Damn, it took me a while but I got it:

main.c:

#include <CoreFoundation/CoreFoundation.h> #include <objc/runtime.h> #include <objc/message.h> // This is a hack. Because we are writing in C, we cannot out and include // <UIKit/UIKit.h>, as that uses Objective-C constructs. // however, neither can we give the full function declaration, like this: // int UIApplicationMain (int argc, char *argv[], NSString *principalClassName, NSString *delegateClassName); // So, we rely on the fact that for both the i386 & ARM architectures, // the registers for parameters passed in remain the same whether or not // you are using VA_ARGS. This is actually the basis of the objective-c // runtime (objc_msgSend), so we are probably fine here, this would be // the last thing I would expect to break. extern int UIApplicationMain(int, ...); // Entry point of the application. If you don't know what this is by now, // then you probably shouldn't be reading the rest of this post. int main(int argc, char *argv[]) { // Create an @autoreleasepool, using the old-stye API. // Note that while NSAutoreleasePool IS deprecated, it still exists // in the APIs for a reason, and we leverage that here. In a perfect // world we wouldn't have to worry about this, but, remember, this is C. id autoreleasePool = objc_msgSend(objc_msgSend(objc_getClass("NSAutoreleasePool"), sel_registerName("alloc")), sel_registerName("init")); // Notice the use of CFSTR here. We cannot use an objective-c string // literal @"someStr", as that would be using objective-c, obviously. UIApplicationMain(argc, argv, nil, CFSTR("AppDelegate")); objc_msgSend(autoreleasePool, sel_registerName("drain")); }

AppDelegate.c:

#import <objc/runtime.h> #import <objc/message.h> // This is equivalent to creating a @class with one public variable named 'window'. struct AppDel { Class isa; id window; }; // This is a strong reference to the class of the AppDelegate // (same as [AppDelegate class]) Class AppDelClass; // this is the entry point of the application, same as -application:didFinishLaunchingWithOptions: // note the fact that we use `void *` for the 'application' and 'options' fields, as we need no reference to them for this to work. A generic id would suffice here as well. BOOL AppDel_didFinishLaunching(struct AppDel *self, SEL _cmd, void *application, void *options) { // we +alloc and -initWithFrame: our window here, so that we can have it show on screen (eventually). // this entire method is the objc-runtime based version of the standard View-Based application's launch code, so nothing here really should surprise you. // one thing important to note, though is that we use `sel_getUid()` instead of @selector(). // this is because @selector is an objc language construct, and the application would not have been created in C if I used @selector. self->window = objc_msgSend(objc_getClass("UIWindow"), sel_getUid("alloc")); self->window = objc_msgSend(self->window, sel_getUid("initWithFrame:"), (struct CGRect) { 0, 0, 320, 480 }); // here, we are creating our view controller, and our view. note the use of objc_getClass, because we cannot reference UIViewController directly in C. id viewController = objc_msgSend(objc_msgSend(objc_getClass("UIViewController"), sel_getUid("alloc")), sel_getUid("init")); // creating our custom view class, there really isn't too much // to say here other than we are hard-coding the screen's bounds, // because returning a struct from a `objc_msgSend()` (via // [[UIScreen mainScreen] bounds]) requires a different function call // and is finicky at best. id view = objc_msgSend(objc_msgSend(objc_getClass("View"), sel_getUid("alloc")), sel_getUid("initWithFrame:"), (struct CGRect) { 0, 0, 320, 480 }); // here we simply add the view to the view controller, and add the viewController to the window. objc_msgSend(objc_msgSend(viewController, sel_getUid("view")), sel_getUid("addSubview:"), view); objc_msgSend(self->window, sel_getUid("setRootViewController:"), viewController); // finally, we display the window on-screen. objc_msgSend(self->window, sel_getUid("makeKeyAndVisible")); return YES; } // note the use of the gcc attribute extension (constructor). // Basically, this lets us run arbitrary code before program startup, // for more information read here: http://stackoverflow.com/questions/2053029 __attribute__((constructor)) static void initAppDel() { // This is objc-runtime gibberish at best. We are creating a class with the // name "AppDelegate" that is a subclass of "UIResponder". Note we do not need // to register for the UIApplicationDelegate protocol, that really is simply for // Xcode's autocomplete, we just need to implement the method and we are golden. AppDelClass = objc_allocateClassPair(objc_getClass("UIResponder"), "AppDelegate", 0); // Here, we tell the objc runtime that we have a variable named "window" of type 'id' class_addIvar(AppDelClass, "window", sizeof(id), 0, "@"); // We tell the objc-runtime that we have an implementation for the method // -application:didFinishLaunchingWithOptions:, and link that to our custom // function defined above. Notice the final parameter. This tells the runtime // the types of arguments received by the function. class_addMethod(AppDelClass, sel_getUid("application:didFinishLaunchingWithOptions:"), (IMP) AppDel_didFinishLaunching, "i@:@@"); // Finally we tell the runtime that we have finished describing the class and // we can let the rest of the application use it. objc_registerClassPair(AppDelClass); }

View.c

#include <objc/runtime.h> // This is a strong reference to the class of our custom view, // In case we need it in the future. Class ViewClass; // This is a simple -drawRect implementation for our class. We could have // used a UILabel or something of that sort instead, but I felt that this // stuck with the C-based mentality of the application. void View_drawRect(id self, SEL _cmd, struct CGRect rect) { // We are simply getting the graphics context of the current view, // so we can draw to it CGContextRef context = UIGraphicsGetCurrentContext(); // Then we set it's fill color to white so that we clear the background. // Note the cast to (CGFloat []). Otherwise, this would give a warning // saying "invalid cast from type 'int' to 'CGFloat *', or // 'extra elements in initializer'. Also note the assumption of RGBA. // If this wasn't a demo application, I would strongly recommend against this, // but for the most part you can be pretty sure that this is a safe move // in an iOS application. CGContextSetFillColor(context, (CGFloat []){ 1, 1, 1, 1 }); // here, we simply add and draw the rect to the screen CGContextAddRect(context, (struct CGRect) { 0, 0, 320, 480 }); CGContextFillPath(context); // and we now set the drawing color to red, then add another rectangle // and draw to the screen CGContextSetFillColor(context, (CGFloat []) { 1, 0, 0, 1 }); CGContextAddRect(context, (struct CGRect) { 10, 10, 20, 20 }); CGContextFillPath(context); } // Once again we use the (constructor) attribute. generally speaking, // having many of these is a very bad idea, but in a small application // like this, it really shouldn't be that big of an issue. __attribute__((constructor)) static void initView() { // Once again, just like the app delegate, we tell the runtime to // create a new class, this time a subclass of 'UIView' and named 'View'. ViewClass = objc_allocateClassPair(objc_getClass("UIView"), "View", 0); // and again, we tell the runtime to add a function called -drawRect: // to our custom view. Note that there is an error in the type-specification // of this method, as I do not know the @encode sequence of 'CGRect' off // of the top of my head. As a result, there is a chance that the rect // parameter of the method may not get passed properly. class_addMethod(ViewClass, sel_getUid("drawRect:"), (IMP) View_drawRect, "v@:"); // And again, we tell the runtime that this class is now valid to be used. // At this point, the application should run and display the screenshot shown below. objc_registerClassPair(ViewClass); }

It's ugly, but it works.

If you would like to download this, you can get it from my dropbox here

You can get it from my GitHub repository here: