grand-central-dispatch - w3toppers.com

iPhone – Grand Central Dispatch main thread

Dispatching a block to the main queue is usually done from a background queue to signal that some background processing has finished e.g. – (void)doCalculation { //you can use any string instead “com.mycompany.myqueue” dispatch_queue_t backgroundQueue = dispatch_queue_create(“com.mycompany.myqueue”, 0); dispatch_async(backgroundQueue, ^{ int result = <some really long calculation that takes seconds to complete>; dispatch_async(dispatch_get_main_queue(), ^{ [self … Read more

Concurrent vs serial queues in GCD

A simple example: you have a block that takes a minute to execute. You add it to a queue from the main thread. Let’s look at the four cases. async – concurrent: the code runs on a background thread. Control returns immediately to the main thread (and UI). The block can’t assume that it’s the … Read more

How do I write dispatch_after GCD in Swift 3, 4, and 5?

The syntax is simply: // to run something in 0.1 seconds DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) { // your code here } Note, the above syntax of adding seconds as a Double seems to be a source of confusion (esp since we were accustomed to adding nsec). That “add seconds as Double” syntax works because deadline … Read more

Does ARC support dispatch queues?

The short answer: YES, ARC retains and releases dispatch queues. And now for the long answer… If your deployment target is lower than iOS 6.0 or Mac OS X 10.8 You need to use dispatch_retain and dispatch_release on your queue. ARC does not manage them. If your deployment target is iOS 6.0 or Mac OS … Read more

How do I dispatch_sync, dispatch_async, dispatch_after, etc in Swift 3, Swift 4, and beyond?

Since the beginning, Swift has provided some facilities for making ObjC and C more Swifty, adding more with each version. Now, in Swift 3, the new “import as member” feature lets frameworks with certain styles of C API — where you have a data type that works sort of like a class, and a bunch … Read more

Waiting until two async blocks are executed before starting another block

Use dispatch groups: see here for an example, “Waiting on Groups of Queued Tasks” in the “Dispatch Queues” chapter of Apple’s iOS Developer Library’s Concurrency Programming Guide Your example could look something like this: dispatch_group_t group = dispatch_group_create(); dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ { // block1 NSLog(@”Block1″); [NSThread sleepForTimeInterval:5.0]; NSLog(@”Block1 End”); }); dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ { // … Read more

Waiting until the task finishes

Use DispatchGroups to achieve this. You can either get notified when the group’s enter() and leave() calls are balanced: func myFunction() { var a: Int? let group = DispatchGroup() group.enter() DispatchQueue.main.async { a = 1 group.leave() } // does not wait. But the code in notify() gets run // after enter() and leave() calls are … Read more

NSOperation vs Grand Central Dispatch

GCD is a low-level C-based API that enables very simple use of a task-based concurrency model. NSOperation and NSOperationQueue are Objective-C classes that do a similar thing. NSOperation was introduced first, but as of 10.5 and iOS 2, NSOperationQueue and friends are internally implemented using GCD. In general, you should use the highest level of … Read more

How do you trigger a block after a delay, like -performSelector:withObject:afterDelay:?

I think you’re looking for dispatch_after(). It requires your block to accept no parameters, but you can just let the block capture those variables from your local scope instead. int parameter1 = 12; float parameter2 = 144.1; // Delay execution of my block for 10 seconds. dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC), dispatch_get_main_queue(), ^{ NSLog(@”parameter1: %d parameter2: … Read more

How do I wait for an asynchronously dispatched block to finish?

Trying to use a dispatch_semaphore. It should look something like this: dispatch_semaphore_t sema = dispatch_semaphore_create(0); [object runSomeLongOperationAndDo:^{ STAssert… dispatch_semaphore_signal(sema); }]; if (![NSThread isMainThread]) { dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); } else { while (dispatch_semaphore_wait(sema, DISPATCH_TIME_NOW)) { [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate dateWithTimeIntervalSinceNow:0]]; } } This should behave correctly even if runSomeLongOperationAndDo: decides that the operation isn’t actually long enough … Read more