What is the correct way to fire methods within a completion block (if this is even recommended)? Right now, I have an IBAction that calls a method that downloads information with a completion block signifying if the info was retrieved successfully or not. If it was, I want to push a view controller that will display that information, but at the moment, nothing is happening. I'm guessing it has something to do with main thread, gcd, etc...
__weak YTTMSetupViewController *weakSelf = self;
[mc downloadJson:^(BOOL success) {
if(success){
NSLog(#"sucess. metric count - %i",(int)mc.collection.count);
//info was downloaded. Push new view controller with info
YTTMMetricTableViewController *mtvc = [self.storyboard instantiateViewControllerWithIdentifier:#"YTTMMetricTableViewController"];
mtvc.group = (WAGroup*)[[WAMetricCollection sharedInstance].collection lastObject];
mtvc.hidesBottomBarWhenPushed = YES;
[weakSelf.navigationController pushViewController:mtvc animated:YES];
}
else{
NSLog(#"failure");
//display failure UI
}
NSLog(#"end of downloading");
[HUD dismissAfterDelay:0.5f animated:YES];
}];
Not sure if this is the right way to do it, but it worked.
I added a method that will push the vc on the main thread as so:
[weakSelf performSelectorOnMainThread:#selector(pushDetail) withObject:nil waitUntilDone:YES];
Completed Code:
__weak YTTMSetupViewController *weakSelf = self;
[mc downloadJson:^(BOOL success) {
if(success){
NSLog(#"sucess. metric count - %i",(int)mc.collection.count);
//info was downloaded. Push new view controller with info
[weakSelf performSelectorOnMainThread:#selector(pushDetail) withObject:nil waitUntilDone:YES];
}
else{
NSLog(#"failure");
//display failure UI
}
NSLog(#"end of downloading");
}];
}
-(void)pushDetail{
__weak YTTMSetupViewController *weakSelf = self;
YTTMMetricTableViewController *mtvc = [self.storyboard instantiateViewControllerWithIdentifier:#"YTTMMetricTableViewController"];
mtvc.group = (WAGroup*)[[WAMetricCollection sharedInstance].collection lastObject];
mtvc.hidesBottomBarWhenPushed = YES;
[weakSelf.navigationController pushViewController:mtvc animated:YES];
}
You could simply try wrapping the call with a dispatch_asynch block...
__weak YTTMSetupViewController *weakSelf = self;
[mc downloadJson:^(BOOL success) {
if(success){
NSLog(#"sucess. metric count - %i",(int)mc.collection.count);
dispatch_async(dispatch_get_main_queue(), ^{
//info was downloaded. Push new view controller with info
YTTMMetricTableViewController *mtvc = [self.storyboard instantiateViewControllerWithIdentifier:#"YTTMMetricTableViewController"];
mtvc.group = (WAGroup*)[[WAMetricCollection sharedInstance].collection lastObject];
mtvc.hidesBottomBarWhenPushed = YES;
[weakSelf.navigationController pushViewController:mtvc animated:YES];
});
}
else{
NSLog(#"failure");
//display failure UI
}
NSLog(#"end of downloading");
[HUD dismissAfterDelay:0.5f animated:YES];
}];
All UI updates must be performed on the main thread. Personally I prefer to do this through GCD as it produces more readable code than performSelectorOnMainThread. However, there's nothing wrong with performSelectorOnMainThread aside from personal preference in the case of calling a single UI update on the main thread following the execution of some completion block. Do note that, whichever one you choose, you should be consistent with what you use to guarantee that blocks are enqueued in the order you specified.
Working code aside, however, the convention Apple's frameworks seem to use is to perform all completion blocks on the main thread unless a queue is specified as a method parameter, in which case the completion block should be performed on that queue. So in this case I would recommend you edit your download handler class's downloadJson method to automatically perform the completion block on the main queue.
Related
I have table view that load new data (depend on page) from SQL data base. Problem is, when i load it in main thread, it block UI for a while. When i try to do "hard work" in background, and reload data in main thread, odd things start to happen, for example, table view section header move in wrong place, and i load enormous amount of data.
First case, all work but block UI for while:
[self.tableView addInfiniteScrollingWithActionHandler:^{
#strongify(self)
if (!self.viewModel.isUpdating){
self.viewModel.isUpdating = YES;
[self.tableView.infiniteScrollingView startAnimating];
[self.viewModel nextPage];
[self.tableView reloadData];
self.viewModel.isUpdating = NO;
}
}];
In second case, i tried to do background work, following not work as expected:
if (!self.viewModel.isUpdating){
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Add some method process in global queue - normal for data processing
self.viewModel.isUpdating = YES;
[self.tableView.infiniteScrollingView startAnimating];
[self.viewModel nextPage];
dispatch_async(dispatch_get_main_queue(), ^(){
//Add method, task you want perform on mainQueue
//Control UIView, IBOutlet all here
[self.tableView reloadData];
self.viewModel.isUpdating = NO;
});
//Add some method process in global queue - normal for data processing
});
}
}];
How should i modify my code to not load main thread, and without "weird" things?
have you tried something like this.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
.....
dispatch_async(dispatch_get_main_queue(), ^{
[self performSelectorOnMainThread:#selector(updateView) withObject:nil waitUntilDone:YES];
});
});
......
-(void)updateView{
[self.tableView reloadData];
self.viewModel.isUpdating = NO;
}
user PerformSelectorOnMainThread it may help you.
While loading when we click on back button or any cell of the table the action is called multiple times after loading is finish.Here the the code snippet that what i'm doing when I start the loading and stop the loading.
+(void)showLoader_OnView{
APP_DELEGATE.window.userInteractionEnabled = NO;
[MBProgressHUD showHUDAddedTo:APP_DELEGATE.window animated:YES];
}
To stop the loading:-
+(void)hideLoader {
APP_DELEGATE.window.userInteractionEnabled =YES;
[MBProgressHUD hideAllHUDsForView:APP_DELEGATE.window animated:YES];
}
please help me.
Update
actually i'm taking data from server. whenever user will go to next window then in viewWillAppear function i call a function which will hit the api to get the data.
-(void)performAutoSync
{
#try
{
if(self.shouldPerformAutoSync)//Necessary conditions to check the auto sync
{
[AppConstants showLoader_OnView]; //here i call the loader.
self.shouldPerformAutoSync = NO;
if(!self.isSyncing)
{
if(!syncBl)
{
syncBl = [[SyncBL alloc] init];
syncBl.delegate = self;
}
if(!syncDl)
syncDl = [[SyncDL alloc] init];
// [self saveModifiedDataForCurrentViewController];
[self delayToAutoSync];
NSMutableDictionary *dictMainData = [NSMutableDictionary new];
[dictMainData setObject:[syncDl fetchCompleteDataAndPrepareDictionary:YES] forKey:#"data"];//#"MainData"];
[syncBl performAutoSync:dictMainData];
}
}
}
#catch (NSException *exception) {
BILog(#"%#",exception);
}
}
Don't block the main thread.
Seeing that you invoke [AppConstants showLoader_OnView] from performAutoSync, and that showLoader_OnView in turn executes:
[MBProgressHUD showHUDAddedTo:APP_DELEGATE.window animated:YES]
I can only assume that performAutoSync is executed in the main thread. This, of course, blocks the UI until your operations are completed.
You should redesign so that you won't need all your state variables, globals, global calls, and take advantage of multi-threading.
Also, remove this, as it qualifies as a kludge;
APP_DELEGATE.window.userInteractionEnabled = NO
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 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];
})
})
}];
I have a method like:
- (BOOL)shouldDoSomeWork {
BOOL result = // here I need do hard work with data in background thread and return result, so main thread should wait until the data is calculated and then return result;
return result;
}
How to implement that?
Are you looking for this:
-(void) startWork
{
//Show activity indicator
[NSThread detachNewThreadSelector:#selector(doSomeWork) toTarget:self withObject:nil];
}
-(void) doSomeWork
{
NSAutoreleasePool *pool = [NSAutoreleasePool new];
//Do your work here
[pool release];
[self performSelectorOnMainThread:#selector(doneWork) withObject:nil waitUntilDone:NO];
}
-(void) doneWork
{
//Hide activity indicator
}
Example how to do it with GCD:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Your hard code here
// ...
//BOOL result = ...
dispatch_async(dispatch_get_main_queue(),^{
[self callbackWithResult:result]; // Call some method and pass the result back to main thread
});
});
That's not typically how you would do it. You need something structured more like this:
- (void)doSomeWorkAndThen:(^void)block {
dispatch_async(dispatch_get_global_queue(0, 0), ^ {
// do
// some
// work
dispatch_sync(dispatch_get_main_queue(), ^ {
block();
});
});
That is, you keep the request and what you do afterwards in one place.
Common advice is to use the highest level of abstraction available to you to perform a task. As such NSThread should be relatively low down in the list of things you can do to execute work in the background.
The order you investigate APIs should be like this:
NSOperation / NSOperationQueue
Grand Central Dispatch (libdispatch)
NSThread
POSIX threads
With the first two you write your code as a "unit of work" and then put this work on a queue to be executed at some point. The system takes care of creating and destroying threads for you and the APIs are easy to work with. Here's an example using NSOperationQueue.
NSBlockOperation * blockOperation = [NSBlockOperation blockOperationWithBlock:^{
//Do work
//update your UI on the main thread.
[self performSelectorOnMainThread:#selector(workDone:) withObject:workResults waitUntilDone:NO];
}];
[self.operationQueue addOperation:blockOperation];
easy as that.