Hello World

appDelegate

#import <objc/runtime.h>
#import <objc/message.h>

@implementation AppDelegate

id (*my_msgSend)(id, SEL, ...) = (void *)objc_msgSend;
void (*my_msgSend_)(id, SEL, ...) = (void *)objc_msgSend;;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    
    Class vc = objc_allocateClassPair(NSClassFromString(@"UIViewController"), "ViewController", 0);
    SEL selector = sel_registerName("viewDidLoad");
    
    Method origMethod = class_getInstanceMethod(vc,
                                                selector);
    IMP origIMP = method_getImplementation(origMethod);
    
    id block = ^(id self){
        UIView* v = my_msgSend(self, @selector(view));
        UIColor* c = my_msgSend(NSClassFromString(@"UIColor"), @selector(whiteColor));
        my_msgSend_(v, @selector(setBackgroundColor:), c);
        
        id label = my_msgSend(NSClassFromString(@"UILabel"), @selector(alloc));
        label = my_msgSend(label, @selector(init));
        
        my_msgSend_(label, @selector(setText:), @"Hello World");
        my_msgSend_(label, @selector(sizeToFit));
        
        my_msgSend_(label, @selector(setFrame:), CGRectMake(0, 0, 100, 100));
        my_msgSend_(v, @selector(addSubview:), label);
    };
    
    IMP viewDidLoad = imp_implementationWithBlock(block);
    method_setImplementation(origMethod, viewDidLoad);

    objc_registerClassPair(vc);
    
    id myInstance = [[NSClassFromString(@"ViewController") alloc] init];
    
    self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.window.rootViewController = myInstance;
    [self.window makeKeyAndVisible];
    
    return YES;
}

@end