Block not completing in iOS - ios

Question

I'm relatively new to iOS development but I'm working on an application to get a better understanding of development. I'm working with a web service and want to check the credentials a user enters. To do this I am making a simple get request with their credentials and then checking the http status for 200. Here is my code below:
-(BOOL)checkCredentials:(NSString *)username withPassword:(NSString *)password{
NSString *requestString = #"SOME URL";
NSURL *url = [NSURL URLWithString:requestString];
NSURLRequest *req = [NSURLRequest requestWithURL:url];
NSData *userPasswordData = [[NSString stringWithFormat:#"%#:%#", username, password] dataUsingEncoding:NSUTF8StringEncoding];
NSString *base64EncodedCredential = [userPasswordData base64EncodedStringWithOptions:0];
NSString *authString = [NSString stringWithFormat:#"Basic %#", base64EncodedCredential];
NSURLSessionConfiguration *sessionConfig=[NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.HTTPAdditionalHeaders=#{#"Authorization":authString};
self.session=[NSURLSession sessionWithConfiguration:sessionConfig];
__block BOOL success = NO;
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
NSURLSessionDataTask *dataTask = [self.session dataTaskWithRequest:req completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if(!error){
NSHTTPURLResponse *httpResp = (NSHTTPURLResponse*) response;
if (httpResp.statusCode == 200) {
success = YES;
}
}
NSMutableDictionary *jsonObject = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];
NSLog(#"%#", jsonObject);
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
[dataTask resume];
return success;
}
I was going to use a semaphore to wait for the block to complete so I can check the status code and then return. But first it seems like my code just hangs, and I think that because I don't have a release, but that's not allowed with ARC. I'm not sure why it's hanging. Is there a better way to wait for the block to complete (without a semaphore) so I can return whether my credentials are valid?
Also is there a better way to pass the username and password so that it's not possible for someone to spoof the username and password?
Any help would be greatly appreciated.

Answer 1

Think simple!
Make your own completionHandler so that you won't deal with the return anymore, the caller will take the responsibility of result verification instead.
There's one thing you need to keep in mind, that if you want to modify anything related to UI (User Interface), you need to dispatch your completion block to main queue or you will get unexpected behavior, see more detail here.
Change your return type to void and add a completion block:
-(void)checkCredentials:(NSString *)username withPassword:(NSString *)password completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))myCompletion
{
NSString *requestString = #"http://google.com";
NSURL *url = [NSURL URLWithString:requestString];
NSURLRequest *req = [NSURLRequest requestWithURL:url];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:req completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
// Here you return exactly what the NSURLSessionDataTask downloaded
// and pass it to the caller as an another completion block
myCompletion(data, response, error);
}];
[dataTask resume];
}
Caller's code, I assume that self is the caller:
[self checkCredentials:#"" withPassword:#"" completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if(!error){
// Result verification's here
NSHTTPURLResponse *httpResp = (NSHTTPURLResponse*) response;
if (httpResp.statusCode == 200) {
NSLog(#"SUCESS");
}
}
}];

Answer 2

You code stops waiting for a semaphore and [dataTask resume] is never executed.
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); <=== waits here
[dataTask resume]; <=== never reached
I'd suggest not using the semaphore here. Do the work in your block instead.
As to username/password. If you worry about spoofing then SSL layer on top of HTTP is the answer.

Answer 3

This is a really dangerous pattern, because this call is going to block until the network request completes. If this is on the main thread, your app will stop responding and the watchdog may kill you.
That warning aside, the reason the block doesn't complete is because the network task is never started. You trap on your semaphore before you call resume, so your task never runs. I would also, personally use a dispatch_group to do the waiting.
To make it better, you would need to rewrite it asynchronously. Basically have your app continue to function, maybe disable the inputs, until the call completes, then run a block to re-enable them, or show an error:
// Assume your login button and whatever are exposed as properties here
self.loginButton.enabled = NO;
NSURLSessionDataTask *dataTask = [self.session dataTaskWithRequest:req completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if(!error){
NSHTTPURLResponse *httpResp = (NSHTTPURLResponse*) response;
if (httpResp.statusCode == 200) {
success = YES;
}
}
NSMutableDictionary *jsonObject = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];
NSLog(#"%#", jsonObject);
// Need to be back on the main queue, the call is complete
self.loginButton.enabled = YES;
}];
[dataTask resume];
Or, just to keep it the way you have it, but resolve the immediate issue, re-order your trap so that it happens after the task resumes:
[dataTask resume];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); // might want to time out here instead of waiting forever
return success;