I recommend you to see WWDC Sessions about network application in iPhone OS. WWDC 2010 Session 207 – Network Apps for iPhone OS, Part 1 WWDC 2010 Session 208 – Network Apps for iPhone OS, Part 2 The lecturer said “Threads Are Evil™” for network programming and recommended to use asynchronous network programming with RunLoop. … Read more
FWIW, there are many synchronization techniques that offer better performance than semaphores. For example, you can use the reader-writer pattern with concurrent queue, where reads are done synchronously, but are allowed to run concurrently with respect to other reads, but writes are done asynchronously, but with a barrier (i.e. not concurrently with respect to any … Read more
Why even use GCD? You could just use an NSTimer and invalidate it when your app returns to the foregound.
You can use the __block storage type qualifier for such a case. __block variables are not automatically retained by the block. So you need to retain the object by yourself: __block UIViewController *viewController = [myViewController retain]; dispatch_async(backgroundQueue, ^{ // Do long-running work here. dispatch_async(dispatch_get_main_queue(), ^{ [viewController updateUIWithResults:results]; [viewController release]; // Ensure it’s released on main … Read more
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
GCD does not have built-in support for cancellation; if it’s important to be able to cancel your background task, then checking a flag like you’ve demonstrated is an acceptable solution. However, you may want to evaluate how quickly the cancellation needs to respond; if some of those methods calls are fairly short, you may be … Read more
This limitation on the cooperative thread pool on the simulator has been removed in Xcode 14.3 (without any mention to this change in the release notes). It looks like this curious behavior is a limitation of the simulator in Xcode 14.2 and earlier. If I run it on my physical iPhone 12 Pro Max, the … Read more
x.sync means that the calling queue will pause and wait until the sync block finishes to continue. so in your example: DispatchQueue.global().async { // yada yada something DispatchQueue.main.sync { // update UI } // this will happen only after ‘update UI’ has finished executing } Usually you don’t need to sync back to main, async … Read more
The problem is that UIImage doesn’t actually read and decode the image until the first time it’s actually used/drawn. To force this work to happen on a background thread, you have to use/draw the image on the background thread before doing the main thread -setImage:. This worked for me: dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{ UIImage * img … Read more