iOS App Wait for HTTP Response when calling a method? - objective-c

Question

I am trying to call a class method that takes a string and posts it to a site to receive a JSON response(among some other variables I have stored in the DataClass). I am stuck trying to return the data in the form of a response and can not at this point even NSLog the returned data. The question is, now that I have called my class method, how can the class method wait to return a response from an HTTP POST to return data? Once I return my JSON, I can expand it to a dictionary and process from there. Help is appreciated :)
Class Method:
//
// APISample.m
//
// Created by Sam on 1/6/13.
// Copyright (c) 2013 Sam. All rights reserved.
//
#import "APISample.h"
#import "DataClass.h"
#implementation APISample
#synthesize first_name = _first_name;
#synthesize last_name = _last_name;
#synthesize profile_pic_url = _profile_pic_url;
#synthesize responseData;
-(id)init
{
self = [super init];
return self;
NSLog(#"Loaded APISample and fetching");
}
+(id)getDataAboutUser:(NSString *)user_request_id;
{
DataClass *userdata=[DataClass getInstance];
NSLog(#"Loaded APISample and fetching %#", user_request_id);
NSMutableURLRequest *user_fetch_details = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:#"http://10.0.23.161/users/user_fetch_details.php"]];
[user_fetch_details setHTTPMethod:#"POST"];
NSMutableString *postString = [NSMutableString stringWithString:#"id=123"];
[postString appendString:#"&userrequest_id="];
[postString appendString:[userdata.str_userid copy]];
[postString appendString:#"&user_id="];
[postString appendString:[userdata.str_userid copy]];
[postString appendString:#"&identifier="];
[postString appendString:[userdata.str_identifier copy]];
[user_fetch_details setValue:[NSString stringWithFormat:#"%d", [postString length]] forHTTPHeaderField:#"Content-length"];
[user_fetch_details setHTTPBody:[postString dataUsingEncoding:NSUTF8StringEncoding]];
NSURLConnection *connection=[[NSURLConnection alloc] initWithRequest:user_fetch_details delegate:self];
NSMutableData *responseData=[NSMutableData data];
[responseData appendData:[NSURLConnection connection:didReceiveData];
if (connection) {
// Create the NSMutableData that will hold
// the received data
// receivedData is declared as a method instance elsewhere
NSMutableData *responseData=[NSMutableData data];
} else {
// inform the user that the download could not be made
}
NSLog(#"Received Data %#", [[NSString alloc] initWithData:responseData encoding:NSASCIIStringEncoding]);
return [[NSString alloc] initWithData:responseData encoding:NSASCIIStringEncoding];
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
[responseData appendData:data];
NSString *receivedDataString = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
if ([receivedDataString isEqualToString: #"error"]) {
UIAlertView *errorAlert = [[UIAlertView alloc] initWithTitle:#"Error"
message:#"An error has occured. The application will now exit. Unexpected Response!"
delegate:nil
cancelButtonTitle:#"Close"
otherButtonTitles:nil];
[errorAlert show];
exit(0);
}else{
NSError* error;
NSDictionary* json = [NSJSONSerialization
JSONObjectWithData:responseData
options:kNilOptions
error:&error];
NSString *firstnameResponse = [json objectForKey:#"first_name"];
NSString *lastnameResponse = [json objectForKey:#"last_name"];
NSString *profile_pic_urlResponse = [json objectForKey:#"profile_pic_url"];
NSLog(#"didReceiveData %# analysed " , firstnameResponse);
}
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
NSLog(#"connectionDidFinishLoading");
NSLog(#"Succeeded! Received %d bytes of data",[self.responseData length]);
}
#end
I receive no data in the log after "Received Data" and do not see my error messages. Thanks you

Answer 1

The design pattern you are describing is called a CallBack. You need to be notified of an event occurring at some point in the future. In objective-c there are 4 main forms of callbacks.
Target Action Pairing (this is what is used with buttons, and things of the like. "When this button is pressed notify my target, and tell them to execute this action")
Delegation (you are using a form of delegation in the code above with NSURLConnection. When you see the word 'delegate' i want you to think 'helper object'. You are saying, "hey NSURLConnection, when important events happen, i would like you to tell this delegate (helper object) about these events)
Notifications (these are used a lot when dealing with model objects changing)
and finally... the one i would recommend for your purposes...
Blocks.
A block is a very cool variable. Most variables hold data. A block is a variable which holds code to be executed at some point in the future. So in your situation you could pass a completion block along with your method getDataAboutUser:(NSString *)user_request_id. So it would look like this.
getDataAboutUser:(NSString*)string withCompletion:(void(^)(NSData *finishedData))cBlock
Store that cBlock as an instanceVar. Then when your NSURLConnection finishes downloading all its data, you will execute the cBlock, passing in the finished data as an argument.
Blocks are a fairly complicated things if you have not used them before, so i would reccomend taking 20 minutes and reading this.

Answer 2

Since you need your method to wait for a response before returning, you can use NSURLConnection's convenience class method sendSynchronousRequest to carry out a synchronous request instead of creating and managing an NSURLConnection instance asynchronously.
So instead of your [[NSURLConnection alloc] init...] line you can do this:
NSURLResponse *response = nil;
NSError *error = nil;
NSData *responseData = [NSURLConnection sendSynchronousRequest:user_fetch_details returningResponse:&response error:&error];
Following which you can immediately parse the JSON from responseData instead of doing that in the connection:didReceiveData delegate.
Edit: Just saw user698846's suggestion to modify your method signature to take a completion block. That's also a good and possibly cleaner way to approach your problem if you are at liberty to change your method signature (i.e. nobody is requiring your function to return synchronously). Either way, sendSynchronousRequest is possibly the easiest way out and there's no shame in it especially if there's nothing your app nor your user can do while waiting for the request to complete.

Answer 3

This is some code:
NSURLResponse *response = nil;
NSError *error = nil;
NSData *responseData = [NSURLConnection sendSynchronousRequest:user_fetch_details returningResponse:&response error:&error];