Want to call a function after downloading data from server in objective c - iphone

Question

I am downloading cards from server using asynchronous request and I want the moment I finished downloading, I should be navigated to some other screen. I am downloading cards using this set of code.
NSMutableArray *array=[[NSMutableArray alloc]initWithObjects:[dictTemp objectForKey:#"image_1_url"],[dictTemp objectForKey:#"image_2_url"],[dictTemp objectForKey:#"image_3_url"], nil];
for(int i=0;i< [array count];i++)
{
NSURL* url = [NSURL URLWithString:[array objectAtIndex:i]];
NSURLRequest* request = [NSURLRequest requestWithURL:url];
[NSURLConnection sendAsynchronousRequest:request
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse * response,
NSData * data,
NSError * error) {
if (!error){
NSString *stringName=[NSString stringWithFormat:#"downloadimage%d",i+1];
UIImage *tempImage=[[UIImage alloc]initWithData:data];
[self saveLocally:tempImage andNAme:stringName];
}
[self performSelectorOnMainThread:#selector(update) withObject:nil waitUntilDone:YES];
}];
But the problem is, I am navigating to some other screen in update method, but it gets called before completion of asynchronous request.
I am downloading three images one by one using for loop as specified in the code, and I want to call update method after downloading all the three cards.
Thanks in advance

Answer 1

You can do the below if don't want want to change:
NSMutableArray *array=[[NSMutableArray alloc]initWithObjects:[dictTemp objectForKey:#"image_1_url"],[dictTemp objectForKey:#"image_2_url"],[dictTemp objectForKey:#"image_3_url"], nil];
NSInteger reqCounts = [array count];
for(int i=0;i< [array count];i++)
{
NSURL* url = [NSURL URLWithString:[array objectAtIndex:i]];
NSURLRequest* request = [NSURLRequest requestWithURL:url];
[NSURLConnection sendAsynchronousRequest:request
queue:[NSOperationQueue mainQueue]
completionHandler:^(NSURLResponse * response,
NSData * data,
NSError * error) {
if (!error){
NSString *stringName=[NSString stringWithFormat:#"downloadimage%d",i+1];
UIImage *tempImage=[[UIImage alloc]initWithData:data];
[self saveLocally:tempImage andNAme:stringName];
reqCounts --;
}
if (reqCounts == 0) {
[self performSelectorOnMainThread:#selector(update) withObject:nil waitUntilDone:YES];
}
}];
Better to check this awesome answer.

Answer 2

Instead of using the NSURLRequest class, the NSSession Object is recommended in iOS7. Try rewriting your code this way.
.......
NSURL* url = [NSURL URLWithString:[array objectAtIndex:i]];
NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDownloadTask *task = [session downloadTaskWithRequest:request completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
NSData *data = [[NSData alloc] initWithContentsOfURL:location];
NSArray *array = [NSJSONSerialization JSONObjectWithData:data options:0 error:&error];
..........
}
dispatch_async(dispatch_get_main_queue(), ^{
//call your update method here
});
Hope it helps.