[iOS] Custom NSError

With the objective-c, some functions may have some errors, for example connection errors.
In this situation, we can pass a NSError* pointer to the function and check the error after calling the function.

Code example:

- (void)getDataWithError:(NSError **)error {
    if(error) { //Check if the parameter is nil
        *error = [[NSError alloc] initWithDomain:@"ConnectionError"
                                            code:1001
                                        userInfo:nil];
    }
}

Also can do something like this:

NSError *urlConnectionError = nil;
NSData *data = [NSURLConnection sendSynchronousRequest:request
                                     returningResponse:&response
                                                 error:&urlConnectionError];
*error = urlConnectionError;