gistfile1.m

#import "UIViewController+RPRIS.h"  
#import <objc/runtime.h>

@implementation UIViewController (Foo)

+ (void)swizzlingMethodViewWillApper
{
    Method originalMethod = class_getInstanceMethod(self, viewWillApper:);
    Method swizzledMethod   = class_getInstanceMethod(self, foo_viewWillApper:);
    method_exchangeImplementations(originalMethod, swizzledMethod);
}

- (void)foo_viewWillAppear:(BOOL)animated
{
    [self foo_viewWillAppear:animated];
    NSLog(@"swizzled viewWillApper Yeah!!");
}

@end