I have a piece of code that firstly calls a method that does quite a bit of thumbnail generation so it slows the device down for about a second. I was hoping to run a method that generates a "loading message" before the first method is called and then remove it when the first method is finished.
[picker dismissViewControllerAnimated:YES completion:^{
NSLog(#"Loading");
[self generatingThumbnailMessageShow];
[self loadAllEffects];
}];
The problem seems to be that although the "generatingThumbnailMessageShow" method is before the "loadAllEffects" method it still seems to get called after the "loadAllEffects" message is finished. What is the best method to call the "loadAllEffects" method only when the first method is finished?
The problem is that [self loadAllEffects] runs on the main thread and blocks the UI. Changes to the UI become only visible after program control has returns to the main runloop.
You have to move the execution of [self generatingThumbnailMessageShow]; to a background thread, something like
[picker dismissViewControllerAnimated:YES completion:^{
NSLog(#"Loading");
// Show "loading" message (must be done on main thread)
[self generatingThumbnailMessageShow];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// perform long running task on background thread
[self loadAllEffects];
dispatch_sync(dispatch_get_main_queue(), ^{
// Hide "loading" message (must be done on main thread again).
[self generatingThumbnailMessageHide];
})
})
}];
Related
I am having some trouble updating my UI using performSelectorOnMainThread. Here is my situation. In my viewDidLoad I set up an activity indicator and a label. Then I call a selector to retrieve some data from a server. Then I call a selector to update the UI after a delay. Here's the code:
- (void)viewDidLoad
{
[super viewDidLoad];
self.reloadSchools = [[UIAlertView alloc] init];
self.reloadSchools.message = #"There was an error loading the schools. Please try again.";
self.reloadSchools.title = #"We're Sorry";
self.schoolPickerLabel = [[UILabel alloc]init];
self.schoolPicker = [[UIPickerView alloc] init];
self.schoolPicker.delegate = self;
self.schoolPicker.dataSource = self;
self.server = [[Server alloc]init];
schoolList = NO;
_activityIndicator = [[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
[self.view addSubview:_activityIndicator];
[self.view bringSubviewToFront:_activityIndicator];
[_activityIndicator startAnimating];
[NSThread detachNewThreadSelector: #selector(getSchoolList) toTarget: self withObject: nil];
[self performSelector:#selector(updateUI) withObject:nil afterDelay:20.0];
}
The selector updateUI checks to see if the data was retrieved, and calls a selector on the main thread to update the UI accordingly. Here is the code for these parts:
-(void)updateUI
{
self.schools = [_server returnData];
if(!(self.schools == nil)) {
[self performSelectorOnMainThread:#selector(fillPickerView) withObject:nil waitUntilDone:YES];
}
else {
[self performSelectorOnMainThread:#selector(showError) withObject:nil waitUntilDone:YES];
}
}
-(void)showError {
NSLog(#"show error");
[_activityIndicator stopAnimating];
[self.reloadSchools show];
}
-(void)fillPickerView {
NSLog(#"fill picker view");
schoolList = YES;
NSString *schoolString = [[NSString alloc] initWithData:self.schools encoding:NSUTF8StringEncoding];
self.schoolPickerLabel.text = #"Please select your school:";
self.shoolArray = [[schoolString componentsSeparatedByString:#"#"] mutableCopy];
[self.schoolPicker reloadAllComponents];
[_activityIndicator stopAnimating];
}
When the selector fillPickerView is called the activity indicator keeps spinning, the label text doesn't change, and the picker view doesn't reload its content. Can someone explain to me why the method I am using isn't working to update my ui on the main thread?
dispatch_async(dispatch_get_global_queue(0, 0), ^{
//load your data here.
dispatch_async(dispatch_get_main_queue(), ^{
//update UI in main thread.
});
});
First of all you should not be using detachNewThreadSelector. You should use GCD and submit your background task to an async queue. Threads are costly to create. GCD does a much better job of managing system resources.
Ignoring that, your code doesn't make a lot of sense to me. You submit a method, getSchoolList, to run on a background thread. You don't show the code that you are running in the background.
Then use performSelector:withObject:afterDelay to run the method updateUI on the main thread after a fixed delay of 20 seconds.
updateUI checks for self.schools, which presumably was set up by your background thread, and may or may not be done. If self.schools IS nil, you call fillPickerView using performSelectorOnMainThread. That doesn't make sense because if self.schools is nil, there is no data to fill the picker.
If self.schools is not nil, you display an error, again using performSelectorOnMainThread.
It seems to me that the logic on your check of self.schools is backwards. If it is nil you should display an error and if it is NOT nil you should fill the picker.
Next problem: In both cases you're calling performSelectorOnMainThread:withObject:waitUntilDone: from the main thread. Calling that method from the main thread doesn't make sense.
Third problem: It doesn't make sense to wait an arbitrary amount of time for a background task to run to completion, and then either succeed or fail. You won't have any idea what's going on for the full 20 seconds. If the background task finishes sooner, you'll never know.
Instead, you should have your background task notify the main thread once the task is done. That would be a valid use of performSelectorOnMainThread:withObject:waitUntilDone:, while calling it from the main thread is not. (Again, though, you should refactor this code to use GCD, not using threads directly.
It seems pretty clear that you are in over your head. The code you posted needs to be rewritten completely.
I'm using the MBProgressHUD library in my app, but there are times that the progress hud doesn't even show when i query extensive amount of data, or show right after the processing of data is finished (by that time i don't need the hud to be displayed anymore).
In another post i found out that sometimes UI run cycles are so busy that they don't get to refresh completely, so i used a solution that partially solved my problem: Now every request rises the HUD but pretty much half the times the app crashes. Why? That's where I need some help.
I have a table view, in the delegate method didSelectRowAtIndexPath i have this code:
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
[NSThread detachNewThreadSelector:#selector(showHUD) toTarget:self withObject:nil];
...
}
Then, I have this method:
- (void)showHUD {
#autoreleasepool {
[HUD show:YES];
}
}
At some other point I just call:
[HUD hide:YES];
And well, when it works it works, hud shows, stays and then disappear as expected, and sometimes it just crashes the application. The error: EXC_BAD_ACCESS . Why?
By the way, the HUD object is already allocated in the viewDidLoad:
- (void)viewDidLoad
{
[super viewDidLoad];
...
// Allocating HUD
HUD = [[MBProgressHUD alloc] initWithView:self.navigationController.view];
[self.navigationController.view addSubview:HUD];
HUD.labelText = #"Checking";
HUD.detailsLabelText = #"Products";
HUD.dimBackground = YES;
}
You need to perform your processing on another thread, otherwise the processing is blocking MBProgressHud drawing until it completes, at which point MBProgressHud is hidden again.
NSThread is a bit too low-level for just offloading processing. I'd suggest either Grand Central Dispatch or NSOperationQueue.
http://jeffreysambells.com/2013/03/01/asynchronous-operations-in-ios-with-grand-central-dispatch
http://www.raywenderlich.com/19788/how-to-use-nsoperations-and-nsoperationqueues
/* Prepare the UI before the processing starts (i.e. show MBProgressHud) */
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
/* Processing here */
dispatch_async(dispatch_get_main_queue(), ^{
/* Update the UI here (i.e. hide MBProgressHud, etc..) */
});
});
This snippet will let you do any UI work on the main thread, before dispatching the processing to another thread. It then returns to the main thread once the processing is done, to allow you to update the UI.
I'm trying to update my textView on screen before it starts downloading data. Right now, it only updates the view after all of the downloads are complete. How can I do it before or in between the downloads?
Edit: I want the self.textView.text = #"Connection is good, start syncing..."; to update the UI before the downloading starts. But right now, it only updates after the download finishes.
Here is what the code looks like.
if ([self.webApp oAuthTokenIsValid:&error responseError:&responseError]) {
self.textView.text = #"Connection is good, start syncing...";
[self.textView setNeedsDisplay];
[self performSelectorInBackground:#selector(downloadCustomers:) withObject:error];
}
I'm new to this and have yet to learn how threads work, but from what I read, the downloadCustomers function should be using a background thread leaving the main thread to update the UI.
if ([self.webApp oAuthTokenIsValid:&error responseError:&responseError]) {
self.textView.text = #"Connection is good, start syncing...";
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self downloadCustomers];
dispatch_async(dispatch_get_main_queue(), ^{
//Do whatever you want when your download is finished, maybe self.textView.text = #"syncing finished"
});
});
}
The pattern here is to initialize your download on background thread and then call back to main thread for UI update.
Below is an example using GCD. The advantage of GCD version is that you can consider using whatever you do in -downloadCustomers, to insert in-line where you call it.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self downloadCustomers];
dispatch_async(dispatch_get_main_queue(), ^{
[self.textView setNeedsDisplay];
});
});
In my application i am using back ground thread for hitting multiple service and perform operation with core data. I have used main thread for back ground process ,Its working fine.
Here is my code
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_async(main,
^{
[self backGroundCall];
});
-(void)backGroundCall
{
NSLog(#"Done");
if([CacheManager refreshDBforFirstTimeUseWithDelegate:self])
{
[[NSUserDefaults standardUserDefaults] setBool:YES forKey:#"IsDBInitialized"];
ContainerViewController *containerViewControllerInstance = [ContainerViewController getContainerInstance];
[containerViewControllerInstance setUserId:_userID];
[progressView setHidden:YES];
[self.view setUserInteractionEnabled:YES];
[self.navigationController setDelegate:containerViewControllerInstance];
[self.navigationController pushViewController:containerViewControllerInstance animated:YES];
}
}
once i initialize the data base , i need to navigate to the container view.During the initialization i will display one progress bar. That is working fine, when the entire background process is completed(app is in minimized state). During the background process if i come to the foreground progress bar is not showing at that time black screen is display instead of progress view . After the completion of the main threat container view all not display[if i comes to foreground of main thread process].
i need to show the progress bar, if i come back to the app in the middle of the main thread process. Please guide me to fix this issue.
Thanks.
dispatch_queue_t main = dispatch_get_main_queue();
dispatch_async(main,
^{
[self backGroundCall];
});
This is a bit misleading... You call the method backGroundCall, but you are actually doing this on the main thread. If you want to make some operation on a working thread, you can do this:
// Declare the queue
dispatch_queue_t workingQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(workingQueue,
^{
// My background job
dispatch_async(dispatch_get_main_queue(),
^{
// Update the UI
}
);
});
I have an app that I'm accessing a remote website with NSURLConnection to run some code and then save out some XML files. I am then accessing those XML Files and parsing through them for information. The process works fine except that my User Interface isn't getting updated properly. I want to keep the user updated through my UILabel. I'm trying to update the text by using setBottomBarToUpdating:. It works the first time when I set it to "Processing Please Wait..."; however, in the connectionDidFinishLoading: it doesn't update. I'm thinking my NSURLConnection is running on a separate thread and my attempt with the dispatch_get_main_queue to update on the main thread isn't working. How can I alter my code to resolve this? Thanks! [If I need to include more information/code just let me know!]
myFile.m
NSLog(#"Refreshing...");
dispatch_sync( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self getResponse:#"http://mylocation/path/to/file.aspx"];
});
[self setBottomBarToUpdating:#"Processing Please Wait..."];
queue = dispatch_queue_create("updateQueue", DISPATCH_QUEUE_CONCURRENT);
connectionDidFinishLoading:
if ([response rangeOfString:#"Complete"].location == NSNotFound]) {
// failed
} else {
//success
dispatch_async(dispatch_get_main_queue(),^ {
[self setBottomBarToUpdating:#"Updating Contacts..."];
});
[self updateFromXMLFile:#"http://thislocation.com/path/to/file.xml"];
dispatch_async(dispatch_get_main_queue(),^ {
[self setBottomBarToUpdating:#"Updating Emails..."];
});
[self updateFromXMLFile:#"http://thislocation.com/path/to/file2.xml"];
}
In my connectionDidFinishLoading: I would try something like this:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^ {
if ([response rangeOfString:#"Complete"].location == NSNotFound]) {
// failed
} else {
//success
dispatch_async(dispatch_get_main_queue(),^ {
[self setBottomBarToUpdating:#"Updating Contacts..."];
});
[self updateFromXMLFile:#"http://thislocation.com/path/to/file.xml"];
dispatch_async(dispatch_get_main_queue(),^ {
[self setBottomBarToUpdating:#"Updating Emails..."];
});
[self updateFromXMLFile:#"http://thislocation.com/path/to/file2.xml"];
}
});
Then all that file access is happening in a background queue so the main queue is not locked up. The main queue will also complete this call to connectionDidFinishLoading much more quickly, since you're throwing all the hard work onto the default queue instead, which should leave it (and the main thread) ready to accept your enqueuing of the updates to the UI which will be done by the default queue as it processes the block you just enqueued to it.
The queue handover becomes
main thread callback to connectionDidFinishLoad:
rapid handoff to default global queue releasing main thread
eventual hand off to main queue for setBottomBarToUpdating: calls
performing main queue blocks on main thread to properly update UI
eventual completion of blocks on main queue
eventual completion of blocks on default queue
You've increased concurrency (good where you've good multi-core devices) and you've taken the burden of I/O off the main thread (never a good place for it) and instead got it focused on user interface work (the right place for it).
Ideally you woud run the NSURLConnection run loop off the main thread too, but this will might be enough for you to get going.
Which run loop are you running the NSURLConnection in? If it's the main loop, you're queueing up the setBottomBarToUpdating: calls behind the work you're already doing, hence the probable reason why you're not seeing the UI update.
You could also give performSelectorOnMainThread try like so:
if ([response rangeOfString:#"Complete"].location == NSNotFound]) {
// failed
} else {
//success
[self performSelectorOnMainThread:#selector(setBottomBarToUpdating) withObject:#"Updating Contacts..." waitUntilDone:false];
[self updateFromXMLFile:#"http://thislocation.com/path/to/file.xml"];
[self performSelectorOnMainThread:#selector(setBottomBarToUpdating) withObject:#"Updating Emails..." waitUntilDone:false];
[self updateFromXMLFile:#"http://thislocation.com/path/to/file2.xml"];
}