Question

It is possible to make universal binary for os 2.x and 3.x, you can see sample code for MFMailComposer But how to make it for iOS4 and iOS3 and XCode 3.2.3?

Answer 1

yes, there’s some documentation on the process but if you want to instantiate a class which might not be present, you must test first.

Using NSClassFromString returns null if you’re running on 3.x without the class, and returns a class you can use to allocate a new instance if the class exists while running on a later OS. In this way, you can test and fallback to 3.x code and make use of 4.x classes and features if available.

Be sure you compile against newer (4.x) libraries while you also set your Target OS to be lower (3.x) so you can cast a new class in your code without compile time warning.

If you want to use a new API or support a change in API, you might test for method presence using respondsToSelector and optionally fire the method via performSelector etc.

There are many different cases, but the idea revolves around setting a the target OS and testing runtime events in order to fallback or avoid features only available in a later OS’

For instance, you might check for UILocalNotification class’s existence and if it is available, you put a button on the toolbar and if it doesn’t exist, you don’t – thus altogether avoiding code which would crash at runtime.