I think the title explains my question pretty well, I'm currently working on a small framework for my personal needs, and thought about building it with ARC (if thats possible at all?), and use it in old projects that were built before ARC?
Yes, with one caveat: if you have iOS 4.x as a deployment target, a supplemental library is necessary to deal with ARC-compiled code on the older runtime. For an application built using ARC, this is packaged in with the application. This is not included if you are using an ARC-compiled library in a non-ARC application.
You can manually link this library into your non-ARC application to avoid runtime problems on the older OS versions by adding -fobjc-arc to your Other Linker Flags for the application.
See this Apple Developer Forums thread for some more discussion on this.
You can link against the glue library provided by Apple (as Brad has said in his answer above). However, the __weak variable qualifier is not compatible with operating systems below 4.3 (As mentioned here: iOS 5 Best Practice (Release/retain?) in a comment by sudo).
Should be fine. The library is already compiled, so ARC/NonARC shouldn't matter anymore.
来源：https://stackoverflow.com/questions/7967415/could-i-build-a-arc-framework-and-use-it-in-a-non-arc-project