IPhone OS4 feature link error while run on OS3.x - iphone

Question

I've compiled an app with IPhone base SDK 4.0, deployment target on iPhone OS 3.0. This app contains OS 4.0 new feature: local notification.
It works well on iPod 2G with OS 4.0; however it crashes every time the app start up on iPhone 1G with OS 3.0. It appears to be runtime reference error:
"dyld: Symbol not found: _OBJC_CLASS_$_UILocalNotification
Referenced from: /var/mobile/Applications/73A3FAB1-63AE-4A71-8C6B-932142A728FE/Tapatalk X.app/Tapatalk X
Expected in: /System/Library/Frameworks/UIKit.framework/UIKit"
If the UIKit framework is different between SDK3.0 & SDK4.0, why it doesn't report while compiling? How can I apply local notification feature on this app, while the app can still running on devices with OS3.0? Thanks.

Answer 1

This answer solved the problem for me:
That error is being triggered because
you didn't weak-link the UIKit
framework. The UIKit framework in
iPhone OS 3.2 added the
UISplitViewController, and if you link
it in as normal your application will
assume those symbols exist on 3.0,
where they don't.
To weak-link a framework, find your
application target in Xcode, inspect
it, and go to the General tab. At the
bottom of that tab should be a list of
frameworks, with a column for Type.
Change the Type for UIKit from
Required to Weak and rebuild your
application. That should take care of
the runtime errors.
This seems safe to me, given that UIKit is always going to be on the devices we're targetting.

Answer 2

If you use a 4.0 SDK feature and you want to support 3.0 devices, you need to check that the functionality exists before you use it.
If you're using a new class (as you are) something like the following should work:
Class localNotificationC = NSClassFromString(#"UILocalNotification");
if (localNotificationC) {
UILocalNotification* localNotification = [[localNotificationC alloc] init];
// do stuff
[localNotification release];
}
else {
// what to do with the 3.0 SDK
}
As for why the compiler doesn't tell you, well, you told the compiler that you were using the 4.0 SDK and those classes/methods work on 4.0.

Answer 3

I found this information really useful. However my question is how to test if the application crashes or not after this change.
The background is I had the same issue with my iPad application. I have my BaseSDK set as 4.3 and Deployment target is 3.2. I never got this crash while doing ad-hoc provisioning and running the app on 3.2 iPad and it got approved by Apple too. But users reported that when they started the application after download on iPad 3.2 it crashes before showing the main screen even.
I was using [UILocalNotifition alloc] in my code which I think could be the result of below crash:
Exception Type: EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x00000001, 0xe7ffdefe
I seem to have fixed this by converting the above statement as per Stephen's suggestion.
However I really don't know how to test it. I am not getting any crash earlier too while testing the application before submission on 3.2 and currently also I am not getting any crash on start up.
I really need to validate this fix before I push the update to app store. Could someone please shed more light on how to test this? It would be really helpful if I can reproduce this crash with the previous version of my application and the same is not the case with current version after this fix.