UI still delayed even using GCD - ios

Question

In my modal UI there is a "DONE" button linked with IBAction -done:, it will upload a text to (lets say Dropbox server). Its code looks like this
- (IBAction)done:(id)sender {
// must contain text in textview
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];
if (![_textView.text isEqualToString:#""]) {
// check to see if we are adding a new note
if (!self.note) {
DBFile *newNote = [[DBFile alloc] init];
newNote.root = #"dropbox";
self.note = newNote;
}
_note.contents = _textView.text;
_note.path = _filename.text;
// - UPLOAD FILE TO DROPBOX - //
NSLog(#"Initializing URL...");
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
NSURL *url = [Dropbox uploadURLForPath:self.note.path];
NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:url];
[request setHTTPMethod:#"PUT"];
NSData *noteContents = [self.note.contents dataUsingEncoding:NSUTF8StringEncoding];
NSLog(#"Creating session task...");
NSURLSessionUploadTask *uploadTask = [self.session uploadTaskWithRequest:request
fromData:noteContents
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSHTTPURLResponse *resp = (NSHTTPURLResponse *) response;
if (!error && resp.statusCode == 200) {
NSLog(#"OK");
dispatch_async(dispatch_get_main_queue(), ^{
[self.delegate noteDetailsViewControllerDoneWithDetails:self];
});
} else {
NSLog(#"Status code: %d", resp.statusCode);
}
}];
[uploadTask resume];
});
} else {
UIAlertView *noTextAlert = [[UIAlertView alloc] initWithTitle:#"No text"
message:#"Need to enter text"
delegate:nil
cancelButtonTitle:#"Ok"
otherButtonTitles:nil];
[noTextAlert show];
}
}
The delegate method noteDetailsViewControllerDoneWithDetails: of this class is look like this
-(void)noteDetailsViewControllerDoneWithDetails:(NoteDetailsViewController *)controller{
// refresh to get latest
[self dismissViewControllerAnimated:YES completion:nil];
[self notesOnDropbox];}
(notesOnDropbox is a time-consuming task). When DONE button is tapped, I expect this modal VC/UI to dismiss immediately and it fetches data on background (by notesOnDropbox method). However, when I try tapping DONE button, my UI stop responding for about seconds, after that the modal UI is dismissed. I cannot figure out where I misuse the GCD. Please help me.

Answer 1

if you want to dismiss your modal VC/UI immediately, just ask the delegate to dismiss,
like is:
- (IBAction)done:(id)sender {
[self.delegate noteDetailsViewControllerDoneWithDetails:self];
// ...
}
In your sample code,
you do the dismiss action after the upload task completed, but the upload task is asynchronous.
and you ask the delegate to dismiss use GCD dispatch_async, this is asynchronous task, too.
After all, you have to consider the what time to do upload, who to do upload task and what time to invoke notesOnDropbox.

Answer 2

First, if notesOnDropbox is a time-consuming task, then you should not be performing it on the main thread (as you are doing). If it is sufficiently time-consuming and you do it on the main thread, the WatchDog process will kill your app dead right before the user's eyes.
Second, there is no need to get off the main thread to do an upload. If you use NSURLSession correctly, it will be asynchronous.

Answer 3

Your code only calls noteDetailsViewControllerDoneWithDetails when the whole upload task is completed, because that's how you wrote your code. Actually, the situation seems worse. If the upload task has any kinds of problems, noteDetailsViewControllerDoneWithDetails will never be called.
You need to call noteDetailsViewControllerDoneWithDetails as soon as possible, and then think about what you are going to do when the upload fails - which might easily happen a long time later.