How to link Device Frameworks dynamically in iOS app?

Now My app is using Assets library Frameworks which is only available on iOS 4.0 and later for multi-choosing photos, but I want my app runs on devices with iOS 3.2.

Could I link the Assets library dynamically? If so I could judge开发者_开发知识库 whether the iOS SDK version is earlier than 4.0, if earlier I would use UIImagePickerController instead of Assets library, if later I could load Assets library dynamically and implement multi-choosing using it.

You can set the framework to weak linking. When you select a framework in Xcode 4 you can switch the target membership in the right pane between optional / required.

You should read Adding Runtime Checks for Newer Symbols in the docs.

Basically you can check the runtime availability of a class :

if ([INeedThisClass class]) ...  // SDK 4.2 and later

or

Classe ineedthis = NSClassFromString(@"INeedThisClass");   // SDK 4.1
if (ineedthis) ...