Hooking UIApplicationMain in Swift or Objc apps with Fishhook

HookUIApplicationMain.m

// UIApplicationMain accepts Swift.String in Swift apps; a C forward declaration is needed
struct SwiftString {
    uint8_t reserved[16];
};
typedef struct SwiftString SwiftString;

int (*orig_UIApplicationMain_objc)(int argc, char *argv[], NSString *_, NSString *delegateClassName) = nil;
int (*orig_UIApplicationMain_swift)(int argc, char *argv[], SwiftString _, SwiftString delegateClassName) = nil;
NSString *(*FoundationBridgeSwiftStringToObjC)(SwiftString str) = nil;

// Our hooks //

static int UIApplicationMain_objc(int argc, char *argv[], NSString *_, NSString *delegateClassName) {
    Class appDelegateClass = NSClassFromString(delegateClassName)];
    // ...
    return orig_UIApplicationMain_objc(argc, argv, _, delegateClassName);
}

static int UIApplicationMain_swift(int argc, char *argv[], SwiftString _, SwiftString delegate) {
    NSString *delegateClassName = FoundationBridgeSwiftStringToObjC(delegate);
    Class appDelegateClass = NSClassFromString(delegateClassName)];
    // ...
    return orig_UIApplicationMain_swift(argc, argv, _, delegate);
}

// Hooking logic //

void ctor() {
    // UIApplicationMain objc
    __unused BOOL didHookObjcMain = flex_rebind_symbols((struct rebinding[1]) {{
        "UIApplicationMain",
        (void *)UIApplicationMain_objc,
        (void **)&orig_UIApplicationMain_objc
    }}, 1) == 0;
    
    // UIApplicationMain Swift shim
    void *mainBinary = dlopen(NSBundle.mainBundle.executablePath.UTF8String, RTLD_LAZY);
    void *swiftmain = dlsym(mainBinary, "$s5UIKit17UIApplicationMainys5Int32VAD_SpySpys4Int8VGGSgSSSgAJtF");
    // For convering Swift.String (a struct) to an NSString
    void *stringBridge = dlsym(mainBinary, "$sSS10FoundationE19_bridgeToObjectiveCSo8NSStringCyF");
    
    // If the shim exists, hook it as well. Only one will be called when using Fishhook
    if (swiftmain && stringBridge) {
        // For convering Swift.String (a struct) to an NSString
        FoundationBridgeSwiftStringToObjC = stringBridge;
        // Hook UIApplicationMain(Int32, UnsafeMutablePointer<…>?, Swift.String?, Swift.String?) -> Int32
        __unused BOOL didHookSwiftMain = rebind_symbols((struct rebinding[1]) {{
            "$s5UIKit17UIApplicationMainys5Int32VAD_SpySpys4Int8VGGSgSSSgAJtF",
            (void *)UIApplicationMain_swift,
            (void **)&orig_UIApplicationMain_swift
        }}, 1) == 0;
    }
}