DownPicker is an extremely light-weight class library for creating DropDownList / ComboBox controls for iOS that will behave like their HTML / Android counterparts. You'll only need a standard UITextField and few lines of code.
DownPicker is basically a control interface wrapper, meaning that you won't use it as a control - it will use an existing UITextField control instead. This is a good thing, because you'll be able to design, positioning and skin your UITextField like you always do, programmatically or inside a Storyboard UI, depending on how you are used to work. You won't change your style, as it will adapt to suit yours.
However, if you don't like the control wrapper pattern, you can just use it as a custom control using the included UIDownPicker class. It's entirely up to you (and very easy to install in both scenarios).
DownPicker is released under MIT License. Installation instructions, examples and documentation are available in the included README.md file.