Asynchronous url requests inside dispatch_async - ios

Question

I am trying to implement asynchronous url requests in a particular function, I want all these requests to complete and then do a particular action but the action precedes the requests i.e, it is getting called before the requests complete.
dispatch_queue_t fetchQ = dispatch_queue_create("Featured Doc Downloader", NULL);
dispatch_async(fetchQ, ^{
[self myAsyncMultipleURLRequestFunction];
dispatch_sync(dispatch_get_main_queue(), ^{
[self updateUIFunction];
});
});
-(void)myAsyncMultipleURLRequestFunction
{
for (int i=0; i<count; i++)
{
NSURLConnection *loginConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
}
}
now updateUIFunction gets called before myAsyncMultipleURLRequestFunction completes all requests. Also tried this with NSOperaitonQueue but could not do what I really want.
[_operationQ addOperationWithBlock: ^ {
for (int i=0; i<count; i++)
{
NSURLConnection *loginConnection = [[NSURLConnection alloc] initWithRequest:theRequest delegate:self];
}
}
[[NSOperationQueue mainQueue] addOperationWithBlock: ^ {
// updating UI
[self updateUIFunction];
}];
}];
I know this is simple but I am running outta time, any help is appreciated.

Answer 1

#tkanzakic is on the right path. The correct construct to use is the dispatch_group_t. But the implementation could be improved. By using a semaphore you can launch all your downloads asynchronously and still make sure that you don't have too many running concurrently. Here is a code sample that illustrates how you should use dispatch_group_t as well as make all your downloads parallel:
dispatch_queue_t fetchQ = dispatch_queue_create("Featured Doc Downloader", NULL);
dispatch_group_t fetchGroup = dispatch_group_create();
// This will allow up to 8 parallel downloads.
dispatch_semaphore_t downloadSema = dispatch_semaphore_create(8);
// We start ALL our downloads in parallel throttled by the above semaphore.
for (int i=0; i<count; i++) {
dispatch_group_async(fetchGroup, fetchQ, ^(void) {
dispatch_semaphore_wait(downloadSema, DISPATCH_TIME_FOREVER);
NSURLConnection *loginConnection = [[NSURLConnection alloc] initWithRequest:requestArray[i] delegate:self];
dispatch_semaphore_signal(downloadSema);
});
}
// Now we wait until ALL our dispatch_group_async are finished.
dispatch_group_wait(fetchGroup, DISPATCH_TIME_FOREVER);
// Update your UI
dispatch_sync(dispatch_get_main_queue(), ^{
[self updateUIFunction];
});
// Release resources
dispatch_release(fetchGroup);
dispatch_release(downloadSema);
dispatch_release(fetchQ);

Answer 2

You can create a dispatch_group_t and then use dispatch_group_notify to execute the updateUIFunction when the previous block of the group finish running, for example:
dispatch_queue_t fetchQ = dispatch_queue_create("Featured Doc Downloader", NULL);
dispatch_async(fetchQ, ^{
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
[self myAsyncMultipleURLRequestFunction];
});
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
dispatch_async(dispatch_get_main_queue(), ^{
[self updateUIFunction];
});
});
});

Answer 3

First Configure run loop.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSURLRequest* request = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:10];
[NSURLConnection connectionWithRequest:request delegate:self];
while(!self.finished) {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}
});
Try this