Wait for Asynchronous request before execution of other code - ios

Question

I'am using [NSURLSession sharedSession] dataTaskWithRequest to get data from webserver and display it in a label & set button images according to it
But my problem is first label code is executed and they are displayed as empty then async block code is finished.
if(![self connected])
{
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
arrMyres = [[NSMutableArray alloc] initWithArray:[prefs objectForKey:#"Myres"]];
}
else
{
// POSTrequest with URL to get data from server
[[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:
^(NSData * _Nullable responseData,
NSURLResponse * _Nullable urlResponse,
NSError * _Nullable error) {
if (error) {
//Display error
}
else
{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)urlResponse;
if([httpResponse statusCode ]>=200 && [httpResponse statusCode]<300)
{
dispatch_async(dispatch_get_main_queue(), ^{
NSDictionary *dataDictionary = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&error];
NSArray *array=[[dataDictionary objectForKey:#"GetExistingFavorites"] isKindOfClass:[NSNull class]]? nil:[dataDictionary objectForKey:#"GetExistingFavorites"];
arrMyres=[[NSMutableArray alloc]initWithArray:array];
});
}
}
}] resume];
}
//This block of code executes first and displays empty label
if([arrMyres count]>0 )
{
//Set Button Image and display label
}
else
{
// Do something else
}
How to wait for asynchrous request to complete execution and use it results somewhere after it? During research I found about dispatch groups and completion handlers. but couldnt understand how to implement
Any suggestions would be helpful.

Answer 1

Update firstLabel UI code inside of async code using main thread. Refer below code
if(![self connected])
{
NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];
arrMyres = [[NSMutableArray alloc] initWithArray:[prefs objectForKey:#"Myres"]];
[self updateUI];
}
else
{
// POSTrequest with URL to get data from server
[[[NSURLSession sharedSession] dataTaskWithRequest:request completionHandler:
^(NSData * _Nullable responseData,
NSURLResponse * _Nullable urlResponse,
NSError * _Nullable error) {
if (error) {
//Display error
}
else
{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)urlResponse;
if([httpResponse statusCode ]>=200 && [httpResponse statusCode]<300)
{
dispatch_async(dispatch_get_main_queue(), ^{
NSDictionary *dataDictionary = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:&error];
NSArray *array=[[dataDictionary objectForKey:#"GetExistingFavorites"] isKindOfClass:[NSNull class]]? nil:[dataDictionary objectForKey:#"GetExistingFavorites"];
arrMyres=[[NSMutableArray alloc]initWithArray:array];
//This block of code executes first and displays empty label
if([arrMyres count]>0 )
{
[self updateUI];
}
}
}
}] resume];
}
-(void)updateUI {
dispatch_async(dispatch_get_main_queue(), ^{
//update UI in main thread.
//Add your ui updates
});
}