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);
