How to write iOS app purely in C

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:

Objective-C is a superset of the C-language, so it is theoretically possible to write a program entirely in C, however, unless you are thoroughly versed in OpenGL ES, You'll need to do at least some objC (Even Rich's sample has a const NSString* in it), else you'll have to write the views yourself.

OK, the above is completely wrong. Let me say, I'm astounded Rich achieved this lofty goal, so I ported it over to the mac (source here). The files below have no headers, do not link to Cocoa, nor does the project have a nib:

AppDelegate.m

#include <objc/runtime.h>#include <objc/message.h>extern id NSApp;struct AppDel{    Class isa;    //Will be an NSWindow later, for now, it's id, because we cannot use pointers to ObjC classes    id window;};// This is a strong reference to the class of the AppDelegate// (same as [AppDelegate class])Class AppDelClass;BOOL AppDel_didFinishLaunching(struct AppDel *self, SEL _cmd, id notification) {    //alloc NSWindow    self->window = objc_msgSend(objc_getClass("NSWindow"),                                sel_getUid("alloc"));    //init NSWindow    //Adjust frame.  Window would be about 50*50 px without this    //specify window type.  We want a resizeable window that we can close.    //use retained backing because this thing is small anyhow    //return no because this is the main window, and should be shown immediately    self->window = objc_msgSend(self->window,                                sel_getUid("initWithContentRect:styleMask:backing:defer:"),(NSRect){0,0,1024,460}, (NSTitledWindowMask|NSClosableWindowMask|NSResizableWindowMask|NSMiniaturizableWindowMask),NSBackingStoreRetained,NO);    //send alloc and init to our view class.  Love the nested objc_msgSends!    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 window.    objc_msgSend(self->window, sel_getUid("setContentView:"), view);    objc_msgSend(self->window, sel_getUid("becomeFirstResponder"));    //makeKeyOrderFront: NSWindow to show in bottom left corner of the screen    objc_msgSend(self->window,                 sel_getUid("makeKeyAndOrderFront:"),                 self);    return YES;}static void initAppDel(){    //Our appDelegate should be NSObject, but if you want to go the hard route, make this a class pair of NSApplication and try initing those awful delegate methods!    AppDelClass = objc_allocateClassPair((Class)                                         objc_getClass("NSObject"), "AppDelegate", 0);    //Change the implementation of applicationDidFinishLaunching: so we don't have to use ObjC when this is called by the system.    class_addMethod(AppDelClass,                    sel_getUid("applicationDidFinishLaunching:"),                    (IMP) AppDel_didFinishLaunching, "i@:@");    objc_registerClassPair(AppDelClass);}void init_app(void){    objc_msgSend(                 objc_getClass("NSApplication"),                 sel_getUid("sharedApplication"));    if (NSApp == NULL)    {        fprintf(stderr,"Failed to initialized NSApplication...  terminating...\n");        return;    }    id appDelObj = objc_msgSend(                                objc_getClass("AppDelegate"),                                sel_getUid("alloc"));    appDelObj = objc_msgSend(appDelObj, sel_getUid("init"));    objc_msgSend(NSApp, sel_getUid("setDelegate:"), appDelObj);    objc_msgSend(NSApp, sel_getUid("run"));}//there doesn't need to be a main.m because of this little beauty here.int main(int argc, char** argv){    //Initialize a valid app delegate object just like [NSApplication sharedApplication];    initAppDel();    //Initialize the run loop, just like [NSApp run];  this function NEVER returns until the app closes successfully.    init_app();    //We should close acceptably.    return EXIT_SUCCESS;}

View.m

#include <objc/runtime.h>#include <objc/message.h>#include <ApplicationServices/ApplicationServices.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, CGRect rect){    //make a red NSColor object with its convenience method    id red  = objc_msgSend(objc_getClass("NSColor"), sel_getUid("redColor"));    // fill target rect with red, because this is it!    NSRect rect1 = NSMakeRect ( 21,21,210,210 );    objc_msgSend(red, sel_getUid("set"));    NSRectFill ( rect1 );}// 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((Class) objc_getClass("NSView"), "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);}

prefix.pch

//// Prefix header for all source files of the 'CBasedMacApp' target in the 'CBasedMacApp' project//#ifdef __OBJC__#import <Foundation/Foundation.h>#import <AppKit/AppKit.h>#endif

I read here Learn C Before Objective-C?

Usually I then replace some Obj-C code with pure C code (after all you can mix them as much as you like, the content of an Obj-C method can be entirely, pure C code)

Is this true?

Could I build an iPhone app purely in the C programming language?

The quoted passage is true, but the answer to your question is no.

To illustrate what answerer Mecki on that other question was talking about:

- (void) drawRect:(CGRect)dirtyRect { //Objective-C    CGContextRef context = UIGraphicsGetCurrentContext();  //C    CGContextSetRGBFillColor(context, 1.0, 0.0, 0.0, 1.0); //C    CGContextFillRect(context, dirtyRect);                 //C} //Objective-C (balances above “- (void) drawRect:…” line)

There is nothing but pure C code within this method, but the method itself is Objective-C code, as is the class that contains this method.

So it is possible to do what Mecki said, but you can't (practically—as Richard J. Ross III showed, it's technically possible but quite a lot of typing) write a whole Cocoa Touch program in pure C.