Implementation injection

ViewControllerTrack.m

//
//  AppDelegate.m
//
//  Created by Yuto Takei on 10/8/14.
//  Copyright (c) 2014 Yuto Takei. All rights reserved.
//

#import <objc/runtime.h>
#import "AppDelegate.h"

@implementation AppDelegate

IMP originalImp = NULL;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    Class vcClass = [UIViewController class];
    SEL sel = @selector(viewDidLoad);
    originalImp = class_getMethodImplementation(vcClass, sel);
    class_replaceMethod(vcClass, sel, (IMP)implInjected, "v@:");
    return YES;
}

void implInjected(id self, SEL _cmd) {
    if (self) {
        Class c = [self class];
        NSLog(@"%@ appear", NSStringFromClass(c));
    }
    if (originalImp) {
        ((void (*)(id, SEL))originalImp)(self, _cmd);
    }
}

@end