If you are working with an NSOperationQueue, it can provide the current dispatch queue for you. NSOperationQueue has the class function [NSOperationQueue currentQueue], which returns the current queue as a NSOperationQueue object. To get the dispatch queue object you can use [NSOperationQueue currentQueue].underlyingQueue, which returns your currrent queue as a dispatch_queue_t. Swift 3: if let … Read more
You should call all code from showAlertMessage on main queue: class func showAlertMessage(message:String, viewController: UIViewController) { DispatchQueue.main.async { let alertMessage = UIAlertController(title: “”, message: message, preferredStyle: .alert) let cancelAction = UIAlertAction(title: “Ok”, style: .cancel) alertMessage.addAction(cancelAction) viewController.present(alertMessage, animated: true, completion: nil) } }
dispatch_sync runs a block on a given queue and waits for it to complete. In this case, the queue is the main dispatch queue. The main queue runs all its operations on the main thread, in FIFO (first-in-first-out) order. That means that whenever you call dispatch_sync, your new block will be put at the end … Read more
From your stack trace, EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) occurred because dispatch_group_t was released while it was still locking (waiting for dispatch_group_leave). According to what you found, this was what happened : dispatch_group_t group was created. group‘s retain count = 1. -[self webservice:onCompletion:] captured the group. group‘s retain count = 2. dispatch_async(…., ^{ dispatch_group_wait(group, …) … }); … Read more
The dispatch_get_global_queue (DispatchQueue.global() in Swift) gets you a background queue upon which you can dispatch background tasks that are run asynchronously (i.e. won’t block your user interface). And if you end up submitting multiple blocks to the global queues, these jobs can operate concurrently. If you have multiple blocks of code that you want to … Read more
You might not be aware of everything that the run loop does in each iteration. (I wasn’t before I researched this answer!) As it happens, CFRunLoop is part of the open-source CoreFoundation package, so we can take a look at exactly what it entails. The run loop looks roughly like this: while (true) { Call … Read more
As Jacob points out, while they may appear the same, they are different things. In fact, there’s a significant difference in the way that they handle sending actions to the main thread if you’re already running on the main thread. I ran into this recently, where I had a common method that sometimes was run … Read more
You must use the __block specifier when you modify a variable inside a block, so the code you gave should look like this instead: __block NSString *textString; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, (unsigned long)NULL), ^(void) { textString = [self getTextString]; }); Blocks capture the state of the variables referenced inside their bodies, so the captured variable must be declared … Read more
No it doesn’t wait and the way you are doing it in that sample is not good practice. dispatch_async is always asynchronous. It’s just that you are enqueueing all the UI blocks to the same queue so the different blocks will run in sequence but parallel with your data processing code. If you want the … Read more
NSOperation* classes are the higher level api. They hide GCD’s lower level api from you so that you can concentrate on getting the task done. The rule of thumb is: Use the api of the highest level first and then degrade based on what you need to accomplish. The advantage of this approach is that … Read more