iPhone OS 4.0 backwards compatible with 3.1/.2

I'm building an iPhone appli开发者_如何学Gocation with a base SDK target of 3.1.x. I've got a friend who offered to test it on the OS 4.0 beta, but I'm wondering in advance if I should bother. Will my application run under 4.0 or is it not backwards compatible? I have to imagine it will since apps built for 2.x run on 3.x just fine AFAIK. Any gotchas?

This information is still under NDA and therefore cannot be discussed here, except what was mentioned at th keynote and is thus public.

That said...

I can see the announced multitasking as a gotcha in terms of memory, performance and application state. Testing on OS 4 seems like a good idea in light of that. However, I your friend's device is not a latest-gen hardware device (iPhone 3GS or iPod touch 3rd generation), you will not get test results that deal with multitasking.

Also, remember that when installing the iPhone OS beta, you will need to add your friends UDID to your list of testing devices in your devloper portal or else the device will lock up when you upgrade.

In case you or someone reading this needs to know: To upgrade, you can either use XCode's Organizer or simply use iTunes. Hold the "option" key and click on the "restore" button. A window will pop up asking you what firmware to use.

Again, thes kind of questions are not yet really legal outside of Apple's develope forum.

I hope this helps. Good luck and happy developing!!

I've been using 4.0 beta for a few weeks now, and haven't run into any problems with my / App Store OS 3 apps. I don't think there were any changes in OS 4 that would break existing OS 3 apps but I can't say so with complete certainty.

Most likely any features from 3.0+ will be in 4.0. Features in 2.x may be removed.

No way to know for certain unless you can use the beta or wait until the official release.

You should always test on as many devices as possible. As mentioned, OS 4 is particularly good to test with just to make sure you interact well with the new multitasking features (which is public knowledge so it's not like saying anything about that would go against the NDA).