As an experiment in using Apple's new Swift programming language, I whipped up a simple tip-calculator app for iOS 8.
Yes, the world really needs Yet Another Tip Calculator, and it also really needs Yet Another Swift App Example.
In addition to using Swift, the app also uses an embedded framework, which is a new feature of iOS 8. The framework contains functions and classes for converting between numbers and text, and it might be useful in other apps.
Source is available on GitHub: https://github.com/kristopherjohnson/KJTipCalculator
If you actually want to use this app, you'll have to build it yourself. But if you have iOS 8 on your phone now, you should know how to do that.