How to wait for completion of NSURLSession? - ios

Question

I have a method that downloads a binary from a server and returns it.
But before the NSURLSession completes, my function is returning the value, so it's coming to be nil each time.
How can I wait till the download is complete and then return the binary?

Answer 1

Try this -
NSURLSession *delegateFreeSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: nil delegateQueue: [NSOperationQueue mainQueue]];
[[delegateFreeSession dataTaskWithURL: [NSURL URLWithString: #"http://www.example.com/"]
completionHandler:^(NSData *data, NSURLResponse *response,
NSError *error) {
NSLog(#"Got response %# with error %#.\n", response, error);
NSLog(#"DATA:\n%#\nEND DATA\n",
[[NSString alloc] initWithData: data
encoding: NSUTF8StringEncoding]);
[self loadDataToView:data]; // << your custom method inside the MyViewControllerClass
}] resume];

Answer 2

Your method should take a callback as an argument. Once the NSURLSession completion handler gets the object you need, you call that callback with the data (or an NSError object if you got an error back from the server)
You can't 'wait' for execution to continue after you get the data. By definition, such network operations are handled asynchronously, hence the need for a callback.
Update: sample code below
- (void)getFeed:(NSDictionary*)parameters fromURL:(NSString*)url withCallback:(void (^)(NSData *data, NSURLResponse *response, NSError *error))callback {
NSURL*newsfeedUrl = [NSURL URLWithString:url];
NSMutableURLRequest *newsfeedRequest = [NSMutableURLRequest requestWithURL:newsfeedUrl];
[newsfeedRequest addValue:#"XMLHTTPRequest" forHTTPHeaderField:#"X-Requested-With"];
newsfeedRequest.HTTPMethod = #"GET";
NSURLSessionDataTask *downloadNeewsfeedDataTask = [instance.session dataTaskWithRequest:newsfeedRequest completionHandler:^(NSData*data, NSURLResponse*response, NSError*error) {
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*) response;
NSLog(#"statusCode %ld", (long)httpResponse.statusCode);
//Do something here in your completion handler and call your callback function when ready (either with the data you expected, or with an error object. Alternatively, you can outright refrain from adding a completion handler block and simply put your callback argument as the completion handler (they must have the same signature).
callback(data, response, error);
}];
[downloadNeewsfeedDataTask resume];
}

Answer 3

Here is the example of completion handler for NSURLSession.
-(void) httpPostWithCustomDelegate{
NSURLSessionConfiguration *defaultConfigObject = [NSURLSessionConfiguration defaultSessionConfiguration];
NSURLSession *defaultSession = [NSURLSession sessionWithConfiguration: defaultConfigObject delegate: nil delegateQueue: [NSOperationQueue mainQueue]];
NSURL * url = [NSURL URLWithString:#"http://hayageek.com/examples/jquery/ajax-post/ajax-post.php"];
NSMutableURLRequest * urlRequest = [NSMutableURLRequest requestWithURL:url];
NSString * params =#"name=Ravi&loc=India&age=31&submit=true";
[urlRequest setHTTPMethod:#"POST"];
[urlRequest setHTTPBody:[params dataUsingEncoding:NSUTF8StringEncoding]];
NSURLSessionDataTask * dataTask =[defaultSession dataTaskWithRequest:urlRequest
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSLog(#"Response:%# %#\n", response, error);
if(error == nil)
{
NSString * text = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];
NSLog(#"Data = %#",text);
}
}];
[dataTask resume];
}
Here is good tutorial for NSURLSession.