使用C的方式进行解决
两个例子，第一个例子拿到类方法；第二个例子拿到实例方法；

//eg1:
        Class watchSessionManager = NSClassFromString(@"TTNAppWCSessionManager");
        SEL sharedInstanceSel = NSSelectorFromString(@"sharedInstance");
        IMP impSharedInstance = [watchSessionManager methodForSelector:sharedInstanceSel];
        id (*funcSharedInstance)(id, SEL) = (void *)impSharedInstance;
        id watchInstance = funcSharedInstance(watchSessionManager, sharedInstanceSel);
        
        SEL startSessionSel = NSSelectorFromString(@"startSession");
        IMP impStartSession = [watchInstance methodForSelector:startSessionSel];
        void (*funcStartSession)(id, SEL) = (void *)impStartSession;
        funcStartSession(watchInstance,startSessionSel);


//eg2:
SEL selector = NSSelectorFromString(@"processRegion:ofView:");
IMP imp = [_controller methodForSelector:selector];
CGRect (*func)(id, SEL, CGRect, UIView *) = (void *)imp;
CGRect result = func(_controller, selector, someRect, someView);