Given that you can request more than one city in parallel, a single global flag on the app for whether there's a pending request seems like it will mess things up pretty bad.
Thanks for the feedback. I've rewritten the codelab from scratch and believe I have fixed this issue. The code has been updated, and the new guide is now live.
If you find any issues with the new codelab, please let us know!
Given that you can request more than one city in parallel, a single global flag on the app for whether there's a pending request seems like it will mess things up pretty bad.