Build iPhone App for multiple devices and versions

Question

Im trying to upgrade my iPhone app from 3.0 to 4.0

Some of the API are deprecated in 4.0, if i use the new API, it might not work on 3.0 right? Therefore i need to have an old API and New API

What is the best way to code for 3.0 and 4.0 for all the pro developers out there?

I know i can check for model and systemVersion from UIDevice.

Do i need to check for which version first and check for which device?

if systemVersion == 3.0 {
//old api code here
}

if systemVersion == 4.0 {

if device == iphone 3G
//new api code here for 3G

if device == iphone 3GS
//new api code here for 3GS

if device == iphone 4
//new api code here for 4
}

Answer 1

See this article by Matt Gallagher, also if you are an iphone dev check the WWDC videos session, there is at least one session that talk about this.