If you have the latest iPhones and you plan to be in mainly Beijing or Shanghai, if you set the region to China, you can add Shanghai and/or Beijing metro cards to your Wallet app. You can sign up and everything right on the iPhone. The digital card is written into the NFC chip on your phone and you can choose your default metro card and use it without having to unlock your phone. Even if your phone powers off due to low battery, the metro card will still work.

(You can change the region back if you want after setup)

https://support.apple.com/en-us/HT208539