The code below is called each time a scrollview scroll and if user scroll it multiple times, it crashed the code. How do i make sure only 1 code execute at a time or threadsafe?
[self.cv addInfiniteScrollingWithActionHandler:^{
[weakSelf loadNextPage];
}];
Here is example
- (void)_startExperiment {
FooClass *foo = [[FooClass alloc] init];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i = 0; i < 4; ++i) {
dispatch_async(queue, ^{
[foo doIt];
});
}
[foo release];
}
Detail is Here
The common pattern is to use a mutex to protect a critical section of code where the structure is accessed and/or modified.
just go through this link->
Does #synchronized guarantees for thread safety or not?
Related
I've some data which is accumulated in a buffer and I need to read the data when buffer is having data. This i need to do with thread synchronisation. I've worked little with GCD, which I'm failing to do. please help how to do a circular buffer with read and write threads in synchronization.
My Code:
- (void)viewDidLoad {
[super viewDidLoad];
readPtr = 0;
writePtr = 0;
currentPtr = 0;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
while(YES){
[self writeToBuffer:buffer[0] withBufferSize:bufferSize];
}
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),^{
while(YES){
float* newBuffer;
if(currentPtr>512){
newBuffer = [self readBuffer];
}else{
continue;
}
[self UseBuffer: newBuffer];
}
});
}
-(void)writeToBuffer:(float*)Values withBufferSize:(int)bSize{
[_lock lock];
for(int i=0;i<bSize;i++){
if(writePtr>1859){
writePtr = 0;
}
globalBuffer[writePtr] = Values[i];
writePtr++;
currentPtr++;
}
NSLog(#"Writing");
[_lock unlock];
}
-(float*)readBuffer{
[_lock lock];
float rBuffer[512];
for(int i=0;i<512;i++){
if(readPtr>1859){
readPtr = 0;
}
rBuffer[i] = globalBuffer[readPtr];
readPtr++;
currentPtr--;
}
NSLog(#"Reading");
[_lock unlock]
return rBuffer;
}
One of the key points of GCD is that it completely replaces the need for locks. So, if you are mixing GCD and mutex locks it is typically a sign that you're doing things wrong or sub-optimally.
A serial queue is, effectively, an exclusive lock on whatever is associated with the serial queue.
There a bunch of problems in your code.
while (YES) {...} is going to spin, burning CPU cycles ad infinitum.
The readBuffer method is returning a pointer to a stack based buffer. That won't work.
It isn't really clear what the goal of the code is, but those are some specific issues.
So with some help, I am more clear on how a nested GCD works in my program.
The original post is at:
Making sure I'm explaining nested GCD correctly
However, you don't need to go through the original post, but basically the code here runs database execution in the background and the UI is responsive:
-(void)viewDidLoad {
dispatch_queue_t concurrencyQueue = dispatch_queue_create("com.epam.halo.queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t serialQueue = dispatch_queue_create("com.epam.halo.queue2", DISPATCH_QUEUE_SERIAL);
for ( int i = 0; i < 10; i++) {
dispatch_async(concurrencyQueue, ^() {
NSLog(#"START insertion method%d <--", i);
dispatch_sync(serialQueue, ^() {
//this is to simulate writing to database
NSLog(#"----------START %d---------", i);
[NSThread sleepForTimeInterval:1.0f];
NSLog(#"--------FINISHED %d--------", i);
});
NSLog(#"END insertion method%d <--", i);
});
}
}
However, when I start refactoring them and putting them into methods and making everything look nice, the UI does not respond anymore:
//some database singleton class
//the serial queues are declared in the class's private extension. And created in init()
-(void)executeDatabaseStuff:(int)i {
dispatch_sync(serialQueue, ^() {
//this is to simulate writing to database
NSLog(#"----------START--------- %d", i);
[NSThread sleepForTimeInterval:1.0f];
NSLog(#"--------FINISHED-------- %d", i);
});
}
-(void)testInsert:(int)i {
dispatch_async(concurrencyQueue, ^() {
[self executeDatabaseStuff:i];
});
}
//ViewController.m
- (void)viewDidLoad {
//UI is unresponsive :(
for ( int i = 0; i < totalNumberOfPortfolios; i++) {
NSLog(#"START insertion method%d <--", i);
[[DatabaseFunctions sharedDatabaseFunctions] testInsert: i];
NSLog(#"END insertion method%d <--", i);
}
}
The only way to make the refactored version work is when I put dispatch_async(dispatch_get_main_queue():
for ( int i = 0; i < totalNumberOfPortfolios; i++) {
dispatch_async(dispatch_get_main_queue(), ^() {
NSLog(#"START insertion method%d <--", i);
[[DatabaseFunctions sharedDatabaseFunctions] testInsert: i];
NSLog(#"END insertion method%d <--", i);
});
}
So my question is, I thought the fact that I use dispatch_async the concurrencyQueue will ensure that my main thread is not touched by the dispatch_sync serialQueue combo. Why is it that when I wrap it in an object/method, I must use dispatch_async(dispatch_get_main_queue()...) ?
Seems that whether my main thread does dispatch_async on a concurrent queue
in viewDidLoad, or within a method, does indeed matter.
I am thinking that the main thread is getting all these testInsert methods pushed onto its thread stack. The methods must then be processed by the main thread. Hence, even though the dispatch_sync is not blocking the main thread, the main thread runs to the end of viewDidLoad, and must wait for all the testInsert methods to be processed and done before it can move onto the next task in the Main Queue??
Notes
So I went home and tested it again with this:
for ( int i = 0; i < 80; i++) {
NSLog(#"main thread %d <-- ", i);
dispatch_async(concurrencyQueue, ^() {
[NSThread isMainThread] ? NSLog(#"its the main thread") : NSLog(#"not main thread");
NSLog(#"concurrent Q thread %i <--", i);
dispatch_sync(serialQueue, ^() {
//this is to simulate writing to database
NSLog(#"serial Q thread ----------START %d---------", i);
[NSThread sleepForTimeInterval:1.0f];
NSLog(#"serial Q thread --------FINISHED %d--------", i);
});
NSLog(#"concurrent Q thread %i -->", i);
});
NSLog(#"main thread %d --> ", i);
} //for loop
When I run the loop from 1 - 63, the UI is not blocked. And I see my database operation processing in the background.
Then when the loop is 64, UI is blocked for 1 database operation, then returns fine.
When I use 65, UI freezes for 2 database operations, then returns fine...
When I use something like 80, it gets blocked from 64-80...so I wait 16 seconds before my UI is responsive.
At the time, I couldn't figure out why 64. So now I know that its 64 concurrent threads allowed at once. ...and has nothing to do with wrapping it in a object/method. :D
Many thanks for the awesome help from the contributors!
There is a hard limit of 64 GCD concurrent operations (per top level concurrent queue) that can be run together.
What's happening is you're submitting over 64 blocks to your concurrent queue, each of them getting blocked by the [NSThread sleepForTimeInterval:1.0f], forcing a new thread to be created for each operation. Therefore, once the thread limit is reached, it backs up and starts to block the main thread.
I have tested this with 100 "database write" operations (on device), and the main thread appears to be blocked until 36 operations have taken place (there are now only 64 operations left, therefore the main thread is now un-blocked).
The use of a singleton shouldn't cause you any problems, as you're calling methods to that synchronously, therefore there shouldn't be thread conflicts.
The simplest solution to this is just to use a single background serial queue for your "database write" operations. This way, only one thread is being created to handle the operation.
- (void)viewDidLoad {
[super viewDidLoad];
static dispatch_once_t t;
dispatch_once(&t, ^{
serialQueue = dispatch_queue_create("com.epam.halo.queue2", DISPATCH_QUEUE_SERIAL);
});
for (int i = 0; i < 100; i++) {
[self testInsert:i];
}
}
-(void)executeDatabaseStuff:(int)i {
//this is to simulate writing to database
NSLog(#"----------START--------- %d", i);
[NSThread sleepForTimeInterval:1.0f];
NSLog(#"--------FINISHED-------- %d", i);
}
-(void)testInsert:(int)i {
NSLog(#"Start insert.... %d", i);
dispatch_async(serialQueue, ^() {
[self executeDatabaseStuff:i];
});
NSLog(#"End insert... %d", i);
}
I don't know why inserting dispatch_async(dispatch_get_main_queue(), ^() {} inside your for loop was working for you... I can only assume it was off-loading the "database writing" until after the interface had loaded.
Further resources on threads & GCD
Number of threads created by GCD?
https://developer.apple.com/library/ios/documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html#//apple_ref/doc/uid/TP40008091-CH102-SW1
https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Multithreading/CreatingThreads/CreatingThreads.html#//apple_ref/doc/uid/10000057i-CH15-SW2
Is there any way to notify when a enumerateLinesUsingBlock is completed? Please check below code. I am calling createFastSearchData method with chunk by chunk data in a while loop and inside that taking each lines and processing it. In while condition I am checking the length of the main string and I want to continue untill it completes the total length. So I want to make sure that enumerateLinesUsingBlock is completed before the while loop trigger again.
while(<checking the length of the mainstring>){
[self createFastSearchData:string];
}
- (void)createFastSearchData:(NSString *)newChunk{
[newChunk enumerateLinesUsingBlock:^(NSString * line, BOOL * stop)
{}];
}
Added:
I am working with blocks and finding difficulty to understand the actual flow. Please check the below code. I want to call fetchCSVData method by passing different values in an array filesToBeFetchedWhat. I want to make sure that, fetchCSVData should not overlap. How can I do that? Please help
dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
#autoreleasepool {
for (int i = 0; i < filesToBeFetched.count; i++) {
[applicationDelegate fetchCSVData:[filesToBeFetched objectAtIndex:i]];
}
}
dispatch_async( dispatch_get_main_queue(), ^{
NSLog(#"Fetching is done *********************");
});
});
To answer the first part of the question:
All enumerate...UsingBlock methods don't work asynchronously.
Regarding the added part:
Assuming fetchCSVData works also synchronously, that's the preferred way to process the data
dispatch_async( dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
#autoreleasepool {
for (int i = 0; i < filesToBeFetched.count; i++) {
[applicationDelegate fetchCSVData:[filesToBeFetched objectAtIndex:i]];
dispatch_async( dispatch_get_main_queue(), ^{
NSLog(#"Fetching is done *********************");
});
}
}
});
I have two methods which run on a serial queue. Each method return a copy of some class. I'm trying to achieve thread safety solution while also mainting data integrity.
for example:
-(Users *) getAllUsers
{
__block copiedUsers;
dispatch_sync(_backgroundQueue, ^{
copiedUsers = [self.users copy]; // return copy object to calling thread.
});
return copiedUsers;
}
-(Orders *) getAllOrders
{
__block copiedOrders;
dispatch_sync(_backgroundQueue, ^{
copiedOrders = [self.Orders copy]; // return copy object to calling thread.
});
return copiedOrders;
}
In addition to this two methods, I have a worker class that add/remove users and orders, all done via a serial queue backgroundQueue.
If in the main thread I call getAllUsers and then getAllOrders right after the other my data integrity isn't safe because between the two calls the worker class might have changed the model.
my question is how can I make to the caller a nice interface that allows multiple methods to run atomically?
Model is only updated from backgroundQueue serial queue.
Client talks to model via a method that receives a block that runs in the background queue.
In addition, not to freeze main thread, I create another queue and run a block that talks with the gateway method.
P.S - attention that dispatch_sync is called only in runBlockAndGetNeededDataSafely to avoid deadlocks.
Code sample:
aViewController.m
ManagerClass *m = [ManagerClass new];
dispatch_queue_t q = dispatch_queue_create("funnelQueue", DISPATCH_QUEUE_SERIAL);
dispatch_block_t block_q = ^{
__Users *users;
__Orders *orders;
[manager runBlockAndGetNeededDataSafely:^
{
users = [manager getUsers];
orders = [manager getOrders];
dispatch_async(dispatch_get_main_queue(),
^{
// got data safely - no thread issues, copied objects. update UI!
[self refreshViewWithUsers:users
orders:orders];
});
}];
}
dispatch_async(q, block_q);
Manager.m implementation:
-(void) runBlockInBackground:(dispatch_block_t) block
{
dispatch_sync(self.backgroundQueue, block);
}
-(Users *) getAllUsers
{
return [self.users copy];
}
-(Orders *) getAllOrders
{
return [self.Orders copy];
}
To answer your question about how to checking the current queue:
First when you create the queue, give it a tag:
static void* queueTag = &queueTag;
dispatch_queue_t queue = dispatch_queue_create("a queue", 0);
dispatch_queue_set_specific(queue, queueTag, queueTag, NULL);
and then run a block like this:
-(void)runBlock:(void(^)()) block
{
if (dispatch_get_specific(queueTag) != NULL) {
block();
}else {
dispatch_async(self.queue, block);
}
}
Your example doesn't work. I suggest to use completion callback. You should have an option to know when the worker finish his job to return to value.
- (void)waitForCompletion:(BOOL*)conditions length:(int)len timeOut:(NSInteger)timeoutSecs {
NSDate *timeoutDate = [NSDate dateWithTimeIntervalSinceNow:timeoutSecs];
BOOL done = YES;
for (int i = 0; i < len; i++) {
done = done & *(conditions+i);
}
do {
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:timeoutDate];
if([timeoutDate timeIntervalSinceNow] < 0.0)
break;
//update done
done = YES;
for (int i = 0; i < len; i++) {
done = done & *(conditions+i);
}
} while (!done);
}
-(void) getAllUsers:(void(^)(User* user, NSError* error))completion
{
dispatch_async(_backgroundQueue, ^{
BOOL condition[2] = [self.userCondition, self.orderCondition];
[self waitForCompletion: &condition[0] length:2 timeOut:60];
if (completion) {
completion([self.users copy], nil);
}
});
}
I am looking for a small scenario that how can we trace the "dispatch_async" is running or not?.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
(unsigned long)NULL), ^(void) {
//back ground process
});
In my case, my app will be in foreground I started the back ground thread and when I bring app from background to foreground I need to check whether it is still running or not. I should not call the same process if it is still running. any idea?
The easiest way to do this (without keeping a reference to every dispatch or a flag for entering/leaving asynchronous tasks) is by using dispatch_group notifications. See the example link and code below:
- (void)downloadPhotosWithCompletionBlock:(BatchPhotoDownloadingCompletionBlock)completionBlock
{
// 1
__block NSError *error;
dispatch_group_t downloadGroup = dispatch_group_create();
for (NSInteger i = 0; i < 3; i++) {
NSURL *url;
switch (i) {
case 0:
url = [NSURL URLWithString:kOverlyAttachedGirlfriendURLString];
break;
case 1:
url = [NSURL URLWithString:kSuccessKidURLString];
break;
case 2:
url = [NSURL URLWithString:kLotsOfFacesURLString];
break;
default:
break;
}
dispatch_group_enter(downloadGroup); // 2
Photo *photo = [[Photo alloc] initwithURL:url
withCompletionBlock:^(UIImage *image, NSError *_error) {
if (_error) {
error = _error;
}
dispatch_group_leave(downloadGroup); // 3
}];
[[PhotoManager sharedManager] addPhoto:photo];
}
dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ // 4
if (completionBlock) {
completionBlock(error);
}
});
}
Note how:
dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ // 4
if (completionBlock) {
completionBlock(error);
}
});
will not be called until after
dispatch_group_leave(downloadGroup); // 3
is called.
You should setup your threading to where you can work with callbacks like this to determine states. You should try to avoid using boolean flags at all costs, as this is exactly what dispatch groups are for. It's also hard to keep track of numerous asynchronous calls using boolean states.
link: dispatch groups
The question is wrong - dispatch_async is running while you call it and stops running when the call returns, which is practically immediately. What you really want to know is whether the dispatched block is running or not. The simplest way is something along the lines of
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
(unsigned long)NULL), ^(void) {
[self blockIsRunning:YES];
// do stuff
[self blockIsRunning:NO];
});
or if you want to know whether the block has run once, you would do something like
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
(unsigned long)NULL), ^(void) {
[self blockStarted];
// do stuff
[self blockFinished];
});
Alternatively, use NSOperationQueue and a subclass of NSOperation so instead of an anonymous block you have a proper object that you can ask whether it is ready, cancelled, executing, or finished.