Replace a method on an instance at runtime

instance-swizzle.m

+ (void)load {
    /* We want to add our method after the application has finished launching
       because that's when the key window should be setup.
       We will add a method to that instance. */

    /*
        WARNING

        This would fail if another object was to key value observe the object
        we are going change the class of before we change it. That means before
        UIApplicationDidFinishLaunchingNotification is fired.
    */

    [[NSNotificationCenter defaultCenter] addObserverForName:UIApplicationDidFinishLaunchingNotification object:[UIApplication sharedApplication] queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification *note) {
        UIApplication *application = [note object];

        UIWindow *window = [application keyWindow];

        Class windowClass = object_getClass(window);
        Method originalMotionMethod = class_getInstanceMethod(windowClass, @selector(motionEnded:withEvent:));
        const char *methodEncoding = method_getTypeEncoding(originalMotionMethod);

        IMP motionImplementation = imp_implementationWithBlock(^(UIWindow *window, UIEventSubtype motion, UIEvent *event) {
            struct objc_super superInfo = {
                window,
                windowClass
            };

            objc_msgSendSuper(&superInfo, @selector(motionEnded:withEvent:), motion, event);

            NSLog(@"It works, implementation should go here.");
        });

        Class mfWindowClass = objc_allocateClassPair(windowClass, "MFWindow", 0);
        class_addMethod(mfWindowClass, @selector(motionEnded:withEvent:), motionImplementation, methodEncoding);
        objc_registerClassPair(mfWindowClass);

        object_setClass(window, mfWindowClass);

        [[NSNotificationCenter defaultCenter] removeObserver:note];
    }];
}