How does an app with lower base sdk work?

Question

In XCode I can specify Base SDK. I am wondering how does that work behind the scenes? If I am running an app, for example, on a device that has iOS 7 and my base SDK is iOS 6, then how come the app has the old 'look and feel'? Does XCode compile the older SDK and include it within my app or does new version of iOS comes with older libraries/SDKs?

In other words, does the run time know this app is compiled with lower base SDK and somewhere in UIKit's code it does:

if (lower SDK) {
  //show old look/feel
} else {
  //show new look/feel
}

or does the app itself include the old library and load it ?

Thanks

Answer 1

iOS applications are forward compatible with new versions of iOS. The reason is :

Almost all changes to the iOS versions are additive and hence an application build using lower version still runs on the higher iOS version.

Though, we need to take care of this point:

As frameworks evolve through various releases, APIs are introduced or deprecated and behaviors of existing APIs may occasionally change. Apple makes every effort to minimize changes that may cause incompatibilities, in some cases providing alternate behaviors based on the framework version. In rare cases, your code needs to determine the framework version and adjust accordingly

To understand more, read this

Answer 2

Apple never changes / deletes / renames classes or methods. They only add new ones.
If they don't want you to use it anymore, they mark it as deprecated.

This is a very important point.
At compile-time, the compiler checks if all classes and method signatures are available in the SDK your building your app with.

If that's the case, you can build and deploy your app. Because those classes and methods will never be deleted from newer versions of the framework, your app will run just fine.

---

On the other hand, you can build apps and deploy them to systems, which do not actually support the current SDK. For example, you can use Autolayout (NSLayoutConstraint class is available since 10.7) and deploy it for Mac OS X 10.6. The compiler will not say a word.

The app will crash though on systems prior to 10.7.

Answer 3

You should set your target to ios 5.0 (via your project target settings) for making sure that none of the ios6 methods are used (or else a compilation error will prevent you from building it).

In order to support new features and check if ios6 is available on the device you have two ways :

During compilation (so you can still build your app with lower targets and newer together) use the following macro

#if __IPHONE_OS_VERSION_MIN_REQUIRED > __IPHONE_6_0
 // Your ios6 code goes here
#endif
2: During runtime : [[[UIDevice currentDevice] systemVersion] floatValue] > 6.0

Answer 4

Your project is built against the Current SDK. If you have an older Deployment Target, then your code base is compiled against that. So if you are building against 7.0, but have a 6.0 deployment target, iOS 7 specific deprecations will not be triggered. Everything will be compiled against the oldest specified deployment target.

This will however put the pressure on you as a developer to make sure you are not using iOS 7 specific code. The compiler will still assume you mean to allow newer users to run your application as well and that all the newest methods are available to you and your latest version users. You can either test your code base against the older SDK with older devices or Simulators to make sure it runs well, or use an application like Deploymate that will test for methods you are using that could potentially cause problems.

If you plan to use any of the latest methods, you will need to wrap them up in the compiler if statement (like Peter Fidemraizer answered) or in normal if statements checking the version in the Foundation framework.

if (floor(NSFoundationVersionNumber) <= NSFoundationVersionNumber_iOS_6_1) {
   // Load resources for iOS 6.1 or earlier
} else {
   // Load resources for iOS 7 or later
}

Answer 5

Base SDK means, the SDK that your app is going to be built on. SDK's have some frameworks etc. that are differantiated as the version of the SDK changes. For example;

Let's say your current Base SDK in your XCode is iOS 6:

• You can have the frameworks and feautres that iOS 6 SDK provided you to.

• Your app will be usable in any iOS SDK that you specify as "Minimum iOS SDK". Minimum iOS device gives you some restrictions on components to use. be aware of that.

• Your app will be usable in iOS 7 too, just like it works in iOS 5 or iOS 6. Because iOS versions have backward compatibility. That means, iOS 7 will run the apps that are running in iOS 6 too.

Let's say your current Base SDK is iOS 6 and you want to make it iOS 7

• Your app will be built with a brand new SDK, so, if the new SDK has some big changes in it, you will see the differences immediately when you run the app. For example, in iOS 7 SDK, you can use status bar (20 px) as a view component too. That may ruin your view hierarchy.

• You need to test your app again to check that your code is compatible with iOS 7

• If you want to use new iOS 7 frameworks or features, you are in the correct way, you can use them now :)

In short, Base iOS SDK is on what iOS version your app is compiled & built on. running it on a iOS X? device is a different concept.

Hope this helps

Answer 6

Base SDK is the SDK that you want to use to build the app. Use "Deployment target" to specify the minimum OS you want your app to run on.

If you want to know the iOS version, check out this question.

Answer 7

While updating the Apple frameworks itself,Apple takes care of support for multiple iOS versions;However you need to follow some basics checks, which are explained here