Xcode 8 says "Do you want to add a stub?" How do I answer?

The answer of raphh is right but xcode is still a little buggy here (at least for me),

If you don't select the Fix-it right after compiling the dot mark transform to exclamation mark and you can't make it appear again unless you try to build one more time.

Right after building : Dot error icon

When you don't do the Fix-it right away : exclamation mark error icon

You have to re-build to get the dot icon again when you have several methods to implement and you did do the Fix-it for the first one.

Simply click on Fix-it and Xcode will add you the stub for that method you need to implement.

Like this. See :

Thank you Xcode 8, finally !