iOS HTTP request - getting the error response message - ios

Question

This is my bit of code doing a GET request to a REST api.
Im not sure how to get back the message if I get an error:
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] init];
NSURL *URL = [NSURL URLWithString:urlString];
[request setURL:URL];
[request setHTTPMethod:#"GET"];
NSError *err = nil;
NSHTTPURLResponse *res = nil;
NSData *retData = [NSURLConnection sendSynchronousRequest:request returningResponse:&res error:&err];
if (err) // This part is never called.
{
NSLog(#"Error: %#", err);
}
else
{
if (res.statusCode != 200)
{
// show the user the status message
NSLog(#"Error: %#", res); // This part is called
}
else
{
}
}
I want to get the error message if it was not successful. But the if (err) block is never called. err is still null, although the statuscode is 400.
And if successful I will get back a json response.
In the code above I get back a statusCode of 400

Answer 1

The error block is not called because the error object is created only if a system level error occurs. This does not happen because the request is sent correctly and the server sends a response. If you are in control of the server, you should probably make it return status code 200 and include an app level status code in the response, that would tell your app that the entered credentials are incorrect.
Edit:
To get status message you can use
+ (NSString *)localizedStringForStatusCode:(NSInteger)statusCode
This is a class method of the NSHTTPURLResponse class.
if (res.statusCode != 200)
{
// show the user the status message
NSLog(#"Error: %#", [NSHTTPURLResponse localizedStringForStatusCode: res.statusCode]); // This part is called
}

Answer 2

Take a look at the NSError class reference:
https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSError_Class/Reference/Reference.html
You can try to log the error message from the localizedDescription.

Answer 3

you are receiving this status code because- The Web server (running the Web site) thinks that the data stream sent by the client (e.g. your Web browser or our CheckUpDown robot) was 'malformed' i.e. did not respect the HTTP protocol completely. So the Web server was unable to understand the request and process it
to log above problem in respect to ios visit this link

Answer 4

If you read the documentation of sendSynchronousRequest...
error
Out parameter used if an error occurs while processing the request. May be NULL.
this mean that erro will be a valid NSError object in case there is a problem to resolve the request, like a malformed URL.
If the request can be resolved error will be NULL and according with HTTP protocol and depending to the server that you are trying to connect, the NSHTTPURLResponse object will contain all the information about the request.
In general is an error think that every status code different than 200 is an error, for example for a REST based API 204 mean empty data, and in this case the request is finished successfully but the requested resource is just empty data, and this is not an error.
So about your question, is absolutely fine that error is NULL most of the time, if is not mean that there is an issue before reach the target server, in general you have to consider both, error and according to the server that you are trying to talk the status code maps, in most of cases the REST pattern