How to wait till asynchronous request returns data before returning value

Question

I have send asynchronous request to a website using the following code:

NSMutableURLRequest *requestSiteToSendData = [NSMutableURLRequest requestWithURL:[[NSURL alloc]initWithString:@"www.example.com"] cachePolicy:NSURLRequestReloadIgnoringCacheData  timeoutInterval:30];
NSURLConnection *connectionToSiteToSendData = [[NSURLConnection alloc]initWithRequest:requestSiteToSendData delegate:self];

Then I used the following method defined inside NSURLConnectionDelegate to get and parse the data after the data fetching is completed.

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    //parse 'data'
    NSString *parsedData = [self parseDataWithData:data];
}

And then in the method in which I send the asynchronous request, I return parsedData. But the returning should only happen after the data fetching is completed and hence parsing is done. I know the question arises if that is what I need then why I am not using synchronous request. It is because I don't want my other methods to hang up when the loading is going on in background.

Answer 1

Quick answer : if it's asynchronous, you don't want to wait the asynchronous method. One of the bests option would be :

• The object calling wanting the data should set itself as the object that runs the asynchronous method, and in didReceiveData, you call a method such as updateData:(NSString *)parsedData, which handles the newly received data

• The object calling the method should use KVO to observe any change on a property of the object that runs the asynchronous method.

Tell me if you need more informations.

Answer 2

Asynchronous requests run on separate thread, So we don't need to worry about handling view lockup. If you want send a synchronous request then you have to use GCD to achieve the same. And various other details like, how much data is send/received etc. will not be available in synchronous request.

Synchronous request are helpful if your code next state is dependent on data received in response of the request.

Answer 3

You have this delegate method which will execute when all the downloading is completed from tha server successfully.Use this method to do the remaining process

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    // do something with the data
    // receivedData is declared as a method instance elsewhere
    NSLog(@"Succeeded! Received %d bytes of data",[receivedData length]);
}

Ok this method gets executed when the all data is recieved.What you should do is to collect all data from -didReceiveData delegate method and then use it to parse in this method.

A must read document for you

Answer 4

As far as i understand you want that to return data after web call is complete. so i would suggest that create any method for webcall that returns NSData and do something like this:

NSHTTPURLResponse* urlResponse = nil;
NSError *error = [[NSError alloc] init];  
NSData *responseData = [NSURLConnection sendSynchronousRequest:theRequest returningResponse:&urlResponse error:&error];  

if ([urlResponse statusCode] >= 200 && [urlResponse statusCode] < 300) {
       // return responseData from here.
}
else {
        NSLog(@"%d",[urlResponse statusCode]);
        NSString *result = [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding];
        NSLog(@"%@",result);

}

and you don't want to hung up your View. so call this method in background thread. like this:

[self performSelectorInBackground:@selector(WebCallMethod) withObject:nil];

Hope it Helps!!