method_setImplementation的问题请教下

[w337643641 (何必呢)]

    Method origMethod = class_getInstanceMethod(NSClassFromString(@"UIViewController"), @selector(viewDidLoad));
    
    IMP orig_fun = method_getImplementation(origMethod);
    
    method_setImplementation(origMethod,imp_implementationWithBlock(^(id self,SEL _cmd){
    
        
        NSLog(@"target = %@",self);
        
        NSLog(@"NSStringFromSelector(_cmd) = %@",NSStringFromSelector(_cmd));
        
        orig_fun(self,_cmd);
    
    }));

为啥闪退,应该怎么写才最好，替换方法后运行原来方法。

[Ken (kinken)]