Although Apple recommended pickers for dropdowns (and surprisingly still recommends), they don't even use them anymore. Spoiler:
In both cases, the "logic" would dictate the use of a dropdown + picker. Apple has chosen a much better solution for its own apps.
Nevertheless, an entire screen on which you can choose between "Female" and "Male" (forgive the binary example) seems exaggerated. Personally, I think the best option is to design a drop-down element that opens an action sheet when you type:
The reason for this is that an action sheet is better than an order picker because:
1) Action sheets do not require scrolling to read and / or select options that are not highlighted.
2) Action sheets darken the background and ensure clarity affordability that clicking outside of the action sheet does not activate other items (while a picker is unsure of where to type to close the picker without accidentally tapping anything else);
3) Action sheets have "Cancel" buttons;
4) Action sheets are 44 points high, have borders between the buttons and can list more options with more space on the screen.
Action sheets are also better than the full-screen mode listed earlier, as they don't guide the user to another screen, making the flow more fluid. and They are better than a tailor-made alternative because they are native and therefore "future-proof". Brad Frost would probably correct me and say that they are actually "future-friendly".
By the way, Luke Wroblewski wrote an excellent article on why dropdowns should be the last resort user interface, and these 4 great videos detail when and why one element works better than the other (and with research to back it up):
• Luke Wroblewski Part 1 – Conversions @ Google 2014
• Luke Wroblewski Part 2 – Conversions @ Google 2014
• Luke Wroblewski – Basics of Mobile Design Part 1 – Conversions @ Google 2015
• Luke Wroblewski – Basics of Mobile Design Part 2 – Conversions @ Google 2015
Put on your headphones and dive in. The videos are worth it.