Background: For some time, I've been running a side business online that has always used PayPal for its browser-based payment processing. More recently, a colleague of mine and I started working on Android and iOS apps to provide a more convenient interface for our users. Earlier this week, I finished testing and debugging the first production-ready version of the iOS app and submitted it to Apple for review. (My first submission! Yay!)
Apple requested three or four changes before they would consider the app as "meeting their guidelines." The biggest of these changes is that I have to get rid of the PayPal link in the app and replace it with Apple's own "in-app purchase" payment processor. Obviously I'll do it, because it's "Apple's house" and that means Apple gets to make the rules for its partners. What leaves me with a bad taste, however, isn't the extra bit of coding or the unnecessary complication to my business' accounting. To me, this rule seems to be anti-competitive. In a free market, shouldn't your vendor partners be able to choose any payment processor they prefer?
I'd like to know what the forum thinks: is Apple making a reasonable demand, or not?
You don't like the guidelines, play elsewhere. Let us know how that works for your pockets. I have 4 grandkids writing iOS apps each pulling down $3-5k annually for part-time work as they learn, from apps that sell for 99¢. That's good money for young teenagers that they can't earn elsewhere for want of opportunities. One has pulled down close to $60k so far this year for a utility that sells for $1.99. This is his third year writing apps, including some he gives away.
Learn Apple's Swift language and you won't want to continue writing for Android. The Apple Developer libraries for Swift generic code are vast and growing. No needs to re-invent the wheel. There are tools, with more coming, that allow you to code in Swift and then port to other platforms. It is an easy learn, especially for those versed in variants of C. Interpreted languages are the future.
As well, when you see your first developer payment statements, you'll quickly learn it simplifies your own business bookkeeping and accounting. It is almost automatically converts to html formatting, and if you're using software for bookkeeping and accounting that doesn't import html, you need to change your software. It's long in the tooth.
MS's in house staff programs on Apple hardware, then ports to MS standards, adding small features that don't exist on Apple. I programmed custom apps for OSX, MS, AIX and other forms of UNIX variants and Linux for decades, mostly for my own purposes and the needs of clients, but tended to right in generic modules and gave them away in open source libraries. Writing for OSX was always faster, porting far easier than I initially suspected.
If you participate in the developer forums, you'll see very few complaints, and then mostly for a glitch most everyone is experiencing, with many work arounds offered. Getting to know which developer forums suit your needs is a worthwhile time investment. I used to do a lot of database communication work with Oracle and 4th Dimension. Without access to their developer forums to learn from, I likely would have walked aways before the first year of programming. People are generous with solutions and where to look for answers to questions. Eventually, I returned as much as I received and still pop in to assist newbies.
Good luck out there. I hope you make a lot of money.
BTW, people don't remember, but Danny Goodman created the first hypertext language. If you can find one of his early out of print books, learning his basic principles will improve your programming foundation. Danny wrote Hypercard, and Supercard for using on Macs when the MacPlus was first released, the first was given away free with every Mac. It was the first multimedia computer program anyone could quickly learn how to use. I knew musicians who used to create digital sound processing boards through a MacPlus on Hypercard before they existed anywhere else. Musicians with no computer experience, including guys like Robert Fripp, Brian Eno, Daniel Lanois among many others. Danny retired at 36, raised three kids with his wife in parts unknown.

He's still sitting on 200k shares of Apple. And more of MS and Oracle. He owned part of Next, and a major part of Disney via the Pixar merger.