omniswizzle.m

@import Foundation;
@import ObjectiveC;


static NSMutableSet *swizzledClasses;
static NSMutableDictionary *swizzledBlocks; // Class -> SEL (as string) -> block
static IMP forwardingIMP;
static dispatch_once_t once;

void Swizzle(Class c, SEL sel, void (^block)(NSInvocation *)) {
    dispatch_once(&once, ^{
        swizzledClasses = [NSMutableSet set];
        swizzledBlocks = [NSMutableDictionary dictionary];
        forwardingIMP = class_getMethodImplementation([NSObject class], @selector(thisReallyShouldNotExistItWouldBeExtremelyWeirdIfItDid));
    });
    
    if(![swizzledClasses containsObject: c]) {
        SEL fwdSel = @selector(forwardInvocation:);
        Method m = class_getInstanceMethod(c, fwdSel);
        __block IMP orig;
        IMP imp = imp_implementationWithBlock(^(id self, NSInvocation *invocation) {
            NSString *selStr = NSStringFromSelector([invocation selector]);
            void (^block)(NSInvocation *) = swizzledBlocks[c][selStr];
            if(block != nil) {
                NSString *originalStr = [@"omniswizzle_" stringByAppendingString: selStr];
                [invocation setSelector: NSSelectorFromString(originalStr)];
                block(invocation);
            } else {
                ((void (*)(id, SEL, NSInvocation *))orig)(self, fwdSel, invocation);
            }
        });
        orig = method_setImplementation(m, imp);
        [swizzledClasses addObject: c];
    }
    
    NSMutableDictionary *classDict = swizzledBlocks[c];
    if(classDict == nil) {
        classDict = [NSMutableDictionary dictionary];
        swizzledBlocks[(id)c] = classDict;
    }
    classDict[NSStringFromSelector(sel)] = block;
    
    Method m = class_getInstanceMethod(c, sel);
    NSString *newSelStr = [@"omniswizzle_" stringByAppendingString: NSStringFromSelector(sel)];
    SEL newSel = NSSelectorFromString(newSelStr);
    class_addMethod(c, newSel, method_getImplementation(m), method_getTypeEncoding(m));
    method_setImplementation(m, forwardingIMP);
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Swizzle([NSBundle class], @selector(objectForInfoDictionaryKey:), ^(NSInvocation *inv) {
            NSLog(@"invocation is %@ - calling now", inv);
            [inv invoke];
            NSLog(@"after");
        });
        
        NSLog(@"%@", [[NSBundle bundleForClass: [NSString class]] objectForInfoDictionaryKey: (__bridge NSString *)kCFBundleVersionKey]);
    }
    return 0;
}