ios - Calling method from different class sometimes works and sometimes doesn't -

i trying call set of methods 1 class reside in class.

the methods reside in viewcontroller.m class , trying call them other class called myclass.m.

sometimes works , doesn't.

the errors are

unrecognized selector sent class 0x1071c0050

terminating app due uncaught exception 'nsinvalidargumentexception', reason: '+[viewcontroller setprogressvalue:]: unrecognized selector sent class 0x1071c0050'

when error suggests went wrong @ [viewcontroller setprogressvalue:] exclusively called within viewcontroller.m resides there.

the flow looks this: viewcontroller.methoda -> myclass.methodb -> viewcontroller.methodb

this not work, following works viewcontroller.methoda -> myclass.methodb -> viewcontroller.methodc

how come first flow doesn't works second does?

the error very descriptive. calling method not exist.

you're calling setprogressvalue: class method, instance method. please note '+' in error description. need call on concrete instance of viewcontroller.