I read that [_tableView reloadData] sends a message to the main queue to update and display the data in the Table View. Based on this I would like to discuss the following case. Suppose another method sends a message to the main queue before [_tableView reloadData] in that case will the second message get processed before [_tableView reloadData] ?
Now this is my case
Suppose I have two threads TA and TB and I have two methods MethodAand MethodB which look like this
This is MethodA
- (void) MethodA
{
dispatch_async(dispatch_get_main_queue(), ^{
[Myarray addObject:#"SomeObject"];
///----------<TimeFrameA>----------------
[_tableView reloadData]
});
}
This is MethodB
- (void) MethodB
{
dispatch_async(dispatch_get_main_queue(), ^{
//Runs under the assumption the "SomeObject" has already been displayed in Tableview
//Make changes to TableView/
});
}
Suppose that MethodB is called by ThreadB and occurs during TimeFRameA.
in that case will MethodB be called before [_tableView reloadData] ?
Is there any way for me to make sure that the MethodB only runs when the tableView is displaying the updated data ?
The main dispatch queue (which is associated with the main thread)
is a serial queue, not concurrent. Therefore it cannot happen
that in
dispatch_async(dispatch_get_main_queue(), ^{
[Myarray addObject:#"SomeObject"];
///----------<TimeFrameA>----------------
[_tableView reloadData]
});
}
any other code executes on the main queue between adding the object
and reloading the table view.
Any other block dispatched to the main queue executes either before
or after this block.
I would recommend you to use "dispatch_group" to synchronize.
Create a dispatch group object by calling:
dispatch_group_t group = dispatch_group_create();
In MethodA use:
-(void) MethodA {
dispatch_group_enter(group);
dispatch_async(dispatch_get_main_queue(), ^{
[Myarray addObject:#"SomeObject"];
///----------<TimeFrameA>----------------
[_tableView reloadData];
dispatch_group_leave();
});
}
In MethodB wait for the group to complete and then perform further operations:
-(void) MethodB {
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
//Runs under the assumption the "SomeObject" has already been displayed in Tableview
//Make changes to TableView/
});
}
Related
I've found this method to work with background thread. My question is that I've run a whole process in background thread which include number of methods. Frist method calls the second one and the the second one makes some data and pass it to the third one.
-(void)firstMethod
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
if(someCondition == 0)
{
[self secondMethod:myArray];
}
}
dispatch_async(dispatch_get_main_queue(), ^(void){
[self.navigationController popViewControllerAnimated:YES];
});
});
}
-(void)secondMethod:(NSArray *)array {
a= a+3;
[self thirdMethod:array[a];
}
So you get the general idea right? So do I have to put the functionality of second and third method in background thread too? Or how this whole process will take place?
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.
In my iOS application, I have a database call that takes some time to complete. I have a spinner visible on the screen while this operation is taking place. I am hitting an error with the app crashing with "com.myapp failed to resume in time" so it seems like it is running the database call on the main thread, causing issues.
Current Code
-(void)timeToDoWork
{
...
[CATransaction flush];
[[DatabaseWorker staticInstance] doWork];
//Additional UI stuff here
...
if([self->myReceiver respondsToSelector:self->myMessage])
{
[self->myReceiver performSelector:self->myMessage];
}
}
To get the doWork function to take place on a background thread, it looks like I can use Grand Central Dispatch:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
[[DatabaseWorker staticInstance] doWork];
});
However, how do I prevent the execution from continuing until it is complete? Should I end the method after the doWork call, and move everything below it to a new function?
Sample
-(void)timeToDoWork
{
...
[CATransaction flush];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
[[DatabaseWorker staticInstance] doWork];
dispatch_async(dispatch_get_main_queue(), ^{
[self doneDoingWork];
});
});
}
-(void)doneDoingWork
{
//Additional UI stuff here
...
if([self->myReceiver respondsToSelector:self->myMessage])
{
[self->myReceiver performSelector:self->myMessage];
}
}
Is there a better way to do this?
Prevent execution in main thread from continuing is really bad idea. iOS will terminate your application since main thread should always work with run loop.
I suggest you following way to handle your problem:
Write a "Locker". Let it show some view with animated spinner and no buttons at all.
When you start dispatch async operation just bring it to the front and let it work with run loop.
When your async operation completes close the locker.
You can also use blocks.
e.g..
- (void)doWorkWithCompletionHandler:(void(^)())handler {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
// do your db stuff here...
dispatch_async(dispatch_get_main_queue(), ^{
handler();
});
});
}
And then use it like that:
[[DatabaseWorker staticInstance] doWorkWithCompletionHandler:^{
// update your UI here, after the db operation is completed.
}];
P.S.
It might be a good idea to copy the handler block.
The error you are receiving suggests that you are doing something in application:didFinishLaunchingWithOptions: or applicationDidBecomeAction: or somewhere else in the launch cycle that is taking too long and the app is getting terminated by the launch watchdog timer. Above all, it is vital that you return as quickly as possible from these methods. I'm not sure where your code fits into the launch cycle; but this explanation seems plausible.
There are all sorts of ways to address this; but taking the lengthy process off the main queue is the first step as you noted. Without knowing more about what main queue objects (e.g. UI) depend on this database transaction, I'd say that your suggested solution is perfectly fine. That is, dispatch the work to a background queue; and on completion dispatch the remaining UI work to the main queue.
Delegates were suggested elsewhere as a solution. That's also workable although you still have to concern yourself with which queue the delegate methods get called on.
I think that you should use a delegate in your DatabaseWorker and the method doWork always run in background, so when the worker finish the work it tell to its delegate that the work is finished. The delegate method must be called in the main thread.
In the case that you have many objects that need to know when the DatabaseWorker finish instead to use a delegate I would use notifications.
EDIT:
In the DatabaseWorker class you need to implement the method doWork like this:
- (void) doWork{
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
//Do the work.
dispatch_async(dispatch_get_main_queue(), ^{
[self.delegate finishWork];
});
});
}
And in the class that implement timeTodoWork:
-(void)timeToDoWork
{
...
[CATransaction flush];
[[DatabaseWorker staticInstance] setDelegate:self];
[[DatabaseWorker staticInstance] doWork];
}
#pragma mark DatabaseWorkerDelegate
- (void) finishWork{
//Additional UI stuff here
...
if([self->myReceiver respondsToSelector:self->myMessage])
{
[self->myReceiver performSelector:self->myMessage];
}
}
Also you can use:
[self performSelectorInBackground:#selector(doWorkInBackground) withObject:nil];
instead of:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^{
//Do the work.
});
And add a method:
- (void) doWorkInBackground{
//Do the work
[self.delegate performSelectorOnMainThread:#selector(finishWork) withObject:nil waitUntilDone:NO];
}
In my app, I have a UITableViewController.
Its tableView is divided in 3 sections.
I download datas for each of those sections from my server. To do this, I have 3 functions (for example f1 f2 and f3). Each updates a corresponding NSArray, used as data source for my table.
Now what I want is to reload datas using this functions and refresh my tableView once this 3 functions are done, but without disturbing the user.
I'm not used with asynchronous request, blocks, threads etc... and I'm looking for tips.
Actually, here is what I do :
-(void)viewDidLoad
{
//some settings
[NSTimer scheduledTimerWithTimeInterval:15.0 target:self selector:#selector(reloadDatas) userInfo:nil repeats:YES];
dispatch_queue_t queue = dispatch_get_main_queue();
dispatch_async(queue, ^{
[self reloadDatas];
});
}
-(void)reloadDatas
{
dispatch_queue_t concurrentQueue = dispatch_get_main_queue();
dispatch_async(concurrentQueue, ^{
[self f1];
[self f2];
[self f3];
[myDisplayedTable reloadData];
});
}
-(void)f1
{
//load datas with a url request and update array1
}
-(void)f2
{
//load datas with a url request and update array2
}
-(void)f3
{
//load datas with a url request and update array3
}
But here, my tableView is "frozen" until it is refreshed.
I don't care about the order of execution of f1 f2 and f3, but I need to wait for this 3 functions to be done before refresh my tableView.
Thanks for your help.
EDIT
Thanks for all your answers.
Here is the working solution :
As mros suggets, I removed the dispatch queue from the viewDidLoad, and replace in reloadDatas:
dispatch_queue_t concurrentQueue = dispatch_get_main_queue();
with
dispatch_queue_t mainThreadQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
And finally, I reload my table in a main thread
dispatch_async(dispatch_get_main_queue(), ^{ [myDisplayedTable reloadData]; });
So your "background thread" is actually your main thread. You have to use dispatch_get_global_queue and specify a priority to actually get a different thread. Also, the dispatch async in viewDidLoad is useless as all view controller lifecycle methods are called in the main thread. I would recommend doing something as follows in your f1, f2 and f3 methods:
Start by launching an asynchronous url request, then in the completion block, update arrayX and reload a particular section of your tableview. This way all three requests can happen simultaneously and the table just updates the necessary data when each one finishes. Alternatively, if you only want to reload once, just replace the concurrentQueue variable you have with a background thread and then perform [tableView reloadData] on the main thread.
The previous answers are absolutely right. However your implementation of reloadDatas & viewDidLoad is a bit problematic.
Just to clarify:
You want to complete the time consuming data loading stuff in a background thread, then update the UI/Cells when your data is ready on the main thread.
Like so:
-(void)viewDidLoad
{
dispatch_queue_t concurrentQueue = dispatch_queue_create("com.my.backgroundQueue", NULL);
dispatch_async(concurrentQueue, ^{
[self reloadDatas];
});
}
-(void)reloadDatas
{
// Expensive operations i.e pull data from server and add it to NSArray or NSDictionary
[self f1];
[self f2];
[self f3];
// Operation done - now let's update our table cells on the main thread
dispatch_queue_t mainThreadQueue = dispatch_get_main_queue();
dispatch_async(mainThreadQueue, ^{
[myDisplayedTable reloadData]; // Update table UI
});
}
One other thing. Pulling data from a server and updating table cells is pretty common.
No need for queues or timers here.
Here's an alternative structure.
Say you're pulling mp3's from your server :
Your model class is : Music.h/m
Your Model manager is : MusicManager.h/m (Singleton) - it will contain an array of music objects - that singleton is basically your datasource;
and finally your UItableViewController : MusicTableVC.h/m
In MusicManager.h/m : You have an NSMutableArray which will be loaded with Music.h objects that you've pull from the server. You can do that as soon as you app loads without even waiting for the TableViewController.
Inside MusicManager you have a few helper methods to add or remove items from the mutableArray and provide the count and of course your networking methods.
Finally : Post a notification in your network code. Your UITableViewController should listen/observe that notification and "reload" accordingly.
[[NSNotificationCenter defaultCenter] postNotificationName:#"NewMusicAdded" object:nil];
You query data from your server, parse the data into Music objects add them to your NSMutable array and post a notification to let the table update itself.
Pretty standard recipe.
In reloadDatas method you should change this line:
dispatch_queue_t concurrentQueue = dispatch_get_main_queue();
To:
dispatch_queue_t concurrentQueue = dispatch_queue_create("some queue", NULL);
But when you call [myDisplayedTable reloadData], you need to call this operation in the main queue.
dispatch_async(dispatch_get_main_queue(), ^{ [myDisplayedTable reloadData]; });
I am trying to process method asynchronously, as per requirements, once the first method has completed, only then the second method should start executing. The Problem is first method itself has code that runs on background thread.
I tried dispatch_semaphore_wait, but that didnt work either.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
[self firstMethod];
NSLog(#"firstMethod Done");
});
dispatch_group_notify(group, queue, ^ {
NSLog(#"1st method completed");
NSLog(#"2nd method starting");
[self secondMethod];
});
FirstMethod itself runs on another worker thread like this
-(void)firstMethod
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//processing here.....
}];
What is the best way to achieve it, I cannot change the definition of firstMethod as it provided by some 3rd party and also changing it means changing lots of existing code from where this method is being called
You can use a completion block. You just need to modify firstMethod this way:
- (void)firstMethodWithOnComplete:(void (^)(void))onComplete {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//processing here.....
onComplete();
});
}
And then use it this way:
[self firstMethodWithOnComplete:^{
[self secondMethod];
}];
Dispatch a Single Queue and call your Methods in order
dispatch_group_async(group, queue, ^{
[self firstMethod];
NSLog(#"firstMethod Done");
[self secondmethod];
});
Or you might dispatch a group of 3 concurrent queues(This is a Wild guess)