sudo zypper dub" while PackageKit is still
running, Zypper will ask me that whether I want PackageKit to stop. 

Unfortunately, even if I say yes, Zypper can't stop it no matter how many times
I say yes.

I have to stop PackageKit manually by: sudo systemctl stop packagekit

Only then, I can use Zypper.

This issue on openSUSE is very severe, since Zypper doesn't work in harmony
with PackageKit, unlike DNF and APT from my experiences with Fedora and Ubuntu.
On those distros, DNF or APT can run along PackageKit, no need to stop
PackageKit before DNF or APT operation. And to make matter worse, PackageKit
(GNOME Software) uses a huge amount of time to start and finish its operation
after the system boot.

To summarize this issue, Zypper should be able to stop PackageKit when the user
tells it to stop PackageKit. I don't know the code behind Zypper, but Zypper
should fire "sudo systemctl stop packagekit" when the user tells it to do so.