// - weak & strong 
#define myWeakify(VAR) \ 
try {} @finally {} \ 
__weak __typeof__(VAR) VAR##_myWeak_ = (VAR) 
 
#define myStrongify(VAR) \ 
try {} @finally {} \ 
__strong __typeof__(VAR) VAR = VAR##_myWeak_ 
 
#define myStrongifyRetVIfNil(VAR, RET) \ 
try {} @finally {} \ 
__strong __typeof__(VAR) VAR = VAR##_myWeak_;\ 
if(VAR == nil) return RET 
 
#define myStrongifyRetNIfNil(VAR) \ 
try {} @finally {} \ 
__strong __typeof__(VAR) VAR = VAR##_myWeak_;\ 
if(VAR == nil) return 

- (void)handleFromVC 
{ 
  NSArray *viewControllers = self.navigationController.viewControllers; 
  @myWeakify(self); 
  [viewControllers enumerateObjectsWithOptions:NSEnumerationReverse usingBlock:^(id _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) { 
    @myStrongifyRetNIfNil(self); 
    //代码 
  }]; 
}