The red dot is to make the results easily distinguishable in the console, especially when we’ll add more queues or tasks to execute. In the Main.storyboard, ViewController scene you’ll find an image view added, and the respective IBOutlet property connected to the ViewController class. We’ll need that image view for a real world example later.
Apart from that, GCD classes inherit by default from DispatchObject() e.g. DispatchQueue() , DispatchGroup(), DispatchWorkItem(), DispatchSource() to name just a few. Most classes, like the first three just mentioned start in activated state, which means they are ready to use right away. Others like DispatchSource() have to be set active first to be useful. Secondly, DipatchObjects can be suspended or resumed if needbe via suspend() or resume(). But mind when dealing with DispatchQueues that a queue excutes all its operations first before transitioning to its suspended state.
Concurrent queues don’t wait for one task to finish executing before starting the next. Both queues process work units in First-In-First-Out order. “x.sync means doing thing in main thread/UI thread and x.async means doing in background thread” — I don’t think that’s true, and you shouldn’t think of queues as threads (they’re not the same). Calling async means it doesn’t block, but you’re calling it on DispatchQueue.main, which is a queue guaranteed to run on the main thread.
Custom concurrent queues always mapped into one of the global queues by specifying a Quality of Service property . In most of the cases if you want to run tasks in parallel it is recommended to use one of the global concurrent queues, you should only create custom serial queues. In all the previous examples we manually created the dispatch queues we used. However, it’s not always necessary Figma Templates, UI Kits to do that, especially if you don’t desire to change the properties of the dispatch queue. As I have already said in the beginning of this post, the system creates a collection of background dispatch queues, also named global queues. You can freely use them like you would do with custom queues, just keep in mind not to abuse the system by trying to use as many global queues as you can.
At the same time, when all threads are busy, a new thread can be brought up in the pool. Grand Central Dispatch offers a task-based paradigm of thinking. There is no explicit thread management in GCD, which allows to write concurrent code without actually thinking about threads.
So literally you are making the ThreadMain into deadlock situation. In first case, you run the code on main and then you use main.sync on the main thread. In essence, you are trying to tell the main queue to wait for itself – which is obviously nonsense and therefore it causes crash. Sometimes it’s necessary to delay the execution of code. GCD allows us to do the same by using a special method .asyncAfter. We need to set the amount of time for which we want to delay the execution.
For example, you can download the data for an image on a secondary, background dispatch queue, but you’ll update the respective image view on the main thread. It’s quite often necessary to tell the system which tasks of your app are more important than others and which need priority in execution when working with the GCD and dispatch queues. Of course, tasks running on the main thread have always the highest priority, as the main queue also deals with the UI and keeps the app responsive. In any case, by providing that information to the system, iOS prioritises the queues properly and gives the needed resources according to what you’ll specify. Needless to say that all the tasks will eventually finish. However, the difference lies to which tasks will finish sooner, and which later.
Dispatch queues are thread-safe which means that we can access them from multiple threads simultaneously. The benefits of GCD are apparent when we understand how dispatch queues provide thread safety to parts of our own code. The key to this is to choose the right kind of dispatch queue and the right dispatching function to submit work to the queue. Comparing to the synchronous execution, this case is quite more interesting; you see that the code on the main queue and the code of our dispatch queue run in parallel. Our custom queue could have actually get more running time at the beginning, but that’s just a matter of priorities (and we’ll see it next).
Dispatch_once was very useful for initialisation code and other functions that were to be executed once and only once. In Swift 3, dispatch_once is deprecated and should be replaced with either global or static variables and constants. Apple/swift-evolution swift-evolution – This maintains proposals for changes and user-visible enhancements to the Swift Programming Language. Written for developers with some iOS programming experience. The book uses a problem-solution approach to discuss the APIs and frameworks of iOS SDK.
If you want to submit the work and proceed, use asynchronous execution. If you need to know that the work has completed, pick the synchronous one. Under the hood GCD is just a list of work items, that you have passed for execution. DispatchWorkItem class represents a single task, which is essentially a Swift closure. These work items are enqueued when DispatchQueue.async is called and dequeued automatically.
The advantages are many, and the most important ones include the execution of demanding tasks in less time, best user experience, no frozen user interfaces, and so on. Every iOS application has a main thread, which is there to display your user interface and listen for events. Complex computations may slow down the main thread and freeze the app. We must move all the heavy lifting to a background thread, and then move the result back to the main.
They can be categorized by a quality-of-service class, which determines the order of execution. Now lets talk about ThreadMain, what it will do from here. Since ThreadMain got a block it starts executing step by step and suddenly it sees ‘DispatchQueue.main.sync’ and submits the inner block on to the same TheradMain queue and keeps onnnnn waitingggggg .
Calling functions from different threads does not work in Playgrounds. Some of our examples would run there of course, but not all. So, we’ll overcome any potential problems by using a normal project, and for your ease just grab this one and open it. For the concurrent queue a new thread is brought up in the pool to service each async operation. Internally, there is a GCD thread pool that services all queues. The threads in the pool do not have any guaranteed lifetime and can be destroyed when a task completes.
Once a queue is created, the operating system is the one that manages it and gives it time to be processed on any core of the CPU. Multiple queues are managed accordingly, and that management is something that developers don’t have to deal with. Queues are following the FIFO pattern , meaning that the queue that comes first for execution https://bitcoin-mining.biz/ will also finish first . We will make that clear in one of our first examples later. So far we’ve seen how dispatch queues work synchronously and asynchronously, and how the Quality of Service class affects the priority that the system gives to them. The common thing to all the previous examples is the fact that our queues are serial.
Since they are both waiting for each other, hence the deadlock occurs. Concurrent queues allow multiple tasks to run at the same time. The queue guarantees tasks start in the order we add them.
This describes the main pattern of work with Grand Central Dispatch. There are some tasks which are more important than others, so we have to make sure that they will be executed first. Tasks running on main / UI thread are always at a high priority because they keep the app responsive, whereas tasks running on background thread are of least priority. Ultimately all tasks complete their execution but priority decides which one will be completed first. Every iOS application has a main thread, which is there to display user interface and listen to events.
The app won’t know about it when it’ll exit the method, we won’t be able to activate the queue, but most importantly we’ll get a crash on runtime. At its core, Grand Central Dispatch is a very efficient runtime for communication between threads and asynchronous execution. It is highly flexible and scales easily to lots of cores on the Mac as well as dual-core Apple Watch 4. It means performing task in Global Queue with using of background thread and when task finish, than global().sync use bring the work from globalQueue to mainQueue which update to UI.
Just like with dispatch groups, you can also use a semaphore object to get notified if multiple tasks are finished. When a work item is executed asynchronously with the async method, the method call returns immediately. When a work item is executed synchronously with the sync method, the program waits until execution finishes before the method call returns. Each work item can be executed either synchronously or asynchronously.
There is no explicit thread management in GCD, which allows to write code without actually thinking about threads. Note that you always have to balance out the enter and leave calls on the group. The dispatch group also allows us to track the completion of different work items, even if they run on different queues. Like getting work off of the main thread onto another queue. A common application of Grand Central Dispatch queues is locking. Not to repeat myself, I recommend reading Atomic Properties in Swift, where I explain the concept of locking and demonstrate different kinds of Swift locking API, including GCD.
On top of that, he’s added macOS programming to his repertoire over the last few years. To put the work in the pipeline, a task must be submitted to a queue. Beside not weakifying self in the first approach, both calls are equivalent. Dispatch queue simply allows you to perform iterations concurrently. Learn the principles of multi-threading with the GCD framework in Swift.