How to write iOS app purely in C

Question

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?

Is it possible to build an iPhone app purely in the C programming language?

Answer 1

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:

Answer 2

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