问题
I've created an iOS app that allows a user to log in with Instagram. I use a web view to display Instagram's authorization page as detailed here. This works great for Instagram users who signed up with a user name and password. However, Instagram also offers the option for new users to create an account using their Facebook account, and users who have gone this route are unable to log into my application because the Instagram login page doesn't give an option to authenticate with Facebook. How do I allow these users to log into my app?
来源：https://stackoverflow.com/questions/23587765/how-do-i-authenticate-an-instagram-user-who-created-an-account-with-facebook-in