如何从NSInvocation获取NSString结果？

NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
[invocation setSelector:@selector(methodName)];

[invocation setArgument:&arg1 atIndex:2];

[invocation invoke];
NSString *result;
[invocation getReturnValue:&result];

NSLog(@"Result: %@", result);

- (NSString *)getStringResult {
    NSMethodSignature *signature = [[self class] instanceMethodSignatureForSelector:@selector(getString)];
    NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
    [invocation setSelector:@selector(getString)];
    [invocation setTarget:self];
    [invocation invoke];
    NSString *result;
    [invocation getReturnValue:&result];
    return result;
}

- (NSString *)getString {
    return @"Hello, World!";
}