Updating iPhone app from OS3 to OS4...

Question

I want to update my app to make use of the multitasking functionality and local notifications available in OS4. My question is, if I update my app and make it only available for OS4 and above does this mean that if someone is running OS3 on their phone they won't get the update? or they will get a warning that they need to update their OS? What I don't want to happen is for them to update their app to find it no longer works? If the final situation is what will happen, how do I write the code to find out what OS is being used?

Thanks

Answer 1

If you update your app to be 4.0 only, and also mark your app as 4.0-only in the Store, then people will not even see your update.

Answer 2

If they try to update on the device, then they won't even see an update that doesn't run under their OS version.

However if a previous customer updates their apps using iTunes on their Mac or PC, then iTunes may show them any new update. If they download that, iTunes will blow away its copy that is compatible with their current device/OS combination. The new copy shouldn't overwrite the working one on their device. But if the user ever needs to do a restore, or deletes the app and wants to reinstall it, they're probably out of luck unless they have really good backups and know how to use them.

Answer 3

You can have the best of both worlds by designing the app to use 4.0-specific features only on 4.0 devices. Weakly link 4.0-only frameworks, and in your code use [object respondsToSelector:@selector(thisOnlyWorksInOS4:)] to test for 4.0 features before using them (or, in the case of multitasking, use the multitaskingSupported property of UIDevice, since not all 4.0 devices support multitasking). Alternatively, UIDevice also provides a systemVersion property.

This blog post explains these techniques fairly well, including weak-linking frameworks: http://blog.federicomestrone.com/2010/07/18/base-sdk-deployment-target-weak-linking-and-import/