iOS - runtime exception of "selector not recognized" when using liberary

Set linker to load every object files in the library that defines Objective-C class or category. In orde to do this we need to pass the flag -ObjC to the linker

1. In Xcode, double-click the target's name under "Targets" in the Project window.
2. Choose the Build pane from the ensuing Info window.
3. Scroll down to the Other Linker Flags build setting under the Linking collection and set its value to -ObjC.