Running NSURLSession completion handler on main thread - ios

Question

I am using a NSURLSession to get the values to populate a TableView. I am updating the TableView in the completion handler, but using [[NSThread currentThread] isMainThread] has shown me that the completion handler isn't running in the main thread. Since I should only updating the UI from the main thread, I know this isn't correct. Is there a way to trigger an action on the main thread from the completion handler? Or is using a NSURLSession the wrong way to go about this?
NSURLSession *session = [NSURLSession sharedSession];
[[session dataTaskWithURL:[NSURL URLWithString:#"http://myurl"]
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSError *jsonError = nil;
NSArray* jsonUsers = [NSJSONSerialization JSONObjectWithData:data options:0 error:&jsonError];
if (jsonError) {
NSLog(#"error is %#", [jsonError localizedDescription]);
// Handle Error and return
return;
}
self.userArray = jsonUsers;
[self.userTableView reloadData];
if ([[NSThread currentThread] isMainThread]){
NSLog(#"In main thread--completion handler");
}
else{
NSLog(#"Not in main thread--completion handler");
}
}] resume];

Answer 1

Yes, just dispatch your main thread stuff using GCD:
NSURLSession *session = [NSURLSession sharedSession];
[[session dataTaskWithURL:[NSURL URLWithString:#"http://myurl"]
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSError *jsonError = nil;
NSArray* jsonUsers = [NSJSONSerialization JSONObjectWithData:data options:0 error:&jsonError];
if (jsonError) {
NSLog(#"error is %#", [jsonError localizedDescription]);
// Handle Error and return
return;
}
dispatch_async(dispatch_get_main_queue(), ^{
self.userArray = jsonUsers;
[self.userTableView reloadData];
if ([[NSThread currentThread] isMainThread]){
NSLog(#"In main thread--completion handler");
}
else{
NSLog(#"Not in main thread--completion handler");
}
});
}] resume];

Answer 2

#graver's answer is good. Here's another way you can do it:
NSURLSession* session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]
delegate:nil
delegateQueue:[NSOperationQueue mainQueue]];
[[session dataTaskWithURL:[NSURL URLWithString:#"http://myurl"]
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSError *jsonError = nil;
NSArray* jsonUsers = [NSJSONSerialization JSONObjectWithData:data options:0 error:&jsonError];
if (jsonError) {
NSLog(#"error is %#", [jsonError localizedDescription]);
// Handle Error and return
return;
}
self.userArray = jsonUsers;
[self.userTableView reloadData];
if ([[NSThread currentThread] isMainThread]){
NSLog(#"In main thread--completion handler");
}
else{
NSLog(#"Not in main thread--completion handler");
}
}] resume];
This way you create a session that calls the completion block and any delegate methods on the main thread. You may find this more aesthetically pleasing, but you do lose the advantage of running the "hard work" in the background.

Answer 3

Here is the best way to update UI from blocks and completion handler, and also when you not confrim which thread running your code.
static void runOnMainThread(void (^block)(void))
{
if (!block) return;
if ( [[NSThread currentThread] isMainThread] ) {
block();
} else {
dispatch_async(dispatch_get_main_queue(), block);
}
}
This is static method which will have a block, and will run on main thread, it will act like a
runOnMainThread(^{
// do things here, it will run on main thread, like updating UI
});

Answer 4

You can try this:
[self.userTableView performSelectorOnMainThread:#selector(reloadData) withObject:nil waitUntilDone:NO];

Answer 5

send a notification on completion that will be observed by the table view controller which will then do a reloadData;
this has the advantage that if you later move this download code off to a separate class, e.g. a model object, then no changes are required and also the code can be reused in other projects without making changes

Answer 6

Swift 3.1
DispatchQueue.main.async {
tableView.reloadData()
}