AFNetworkingErrorHandling

A simple sample app that shows how to get HTTP error responses in AFNetworking 2.x

AFNetworking is one of the most popular 3rd party, open-source Cocoa frameworks that takes the pain out of complex many complex networking tasks iOS/OSX developers frequently encounter. Among other things, AFNetworking makes it easy to process data from RESTful APIs, giving you an id responseObject which you can serialize into Foundation NSArray, NSDictionary, NSString, and NSNumber data structures your model classes can use. However, not all API calls are successful (sometimes the server is down!) so we need to properly handle errors should they occur.

For example, when making a request to the Twitter API without any authentication, we get a JSON reponse that looks like this:

{
    "errors": [
        {
            "message": "Bad Authentication data",
            "code": 215
        }
    ]
}

In our simple sample app, we want to extract the message key to present to the user for why the GET request failed, however in AFNetworking, the response object is not passed as a parameter in the failure block like it is in the success block. We've all tried logging and inspecting the NSError we do have access to only to get this not so useful block of text:

2015-01-30 00:09:22.913 AFNetworkingErrorHandling[12816:637494] AFNetworking error response: Error Domain=com.alamofire.error.serialization.response Code=-1011 "Request failed: bad request (400)" UserInfo=0x7fdc4c3627f0 {com.alamofire.serialization.response.error.response=<NSHTTPURLResponse: 0x7fdc4a71ba10> { URL: https://api.twitter.com/1.1/statuses/mentions_timeline.json?count=2&since_id=14927799 } { status code: 400, headers {
"Content-Encoding" = deflate;
"Content-Length" = 74;
"Content-Type" = "application/json;charset=utf-8";
Date = "Fri, 30 Jan 2015 06:09:22 UTC";
Server = "tsa_a";
"Strict-Transport-Security" = "max-age=631138519";
"x-connection-hash" = 3d666c379f25e7a1d8ad46e88ce2be93;
"x-response-time" = 2;
"x-spdy-version" = "3.1-NPN";
} }, NSErrorFailingURLKey=https://api.twitter.com/1.1/statuses/mentions_timeline.json?count=2&since_id=14927799, com.alamofire.serialization.response.error.data=<7b226572 726f7273 223a5b7b 226d6573 73616765 223a2242 61642041 75746865 6e746963 6174696f 6e206461 7461222c 22636f64 65223a32 31357d5d 7d>, NSLocalizedDescription=Request failed: bad request (400)}

When instead, what you as a developer would really like to see is an NSDictionary like this which matches the JSON response body from above:

2015-01-30 00:09:22.913 AFNetworkingErrorHandling[12816:637494] 
AFNetworking error response body: {
    errors =     (
        {
            code = 215;
            message = "Bad Authentication data";
        }
    );
}

Searching the Internet has several solutions to this problem, including subclassing AFJSONResponseSerializer and implementing a custom version of this method:

- (id)responseObjectForResponse:(NSURLResponse *)response data:(NSData *)data error:(NSError *__autoreleasing *)error

These two blogposts go into good detail with how to solve this problem that way:

• Retrieving Response Body via AFNetworking with an HTTP Error Status Code
• Accessing the response body in failure blocks with AFNetworking 2

However, there actually exists a much simpler solution that doesn't require subclassing! If you peek into the userInfo dictionary and look at the AFNetworkingOperationFailingURLResponseDataErrorKey key, you get the response you're looking for as an NSData object. You can then transform that data into an NSDictionary using the NSJSONSerialization class.

Here's how that would be implemented:

NSData *errorData = error.userInfo[AFNetworkingOperationFailingURLResponseDataErrorKey];
NSDictionary *serializedData = [NSJSONSerialization JSONObjectWithData: errorData options:kNilOptions error:nil];

To recap, within this simple sample app, I send a GET request to Twitter from their developer documentation that fails due to a lack of authentication. I subclass AFHTTPSessionManager as my custom ErrorSessionManager and set the base URL to: https://api.twitter.com/ so all requests have that base URL. I then make the request, which I expect to fail, print out the NSError object, then the failure response body, and finally access the dictionary to get a clear message that is presentable to a user.

Here's the complete request:

NSString *samplePath = @"1.1/statuses/mentions_timeline.json?count=2&since_id=14927799";

// This call will fail without proper Twitter authenticaion
[[ErrorSessionManager sharedInstance] GET:samplePath parameters:nil success:^(NSURLSessionDataTask *task, id responseObject) {
    // Log success response
    NSLog(@"AFNetworking success response body: %@", responseObject);
} failure:^(NSURLSessionDataTask *task, NSError *error) {   
    // Log error object
    NSLog(@"AFNetworking error response: %@\n\n\n", error);

    // Use the appropriate key to get the error data
    NSData *errorData = error.userInfo[AFNetworkingOperationFailingURLResponseDataErrorKey];

    // Serialize the data into JSON
    NSDictionary *serializedData = [NSJSONSerialization JSONObjectWithData: errorData options:kNilOptions error:nil];

    // Print out the error JSON body
    NSLog(@"AFNetworking error response body: %@\n\n\n", serializedData);

    // Get the first error cause and log it
    NSDictionary *failureError = serializedData[@"errors"][0];
    NSLog(@"Error reason: %@", failureError);

    // Present that same error cause to the user
    NSString *message = [NSString stringWithFormat:@"Failed to GET Twitter data with reason: %@", failureError[@"message"]];
    UIAlertController *errorController = [UIAlertController alertControllerWithTitle:@"Error" message:message preferredStyle:UIAlertControllerStyleAlert];
    [errorController addAction:[UIAlertAction actionWithTitle:@"Ok" style:UIAlertActionStyleDefault handler:nil]];

    [self presentViewController:errorController animated:YES completion:nil];
}];

That's it! Hopefully this simplifies your error handling code when using AFNetworking 2.x in your iOS and OS X apps.