Ios grand central dispatch cancel

What Sets Them Apart

An operation is ready when every dependency has finished executing. This is another important benefit if you want to monitor the state of an operation or operation queue.


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Operations can be paused, resumed, and cancelled. Once you dispatch a task using Grand Central Dispatch, you no longer have control or insight into the execution of that task. The NSOperationQueue also adds a number of benefits to the mix. For example, you can specify the maximum number of queued operations that can run simultaneously.

Swift - Grand Central Dispatch and Multi Threading Tutorial

This makes it easy to control how many operations run at the same time or to create a serial operation queue. In general, Apple advises developers to use the highest level of abstraction that is available. But why does Apple advise developers to use the highest level of abstraction? With every release, Apple tweaks and optimizes the frameworks and libraries that power the operating system. This usually involves changes affecting low er -level APIs.

grand central dispatch - How to stop a DispatchWorkItem in GCD? - Stack Overflow

But when do you use which technology? You can use Grand Central Dispatch whenever and wherever you like. You could, for example, use an NSOperation subclass to encapsulate the login sequence of an application. Dependency management is the icing on the cake.

Ultimate Grand Central Dispatch tutorial in Swift

So executing a task on a background queue and updating the UI on the main queue after the task finished is a pretty easy one using Dispatch queues. There is no big difference between sync and async methods on a queue. Sync is just an async call with a semaphore explained later that waits for the return value. A sync call will block, on the other hand an async call will immediately return. Basically if you need a return value use sync, but in every other case just go with async.

Oh, by the way it's just for debugging purpose, but you can return the name of the current queue by using this little extension. Do not use in production code!!! DispatchWorkItem encapsulates work that can be performed. A work item can be dispatched onto a DispatchQueue and within a DispatchGroup. So you just like with operations by using a work item you can cancel a running task. Also work items can notify a queue when their task is completed.

When to Use Which

So you need to perform multiple network calls in order to construct the data required by a view controller? This is where DispatchGroup can help you. All of your long running background task can be executed concurrently, when everything is ready you'll receive a notification. Just be careful you have to use thread-safe data structures, so always modify arrays for example on the same thread!

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. One more thing that you can use dispatch groups for: It might happens that the work is done faster than you'd expect and the indicator animation could not finish. To solve this situation you can add a small delay task so the group will wait until both of the tasks finish.

A semaphore is simply a variable used to handle resource sharing in a concurrent system. It's a really powerful object, here are a few important examples in Swift. If you want to avoid race condition you are probably going to use mutual exclusion. This could be achieved using a semaphore object, but if your object needs heavy reading capability you should consider a dispatch barrier based solution.

Just like with dispatch groups, you can also use a semaphore object to get notified if multiple tasks are finished. You just have to wait for it You can create a thread pool like behavior to simulate limited resources using a dispatch semaphore. So for example if you want to download lots of images from a server you can run a batch of x every time.

Quite handy. A dispatch source is a fundamental data type that coordinates the processing of specific low-level system events. Signals, descriptors, processes, ports, timers and many more. Everything is handled through the dispatch source object. I really don't want to get into the details, it's quite low-level stuff. You can monitor files, ports, signals with dispatch sources. Please just read the offical Apple docs.


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I'd like to make only one example here using a dispatch source timer. Thread safety is an inevitable topic if it comes to multi-threaded code. In the beginning I mentioned that there is a thread pool under the hood of GCD. Every thread has a run loop object associated with it, you can even run them by hand.

If you create a thread manually a run loop will be added to that thread automatically. A GCD queue is not a thread, if you run multiple async operations on a concurrent queue your code can run on any available thread that fits the needs. My 1 follower, The Bao , said. I have learned it and saw some tutorial but these do not explain good enough about advanced GCD.

However, I will cover the fundamental first. Imagine you are an iOS developer at Snapchat. The first part deals with flipping the image. The second networking with the server. Here comes our savior, Grand Central Dispatch. If I were to explain the concept to a 6-year old, GCD gives you iPhone the ability to text your friend main while taking a poop background with a couple lines of code. GCD is a magic.