[liyanchang]

If you're looking to learn iOS, I'd suggest contributing to an open source application.

An app requires a bit of setup so taking someones app means that most of the minutia is set up. Therefore you can take on manageable size chunks. Also, and maybe more importantly, you'll have an experienced iOS developer review your code.

https://github.com/nothingmagical/cheddar-ios is a good one. Shameless plug: https://github.com/Ink/ThatInbox, ThatPDF, ThatPhoto, ThatCloud are ones that my company open sourced.

Related: https://learn.thoughtbot.com/ios is a good listing of iOS resources to get started.