TL;DR;
Is it best practice to always make sure to connect to the drone via WIFI-password instead of QR-code to prevent or minimize the risk of being unable to reconnect, if you lose your connection.. in the case locking the iPhone screen for example?

Longer version:
I just bought a MA and have had some scary moments with it using only the iPhone 7+ for controlling the MA.

I connected the iPhone 7+ to the drone with the DJI 4 Go app using QR-code.
I told the drone to follow me using "Active Track" I put the phone in my pocket.
After I noticed that the MA stopped following me I stopped and unlocked the phone only to realize it was disconnected. I thought I could just reconnect, but I couldn't. The drone was hovering 4 meters up in the air mocking me. I tried restarting the app and connect again. The app continued the mocking, telling me to hit the button in the back of the drone and keeping my phone 50 cm or closer to the drone. It was 4 meters up in the air, and I'm quite short. I was f****d.

So, I'm wondering. Since there's a WIFI-password on the side of the drone, would I've been able to reconnect to the drone via WIFI if I've chosen password instead o QR-code to connect to the drone. I mean, the WIFI-password is stored on the phone, but the QR-code is hard to read from the phone if the drone is 4 meters up in the air.

It's important to me to be able to use the iPhone alone as the controller, without the RC, because I want to be able to get rid of the controller fast in my jacket when I'm about to ski and let the drone follow me.

You need to set it up for either phone control or RC control, it's 2 separate modes, you have to put the AC into either mode.To set this you need to press the button on the back of the AC, so no , you would not be able to do this while in flight, unless you came back and landed

Each time you want to use the other mode you have to reset the AC using the button on the back of the AC , for wi-fi only mode using only the phone, see 3.30 minutes into the video.

Thanks for your reply Lannes. However, I'm not wondering about how to connect to the to the AC with RC vs WIFI.  I'm only curious about the two ways of connecting to the AC via WIFI.
A. Reading the QR-code
B. Entering the WIFI-password to the Mavic Airs own Wi-fi network.

Scenario: Let's say you start the drone, get it going into the air. You lock your phone and the connection is lost.. this means your control of the drone is doomed. You can't reconnect, right?

A. With this alternative you can only reconnect to the drone, once it has run out of battery and is back on the ground?
B. When you unlock your phone again and find the drones WIFI-network you should hopefully be able to reconnect, even if the connetion is lost? This would be very helpful if the drone is high up in the air.'

Has anyone tried this or had any problems with reconnecting to the drone after the WIFI connection is lost.

Hi there, it is recommended to connect the aircraft via WiFi password. The password will be saved in the mobile device automatically.

Thank you Susan for clearing that out! I will always use WiFi-password from now on. I have to say that the QR-scanning alternative is very dangerous, in my opinion, since the password isn't stored.