Which version of the iPhone SDK should I use for an OS4 app?

Question

I'm wondering if I will have compatibility issues with OS4.0 if my app is built with the 4.1 SDK, or is this a non-issue?

Answer 1

The 4.1 version will be fine, if you have an iPhone with a version 4.0 of iOS you could be able to run your app on it. On the other hand if you have an iPhone with iOS 4.1 and and sdk with version 4.0 you won't.

Answer 2

Depending on the APIs you will be using. Let's say you want to implemented in-app texting (MFMessageComposeViewController) and want ALL of your end-users (will be defined as EUs) to access that feature then you'll have to compile against iOS 4.0. But let's say you want the in-app texting to be optional and a "plus" for your EUs, you'll just compile your app against iOS 3.0 (let's say).

Hope I answered your question :-)

Answer 3

The short answer to your question is: Use a Base SDK Version of at least 4.0 for your application and a Deployment Target Version of exactly 4.0.

I've downvoted both answers because they are wrong or at least incomplete.

When building applications there are two settings that you should be aware of:

There is the Base SDK Version which defines against which version of the SDK your application will be compiled.

There is also the Deployment Target Version which defines the lowest version of the iOS that your application requires. You can set this to as low as 2.2.1 in Xcode but the App Store will not accept versions lower then 3.1.3 at the moment.

The Base SDK Version can be higher then the Deployment Target Version. This simply means that your application is backward compatible with older version of the iOS. This also means that your app needs to make decisions at runtime to be sure not to use newer functionality when running on an older version.

For example, the MFMessageComposeViewController was introduced in 4.0 so if your app has been configured to also run on 3.1.3 then you should use NSClassFromString() to find if that specific class is actually available before you use it.

There are many questions here on Stack Overflow on how to discover available functionality so I will not repeat those techniques here.