dispatch_async handle event - ios

I know dispatch_async can handle thread.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// handle things that takes a long time
dispatch_async(dispatch_get_main_queue(), ^{
// update UI
});
});
But How can I cancel it the thread?
For example:
I have two viewcontrollers- AViewController and BViewController, A->present B, and a button to dismiss B.
I'll do myMethod in BViewController, the things is I want to dismiss B when exec myMethod, so I use dispatch_async, it did dismiss from B to A. but after going back to AViewController, here comes the audio.
How can I stop the myMethod after dismiss method exec immediately?
- (void)myMethod {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// handles thing A
// play a audio
dispatch_async(dispatch_get_main_queue(), ^{
// update UI
});
});
}

What you need for this is not GCD and dispatch_async, but an NSOperation. It is a high-level wrapper around GCD that allows you to have references to your async operations, check their status, cancel them, etc.
I recommend using a very handy subclass named NSBlockOperation. You should keep a reference to your operation in your class, and if you need to cancel it, just call [self.myOperation cancel].

Related

Execute a method after the completion of another method

In my app i have two methods, storeData and gotoNextView. I want the gotoNextPage to be executed after storeData method has completed execution. In storeData i am saving the token obtained after sucessful login using Egocache, in gotoNextPage i have code which is used to load a new view controller, in the next viewcontroller i have to use the token for fetching the other details. But the problem the method gotoNextView is being executed before the storeData so i am gettin null token in the next view.
I have tried using the following :
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
[self storeData];
});
dispatch_group_notify(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
[self gotoNextPage];
the above code is serving the purpose but when i am using the above code, the NSUrlConnections in the next view are not loading.
[self storeData];
[self performSelector:#selector(gotoNextPage) withObject:nil afterDelay:1.0f];
this code is working and the NSUrlConnections in next view also working, but is there a better way to achieve this purpose
You could pass a completion block to the storeData method. That way storeData can let you know when it's finished doing what it needs to do, instead of you trying to guess.
- (void)storeDataWithCompletion:(void (^)(void))completion
{
// Store Data Processing...
if (completion) {
completion();
}
}
// Calling storeDataWithCompletion...
[self storeDataWithCompletion:^{
dispatch_async(dispatch_get_main_queue(), ^{
[self gotoNextPage];
});
}];
The dispatch_async to the main queue is not required. I added that since gotoNextPage is UI related and it's not clear what thread storeDataWithCompletion: would call the completion block from.
Here's a link to Apple's documentation on blocks
Hope this helps.

Objective-C - Wait for call on another thread to return before continuing

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];
}

How to execute statements in order in objective c

When i tap on my button, my function was called
[myBtn addTarget:self action:#selector(myFunction) forControlEvents:UIControlEventTouchUpInside];
In my function, a collection of complex statement will be executed and take a litte bit time to run, so i want to show Loading (UIActivityIndicatorView) as the following:
-(void) addTradeAction {
//Show Loading
[SharedAppDelegate showLoading];
//disable user interaction
self.view.userInteractionEnabled = NO;
//execute call webservice in here - may be take 10s
//Hide Loading
[ShareAppDelegate hideLoading];
}
When tap on myBtn (my Button) -> after 3s or 4s, [ShareAppDelegate showLoading] was called.
It is unusual when i use [ShareAppDelegate showLoading] on other Function, -> it work very nice, i mean all the statement be executed in order.
All i want, when i tap on My Button, Loading will be called immediatelly.
Tks in advance
A correct way to perform a tasks in background, and in your case showing an activity indicator, is :
-(void)myBackGroundTask
{
//here showing the 'loading' and blocking interaction if you want so
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//here everything you want to perform in background
dispatch_async(dispatch_get_main_queue(), ^{
//call back to main queue to update user interface
});
});
}
With this kind of block, you are sure that your interface do not freeze, and keep a smooth animation.
If your complex statements do not any UI animations or UI related code, then you can execute that part in a different thread(other than the mainThread). Once the statements are done(or in completion block), you can remove the loadingOverlay there.
Put myFunction to run on a background queue as it probably makes the system hang:
- (void)myFunction {
dispatch_queue_t myQueue = dispatch_queue_create("myQueue", NULL);
// execute a task on that queue asynchronously
dispatch_async(myQueue, ^{
// Put the current myFunction code here.
});
}

using dispatch_sync as a mutex lock

Here is what I need to do.
I hope dispatch_sync would be the best way to do it using GCD
I have a certain piece of critical section code that is placed in the applicationDidBecomeActive callback in Appdelegate..
I am wrapping up that method inside a dispatch_sync call so that it gets called only once no matter how many times applicationDidBecomeActive is called
- (void)applicationDidBecomeActive:(UIApplication *)application{
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(#"Thread created");
//crtical code
[self runCriticalSection];
});}
Is this the right way for doing it using dispatch_sync?
dispatch_sync() does not return until the block has finished, which means that
applicationDidBecomeActive does not return until runCriticalSection has finished
execution.
This is probably not what you want, therefore you have to use dispatch_async() (as already
stated in the other answer).
But you don't want another runCriticalSection to start
if the previous one is still running. This can be achieved with a "counting semaphore"
(which are also a feature of GCD):
static dispatch_semaphore_t sema; // The semaphore
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
// Initialize with count=1 (this is executed only once):
sema = dispatch_semaphore_create(1);
});
// Try to decrement the semaphore. This succeeds if the count is still 1
// (meaning that runCriticalSection is not executing), and fails if the
// current count is 0 (meaning that runCriticalSection is executing):
if (dispatch_semaphore_wait(sema, DISPATCH_TIME_NOW) == 0) {
// Success, semaphore count is now 0.
// Start asynchronous operation.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//critical code
[self runCriticalSection];
// Increment the semaphore count (from 0 to 1), so that the next call
// to applicationDidBecomeActive will start a new operation:
dispatch_semaphore_signal(sema);
});
}
The runCriticalSection method will be called multiple times, just not concurrently, so I don't know if this is what you want to achieve.
dispatch_sync just add the specified block to a serial queue (the default priority global queue), so if applicationDidBecomeActive gets fired two times in a row, the queue will contain two blocks that will run runCriticalSection. As the first one starts and finishes its execution, the second one will start, so there will not be any execution of the two blocks at the same time.
Is this the expected behavior? If so, dispatch_sync is the way to go.
As an add-on: if runCriticalSection performs an heavy operation, consider that dispatch_sync will block the thread that runs the applicationDidBecomeActive method (the main one if you don't call the method by hand from another thread) until that operation is finished.
If you want to avoid this, you should do something like:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self runCriticalSectionOnComplete:^{
// If you want to perform something on completion, place it here. This is called asynchronously, without blocking the main thread.
}];
});
dispatch_async will return as soon as the block is added to the queue, while dispatch_sync waits for the code inside the block to be completed.

Parsing JSON data from WebService on a background thread

I have a method that builds a package, sends it to a web service, gets a package back, opens it and returns me a nsdictionary. How can I call it on a background queue in order to display a HUD while it requests the data?
You could detach a new thread like following
- (void) fetchData
{
//Show Hud
//Start thread
[NSThread detachNewThreadSelector:#selector(getDataThreaded)
toTarget:self
withObject:nil];
}
- (void) getDataThreaded
{
//Start Fetching data
//Hide hud from main UI thread
dispatch_async(dispatch_get_main_queue(), ^{
//Update UI if you have to
//Hide Hud
});
}
Grand central dispatch (gcd) provides great support for doing what you ask. Running something in the background using gcd is simple:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_NORMAL, 0) ^{
NSDictionary* data = [self fetchAndParseData];
dispatch_async(dispatch_get_main_queue(), ^{
[self dataRetrieved:data];
});
});
This call will return immediately (so your UI will continue to be responsive) and dataRetrieved will be called when the data is ready.
Now, depending on how fetchAndParse data works it may need to be more complicated. If you NSURLConnection or something similar, you might need to create an NSRunLoop to process data callbacks on the gcd thread. NSURLConnection for the most part is asynchronous anyway (though callbacks like didReceiveData will be routed through the UI thread) so you can use gcd only to do the parsing of the data when all the data has been retrieved. It depends on how asynchronous you want to be.
In addition to previous replies, why don't you use NSOperation and NSOperationQueue classes? These classes are abstractions under GCD and they are very simple to use.
I like NSOperation class since it allows to modularize code in apps I usually develop.
To set up a NSOperation you could just subclass it like
//.h
#interface MyOperation : NSOperation
#end
//.m
#implementation MyOperation()
// override the main method to perform the operation in a different thread...
- (void)main
{
// long running operation here...
}
Now in the main thread you can provide that operation to a queue like the following:
MyOperation *op = [[MyOperation alloc] initWithDocument:[self document]];
[[self someQueue] addOperation:op];
P.S. You cannot start an async operation in the main method of a NSOperation. When the main finishes, delegates linked with that operations will not be called. To say the the truth you can but this involves to deal with run loop or concurrent behaviour.
Here some links on how to use them.
http://www.cimgf.com/2008/02/16/cocoa-tutorial-nsoperation-and-nsoperationqueue/
https://developer.apple.com/cocoa/managingconcurrency.html
and obviously the class reference for NSOperation

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