How can I make my iPhone automatically connect to a VPN when connecting to a WiFi?

When connecting to public WiFi networks, I prefer to have more privacy and also want to be sure that really every network packet is encrypted1. You can achieve that by connecting to a VPN, for example. On an iPhone, you can only connect to a VPN if you already have an internet connection. Well, that's trivial and always the case, but on iPhone you have to connect to a WiFi first, to manually connect to a VPN afterwards. So there will be a short moment, where your apps (background sync etc.), will send the traffic without using a VPN. Furthermore, if you put your iPhone aside or lock it immediately, the WiFi will disconnect and thus the VPN will disconnect. If you unlock your iPhone again or your iPhone wants to perform some background synchronization, WiFi will reconnect, but VPN will not. Again, sensitive data might leak. **So, how can I make my iPhone automatically connect to a VPN when connecting to a WiFi?** ---------- 1Most data will be encrypted nowadays anyway, but: * there might be an app with security issues in their protocol * or there might be an app that just does not encrypt every data * or there might be encrypted data accompanied by unencrypted metadata