Issue in execute a method after completion of another method - iOS - ios

I have two methods as loadTopicPostsFromDB and loadTopicPosts. In the loadTopicPostsFromDB method I am updating the value of a global NSString called strLastTimeStamp which should use in the loadTopicPosts. Thus, I want to execute loadTopicPostsFromDB first and after it finished(global string updated) I want to execute loadTopicPosts method.
This is how I did it. But, currently loadTopicPosts method executes before updating the global strLastTimeStamp, so always I get a wrong strLastTimeStamp.
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
[self performSelectorOnMainThread:#selector(loadTopicPostsFromDB) withObject:nil waitUntilDone:NO];
});
dispatch_group_notify(group, queue, ^{
NSLog(#"LoadDBCompleted");
[self loadTopicPosts];
});
How can I do this, please advice me on what is the wrong in this implementation.

performSelectorOnMainThread: is finished as soon as iOS has put the task into a queue. The selector has most likely not even started running when the call returns. And really, you shouldn't be using performSelectorOnMainThread at all - the function isn't available in Swift, for good reason. The solution is a lot easier (fix the problems yourself):
dispatch_async (dispatch_get_main_queue (), ^{
[self loadTopicsFromDB];
[self loadTopicPosts];
});
You probably want to perform loadTopicsFromDB on a background thread though.

When you are doing something using network connection I advice you to use blocks to handle the endpoint of the call.
It is pretty simple to write in this code
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self loadTopicsFromDB: ^(BOOL success, NSError *error) {
[self loadTopicPosts];
}];
});

Related

Dispatch_sync or completion block difference in objective-c

I'm struggling with GCD and blocks. I'm trying to create a series of methods that require data from a previous one. I was thinking about 2 different ways to achieve it.
dispatch_sync serial queue
nested completion blocks
Don't you think the following 2 options return the same value? AS far as I read in Apple's dispatch queues, DISPATCH_QUEUE_SERIAL runs in FIFO order. So both options should return identical values.
What am I doing wrong here? and which one is the best approach?
Thanks for your help!
//Option 1
dispatch_queue_t delete_queue = dispatch_queue_create("delete_queue", DISPATCH_QUEUE_SERIAL);
dispatch_sync(delete_queue, ^{
[self dosomething];
});
dispatch_sync(delete_queue, ^{
[self dosomething2];
});
dispatch_sync(delete_queue, ^{
[self dosomething3];
});
//Option 2
-(void)dosomething1:(dispatch_block_t)completion;
-(void)dosomething2:(dispatch_block_t)completion;
-(void)dosomething3:(dispatch_block_t)completion;
[self dosomething:^{
[self dosomething2:^{
[self dosomething3:^{}];
}];
}];
-(void)dosomething:(dispatch_block_t)completion {
/*method logic here*/
completion();
}
-(void)dosomething2:(dispatch_block_t)completion {
/*method logic here*/
completion();
}
-(void)dosomething3:(dispatch_block_t)completion {
/*method logic here*/
completion();
}
Both code samples you have shown are equivalent to just:
[self dosomething];
[self dosomething2];
[self dosomething3];
In other words, both ways execute the methods synchronously, in order, and block the thread until they are done.
Also, as Ken Thomases said, none of your methods "return" anything, so your question about returning doesn't make sense.
It doesn't really make sense to do three separate calls to dispatch_sync() here:
dispatch_sync(delete_queue, ^{
[self dosomething];
});
dispatch_sync(delete_queue, ^{
[self dosomething2];
});
dispatch_sync(delete_queue, ^{
[self dosomething3];
});
You should instead just do them all in a single block:
dispatch_sync(delete_queue, ^{
[self dosomething];
[self dosomething2];
[self dosomething3];
});
As for the use of completion blocks, you can certainly obtain a similar result, except that the completion-handler result would need to be asynchronous.
I think you need to take a step back and explain what kind of API you are trying to design in order to determine how you want to use the tools of the system and language to achieve that API design.

Handle concurrency and asynchronous response

I am trying to implement concurrency in objective C. I have a problem with an actions that needs to be run in a synchronized way. The problem here is that I use function that executes a block after completion.
I want to connect to a bluetooth device to run some operations and connect to the next device.
for (Beacon * beacon in beacons) {
[beacon setDelegate:self];
[beacon connectToBeacon];
}
But the connection is asynchronous. The beacon call the delegate (in this case it's the same class) method didConnectSuccess when connection is successful.
I need to wait all my operations in "beaconDidConnect" and deconnection to finish before connecting to the next device.
I currently use a combination of dispatch queue and dispatch semaphore, my semaphore is an ivar
dispatch_queue_t myCustomQueue;
myCustomQueue = dispatch_queue_create("com.example.MyCustomQueue", NULL);
for (Beacon * beacon in beacons) {
[beacon setDelegate:self];
dispatch_async(myCustomQueue, ^{
dispatch_semaphore_wait(semaphoreBluetooth, DISPATCH_TIME_FOREVER);
[beacon connectToBeacon];
});
}
In combination with
- (void)beaconDidDisconnect:(Beacon *)beacon
{
dispatch_semaphore_signal(semaphoreBluetooth);
}
Without the dispatch_async, by blocking the callback (beaconDidConnect), the wait was causing a deadlock.
I wanted to dispatch_semaphore_wait in the for loop and not in the dispatch block but the wait causes the callback to wait again, causing a deadlock.
This way it seems to work but I found it a bit ugly.
My other issue is that in my beaconDidConnect method I need to chain asynchronous call and in each waiting the previous to terminate.
All those calls have a termination block, executing when the call is done. I could write instructions in deeper and deeper block but I'd like to avoid this.
I'd need an equivalent of the javascript "promise" concept.
Currently I have something with dispatch queue and dispatch semaphore but I sometimes have deadlock for unknown reason.
Eg :
- (void)beaconConnectionDidSucceeded:(Beacon *)beacon
{
dispatch_semaphore_t semaphoreEditing = dispatch_semaphore_create(1);
dispatch_queue_t editingQueue = dispatch_queue_create("com.example.MyCustomQueue.Editing", NULL);
// First writing procedure
dispatch_async(editingQueue, ^{
dispatch_semaphore_wait(semaphoreEditing, DISPATCH_TIME_FOREVER);
[beacon writeSomeCaracteristic:caracteristic withValue:value withCompletion:^(void) {
dispatch_semaphore_signal(semaphoreEditing);
}];
});
// A unknow number of writing sequences
dispatch_async(editingQueue, ^{
dispatch_semaphore_wait(semaphoreEditing, DISPATCH_TIME_FOREVER);
[beacon writeSomeCaracteristic:caracteristic withValue:value withCompletion:^(void) {
dispatch_semaphore_signal(semaphoreEditing);
}];
});
//
// ...
//
dispatch_async(editingQueue, ^{
dispatch_semaphore_wait(semaphoreEditing, DISPATCH_TIME_FOREVER);
[beacon writeSomeCaracteristic:caracteristic withValue:value withCompletion:^(void) {
dispatch_semaphore_signal(semaphoreEditing);
}];
});
// Terminate the edition
dispatch_async(editingQueue, ^{
dispatch_semaphore_wait(semaphoreEditing, DISPATCH_TIME_FOREVER);
[beacon disconnectBeacon];
dispatch_semaphore_signal(semaphoreEditing);
});
}
I want to write clear code that execute my instructions in a sequential way.
If your asynchronous methods do have a completion handler, you can "serialize" or "chain" a number of asynchronous calls like shown below:
[self asyncFooWithCompletion:^(id result){
if (result) {
[self asyncBarWithCompletion:^(id result){
if (result) {
[self asyncFoobarWithCompletion:^(id result){
if (result) {
...
}
}];
}
}];
}
}];
Of course, this gets increasingly confusing with the number of chained asynchronous calls, and especially when you want to handle errors, too.
With a third party library which especially helps to overcome these problems (including error handling, cancellation) it may look similar as the code below:
Given:
- (Promise*) asyncFoo;
- (Promise*) asyncBar;
- (Promise*) asyncFoobar;
"Chaining" the three asynchronous methods including error handling:
[self asyncFoo]
.then(^id(id result){
... // do something with result of asyncFoo
return [self asyncBar];
}, nil)
.then(^id (id result){
... // do something with result of asyncBar
return [self asyncFoobar];
}, nil)
.then(^id(id result) {
... // do something with result of asyncFoobar
return nil;
},
^id(NSError*error){
// "catch" any error from any async method above
NSLog(#"Error: %#", error);
return nil;
});
For general info about "Promises", please read wiki article Futures and Promises.
There are number of Objective-C libraries which implement a Promise.
Have you considered use NSOperation and NSOperationQueue?
If you need to wait for every beacon to run a set of operations before continue, you can store every set of operations in a NSOperation and put all the operations inside a NSOperationQueue with a maxConcurrentLimit of 1. It might be easier to cancel/pause/terminate every single operation and the queue will take care of the concurrency.
I kept the dispatch_queue and dispatch_semaphore for the connection but for the writing actions I use a library called Sequencer I found here.
It follows the Promises principle CouchDeveloper talked about.

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];
}

iOS-Managing and Keeping Track of Multiple Concurrent tasks

In my app I need to load up data from multiple sources and put them together in a table view. Gathering each of the sources one after another would take forever. To get around this I need to run all of the download operations together. Since they are download tasks, in theory I could just run them, but the issue is that only part of the code on the thread runs asynchronously, which means it will need the main thread to complete the operation.
So in order to get ALL of it running in the background, I need to use GCD, which I don't have much experience with.
//DataLoader.m
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
[self.webLoader getFeedWithCompletion:self.thatOtherCompletionBlock];
[self.otherDataLoader getDataWithCompletion:self.completionBlock];
[self.thatDataLoader getThatDataWithCompletion:self.anotherCompletionBlock]
dispatch_async(dispatch_get_main_queue(), ^(void){
});
});
However, since part of the task is already asynchronous, I need to figure out where to put GCD code.
I could put it before starting the task, like I did above. This could work, however, since the tasks are already partially run in the background (in some cases I cannot change that), it seems wasteful to be running a task that already runs partially in the background in the background. Why run something that already runs in a background thread in another thread?
Another option would be to use GCD in the actual class that gets the feed (ex. webloader), putting it on all code that isn't running in the background
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
.......
});
Which way is better?
There is also another problem. Since part of the tasks are asynchronous, they use completion blocks. Not only do I need to also run the completion blocks in the background, I need to figure out which one is the last one to finish, so I can run some code to clean up and neatly package and ship the data to the view controller.
The way I thought of would be to use a BOOL for each task, simply changing it to true when it's done. Then in my completion blocks I can check if all the other tasks are complete, and if so, run the clean up code. However, this may not be the most elegant solution.
What would be the best way to deal with these tasks, ensuring that it all happens in the background?
GCD groups could easily be used for this. Groups allow you to track arbitrary "members" of the group, and hook a block up to run when all members of the group have finished. It's quite handy. For example (using your code):
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group); // + 1
[self.webLoader getFeedWithCompletion: ^{
self.thatOtherCompletionBlock();
dispatch_group_leave(group); // - 1
}];
dispatch_group_enter(group); // + 1
[self.otherDataLoader getDataWithCompletion:^{
self.completionBlock();
dispatch_group_leave(group); // - 1
}];
dispatch_group_enter(group); // + 1
[self.thatDataLoader getThatDataWithCompletion:^{
self.anotherCompletionBlock();
dispatch_group_leave(group); // - 1
}];
dispatch_group_notify(group, dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// This will get executed once all three of the prior completion blocks have been run.
// i.e. when the group "count" goes to zero.
});
dispatch_release(group);
});
You could also, albeit a bit circuitously, use NSOperation's inter-operation dependency feature to achieve this. Like this:
NSOperationQueue* q = [[[NSOperationQueue alloc] init] autorelease];
NSOperation* completionA = [NSBlockOperation blockOperationWithBlock: self.thatOtherCompletionBlock];
NSOperation* completionB = [NSBlockOperation blockOperationWithBlock: self.completionBlock];
NSOperation* completionC = [NSBlockOperation blockOperationWithBlock: self.anotherCompletionBlock];
NSBlockOperation* afterAllThree = [[[NSBlockOperation alloc] init] autorelease];
[afterAllThree addDependency: completionA];
[afterAllThree addDependency: completionB];
[afterAllThree addDependency: completionC];
[afterAllThree addExecutionBlock:^{
// This will get executed once all three of the prior completion blocks have been run.
}];
// Kick off the tasks
[q addOperationWithBlock:^{
[self.webLoader getFeedWithCompletion: ^{ [q addOperation: completionA];}];
[self.otherDataLoader getDataWithCompletion:^{ [q addOperation: completionB]; }];
[self.thatDataLoader getThatDataWithCompletion:^{ [q addOperation: completionC]; }];
}];
I personally prefer the dispatch_group method, but they would both get the job done.

Mulithreading: executing method calls only after finished executing other method

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)

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