grand-central-dispatch – Page 7

iPhone – Grand Central Dispatch main thread

November 23, 2022 by Tarik

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

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

October 21, 2022 by Tarik

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

Wait until swift for loop with asynchronous network requests finishes executing

October 17, 2022 by Tarik

You can use dispatch groups to fire an asynchronous callback when all your requests finish. Here’s an example using dispatch groups to execute a callback asynchronously when multiple networking requests have all finished. override func viewDidLoad() { super.viewDidLoad() let myGroup = DispatchGroup() for i in 0 ..< 5 { myGroup.enter() Alamofire.request(“https://httpbin.org/get”, parameters: [“foo”: “bar”]).responseJSON { … Read more

Waiting until two async blocks are executed before starting another block

October 17, 2022 by Tarik

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

In Swift how to call method with parameters on GCD main thread?

October 16, 2022 by Tarik

Modern versions of Swift use DispatchQueue.main.async to dispatch to the main thread: DispatchQueue.main.async { // your code here } To dispatch after on the main queue, use: DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) { // your code here } Older versions of Swift used: dispatch_async(dispatch_get_main_queue(), { let delegateObj = UIApplication.sharedApplication().delegate as YourAppDelegateClass delegateObj.addUIImage(“yourstring”) })

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

October 5, 2022 by Tarik

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

GCD to perform task in main thread

October 3, 2022 by Tarik

No, you do not need to check whether you’re already on the main thread. By dispatching the block to the main queue, you’re just scheduling the block to be executed serially on the main thread, which happens when the corresponding run loop is run. If you already are on the main thread, the behaviour is … Read more

Create singleton using GCD’s dispatch_once in Objective-C

September 26, 2022 by Tarik

This is a perfectly acceptable and thread-safe way to create an instance of your class. It may not technically be a “singleton” (in that there can only ever be 1 of these objects), but as long as you only use the [Foo sharedFoo] method to access the object, this is good enough.

How to create dispatch queue in Swift 3

September 20, 2022 by Tarik

Creating a concurrent queue let concurrentQueue = DispatchQueue(label: “queuename”, attributes: .concurrent) concurrentQueue.sync { } Create a serial queue let serialQueue = DispatchQueue(label: “queuename”) serialQueue.sync { } Get main queue asynchronously DispatchQueue.main.async { } Get main queue synchronously DispatchQueue.main.sync { } To get one of the background thread DispatchQueue.global(qos: .background).async { } Xcode 8.2 beta 2: … Read more

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

September 15, 2022 by Tarik

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