I have a function:
- (void)fetchClassListOnCompletion:(void(^) (BOOL success, NSArray *classlist))completionBlock;
I want to call this function from two different thread (may be simultaneously) and want the classlist from the one that finished last.
Need help to implement this
Try this:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
[self fetchClassListOnCompletion:^(BOOL success, NSArray *classlist) {
self.classList = classlist;
}];
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
[self fetchClassListOnCompletion:^(BOOL success, NSArray *classlist) {
self.classList = classlist;
}];
});
self.classList will be assigned to classlist from the block that finishes last.
Because this runs on background thread, make sure your method doesn't need to be run on main thread.
Related
If I dispatch_async a block on main queue like this:
-(void) myTask {
dispatch_async(dispatch_get_main_queue(), ^{
[self.service fetchData];
});
}
In unit test, I can execute the block passed in main queue by manually run the main loop like this:
-(void)testMyTask{
// call function under test
[myObj myTask];
// run the main loop manually!
[[NSRunLoop mainRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.01]];
// now I can verify the function 'fetchData' in block is called
...
}
Now, I have another similar function which dispatch block to an sequential queue other than main queue:
-(void) myTask2 {
dispatch_async(dispatch_queue_create("my.sequential.queue", NULL), ^{
[self.service fetchData];
});
}
In unit test, how can I execute the block manually now?
-(void)testMyTask2{
// call function under test
[myObj myTask2];
// How to manually execute the block now?
}
=== Clarify ===
The reason why I want to manually execute is because I don't like any Wait-For-Timeout way doing the test. Because waiting time is depending on CPU speed, on different machines could be different. I'd like to manually execute the block passed to queue (the same way as how I did for the main queue test case) and then verify the result.
You could create the queue in your test function.
-(void) myTask2:(dispatch_queue_t*)queue {
dispatch_async(*queue, ^{
[self.service fetchData];
});
}
-(void)testMyTask2{
dispatch_queue_t queue = dispatch_queue_create("my.sequential.queue", NULL);
[myObj myTask2:&queue];
dispatch_sync(queue, ^{
});
}
(Just realised currentRunLoop is not needed)
For execute test in async block use XCTestExpectation class
-(void) myTask2 {
XCTestExpectation *expectation = [self expectationWithDescription:#"catch is called"];
dispatch_async(dispatch_queue_create("my.sequetial.queue", NULL), ^{
[self.serviceClient fetchDataForUserId:self.userId];
[expectation fulfill];
});
[self waitForExpectationsWithTimeout:Timeout handler:^(NSError *error) {
//check that your NSError nil or not
}];
}
Hope this help
I have 3 function in my code.
The code i did for manage queue for function execution.
[self firstMethodWithOnComplete:^{
[self SecongMethodWithOnComplete:^{
dispatch_async(dispatch_get_main_queue(), ^{
[NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:#selector(CallThirdMethod) userInfo:nil repeats:NO];
});
}];
}];
First And Second Function
- (void)firstMethodWithOnComplete:(void (^)(void))onComplete {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//processing here.....
[self CallFirstMethod];
onComplete();
});
}
- (void)SecongMethodWithOnComplete:(void (^)(void))onComplete {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
//processing here.....
[self CallSecondMethod];
onComplete();
});
}
The problem is i am unable to manage their execution. I want execution order such in a way that second function only execute if first is over and third execute if second execution over.
Please help me to solve this or give any appropriate suggestions.
You can use dispatch groups for this kind of requirement, Below i am posting example code which i have used
__block NSError *configError = nil;
__block NSError *preferenceError = nil;
// Create the dispatch group
dispatch_group_t serviceGroup = dispatch_group_create();
dispatch_group_enter(serviceGroup);
// Start the first async service
dispatch_group_leave(serviceGroup);
dispatch_group_enter(serviceGroup);
// Start the second async service
dispatch_group_leave(serviceGroup);
dispatch_group_notify(serviceGroup,dispatch_get_main_queue(),^{
//Update UI in this block of code
});
I have a for loop containing three asynchronous methods, and I want to make some treatment after this 3 async methods are finished.
-(void)getAllUsersInformations{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
for(User *user in users){
[self getUserInfo:user];
}
//Here, I want to reload the table view for example, after finishing the for loop (executing the whole three methods).
});
}
-(void)getUserInfo:(User*)user{
[self getInformations:user];
[self getExperiences:user];
[self getEducation:user];
}
Do you have any technic to have this result?
Thank you very much.
One GCD approach is to use dispatch_group. So, before you start an asynchronous task, call dispatch_group_enter, and then when the asynchronous task finishes, call dispatch_group_leave, and you can then create a dispatch_group_notify which will be called when the asynchronous tasks finish. You can marry this with a completion block pattern (which is a good idea for asynchronous methods, anyway):
If getInformations, getExperiences and getEducation are, themselves, all asynchronous methods, the first thing you need is some mechanism to know when they're done. A common solution is to implement a completion block pattern for each. For example:
// added completionHandler parameter which will be called when the retrieval
// of the "informations" is done.
- (void)getInformations:(User*)user completionHandler:(void (^)(void))completionHandler {
// do whatever you were before, but in the asynchronous task's completion block, call this
// completionHandler()
//
// for example
NSURLRequest *request;
[NSURLConnection sendAsynchronousRequest:request queue:nil completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
// handle the request here
// the important thing is that the completion handler should
// be called _inside_ the this block
if (completionHandler) {
completionHandler();
}
}];
}
Repeat this process for getExperiences and getEducation, too.
Then, you can use a dispatch group to notify you of when each of these three requests are done done, calling a completion block in getUserInfo when that takes place:
// added completion handler that will be called only when `getInformations`,
// `getExperiences` and `getEducation` are all done.
//
// this takes advantage of the completion block we added to those three
// methods above
- (void)getUserInfo:(User*)user completionHandler:(void (^)(void))completionHandler {
dispatch_group_t group = dispatch_group_create();
// start the three requests
dispatch_group_enter(group);
[self getInformations:user completionHandler:^{
dispatch_group_leave(group);
}];
dispatch_group_enter(group);
[self getExperiences:user completionHandler:^{
dispatch_group_leave(group);
}];
dispatch_group_enter(group);
[self getEducation:user completionHandler:^{
dispatch_group_leave(group);
}];
// this block will be called asynchronously only when the above three are done
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
if (completionHandler) {
completionHandler();
}
});
}
And you then repeat this process at the getAllUsersInformations:
// call new getUserInfo, using dispatch group to keep track of whether
// all the requests are done
-(void)getAllUsersInformations {
dispatch_group_t group = dispatch_group_create();
for(User *user in users){
dispatch_group_enter(group);
[self getUserInfo:user completionHandler:^{
dispatch_group_leave(group);
}];
}
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
[self.tableView reloadData];
});
}
Two final thoughts:
Having outlined all of that, I must confess that I would probably wrap these requests in concurrent/asynchronous custom NSOperation subclasses instead of using dispatch groups. See the "Configuring Operations for Concurrent Execution" section of the Concurrency Programming Guide. This is a more radical refactoring of the code, so I won't tackle that here, but it lets you constrain the number of these requests that will run concurrently, mitigating potential timeout issues.
I don't know how many of these user requests are going on, but you might want to consider updating the UI as user information comes in, rather than waiting for everything to finish. This is, again, a more radical refactoring of the code, but might lead to something that feels more responsive.
Try to do a block with completion, you can't do this with a for loop if the methods are async. you have to call getUserInfo one by one after the completion of the previous. I think this gonna be solved your problem.
-(void)getAllUsersInformations{
[self registerUserAtIndex:0];
}
- (void) registerUserAtIndex: (NSInteger ) userIndex
{
RegisterOperation *op = [[RegisterOperation alloc] initWithUser:[users objectAtIndex:userIndex]];
[RegisterOperation setResultCompletionBlock:^(BOOL *finished, NSInteger userIndex) {
dispatch_async(dispatch_get_main_queue(), ^{
if (userIndex++ < [users count] {
[self registerUserAtIndex:userIndex++];
} else {
[myTableView reloadData];
}
}];
[[NSOperationQueue mainQueue] addOperation:op];
}
Hope this will help you.
Rop Answer with swift:
func processData()
{
let group: dispatch_group_t = dispatch_group_create()
for item in data as! Object {
dispatch_group_enter(group)
item.process(completion: {() -> (Void) in
dispatch_group_leave(group)
})
}
dispatch_group_notify(group, dispatch_get_main_queue(), {
//Do whatever you want
})
}
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// Background work
for(User *user in users){
[self getUserInfo:user];
}
dispatch_async(dispatch_get_main_queue(), ^{
//reload tableview , this is on main thread.
});
});
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.
So, I've been searching all over but didn't find a solution, or at least I couldn't apply it.
I've found this thread here on stackoverflow, but didn't succeed in implementing it in my code.
My issue is, that I need to know when nested AFNetworking calls and For loops are done. I've tried it with GCD groups, but with no luck.
The code looks like this:
{
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_async(queue, ^{
[[JSON GET method using AFNetworking 2.0] success:^(NSArray *result) {
dispatch_group_async(group, queue, ^{
//do some work with the result
for (NSDictionary *resultPartDictionary in result) {
dispatch_group_async(group, queue, ^{
//do some more work with parts of the result
[[JSON GET method based on result] success:^(NSArray *result) {
dispatch_group_async(group, queue, ^{
//do some work
for (NSDictionary *resultPartDictionary in result) {
dispatch_group_async(group, queue, ^{
//do some work
[[JSON GET method based on result] success:^(NSArray *result) {
dispatch_group_async(group, queue, ^{
//do some work
for (NSDictionary *resultPartDictionary in result) {
dispatch_group_async(group, queue, ^{
//do some work
});
}
});
}];
});
}
});
}];
});
}
});
}
});
}
Right now, everything works. I'm handling Core Data inside the blocks, so I needed MOCs for every thread, which works as well.
The only thing I'd like to know is how to know when all these blocks finish.
Thank you!
EDIT
So, I've tried using dispatch_group_enter(group) and dispatch_group_leave(group), but it seems to me, that it's just not possible with this embedded architecture. Because of the For loops, the "leave" notifications are either too many, which causes an exception or not enough and the dispatch_group_notify returns too early.
Any ideas on this?
You are looking for dispatch_group_notify and dispatch_group_enter/dispatch_group_leave.
dispatch_group_notify executes the given block in the given queue, when every block in the group is finished.
dispatch_group_enter increases the current count of executing tasks in the group. Every dispatch_group_enter must be balanced with a call to dispatch_group_leave.
dispatch_group_leave decreases the current count of executing tasks in the group.
So, you should trick dispatch_group_notify with increase the number of the tasks in the group before your network calls start and decrease it when everything finished. To achieve this, call dispatch_group_enter before dispatch_async and call dispatch_group_leave in the last thread. Since you know the element count of the every result array, you can check if the current thread is the last one.
dispatch_group_enter(group); // Increases the number of blocks in the group.
dispatch_async(queue, ^{
// Make your AFNetworking calls.
dispatch_group_async(group, queue, ^{
//do some work.
if (isLastThread)
dispatch_group_leave(group); // Decreases the number of blocks in the group.
});
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{ // Calls the given block when all blocks are finished in the group.
// All blocks finished, do whatever you like.
});