I have been wondering the same thing. When you create the new widget with WidgetKit, do you also keep the old widget for older iOS versions? From the info I find they are completely separate, but I might be missing something major here....
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I am pretty new to iOS programming myself, and these are some of the things that have helped me so far:
There is a cool app SoloLearn, where you can learn about different languages and techniques with Swift being one of them. I think it is pretty cool to just get started with some basics.
I would also recommend for you to search in the Apple`s Books app for their official books on programming, app making, etc. They are all free and I find them very useful.
Last but not least joining the Swift / iOS dev community on Twitter, has just been such a great experience for me! You will find great tips, articles, ideas and most importantly support.