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Also notice that while Task 3 started after Task 2, it finished first. The decision of when to start a task is entirely up to GCD.

Instead, you specify a Quality of Service QoS class property. A synchronous function returns control to the caller after the task completes. You can schedule a unit of work synchronously by calling DispatchQueue. An asynchronous function returns immediately, ordering the task to start but not waiting for it to complete. Thus, an asynchronous function does not block the current thread of execution from proceeding on to the next function.

You can schedule a unit of work asynchronously by calling DispatchQueue. For the purposes of this tutorial you can consider a task to be a closure.


Closures are self-contained, callable blocks of code you can store and pass around. You can configure the behavior of a DispatchWorkItem such as its QoS class or whether to spawn a new detached thread. Head back to the app and add some photos from your Photo Library or use the Le Internet option to download a few. Tap on a photo. Notice how long it takes for the photo detail view to show up. The lag is more pronounced when viewing large images on slower devices. Open PhotoDetailViewController. Modify viewDidLoad and replace these two lines:. In two spots, you add [weak self] to capture a weak reference to self in each closure.

Build and run the app. Download photos through Le Internet option.

Programming Java threads in the real world, Part 1 | JavaWorld

This lends a nice before and after effect to the app as the googly eyes appear. In general, you want to use async when you need to perform a network-based or CPU-intensive task in the background and not block the current thread. DispatchQueue allows you to delay task execution. Instead, use this when you want a task to run at a specific time. Consider the user experience of your app for a moment.

If you display a prompt too quickly, they might miss it as their eyes linger on other parts of the view. Open PhotoCollectionViewController. Why not use Timer? You could consider using it if you have repeated tasks which are easier to schedule with Timer. One is readability. To use Timer you have to define a method, then create the timer with a selector or invocation to the defined method.

With DispatchQueue and asyncAfter , you simply add a closure. Timer is scheduled on run loops so you would also have to make sure you scheduled it on the correct run loop and in some cases for the correct run loop modes. In this regard, working with dispatch queues is easier. This concern is justified given their use: singletons are often used from multiple controllers accessing the singleton instance at the same time.

Thread safe code can be safely called from multiple threads or concurrent tasks without causing any problems such as data corruption or app crashes. Code that is not thread safe can only run in one context at a time. There are two thread safety cases to consider: during initialization of the singleton instance and during reads and writes to the instance.

Initialization turns out to be the easy case because of how Swift initializes static variables. It initializes static variables when they are first accessed, and it guarantees initialization is atomic. That is, Swift treats the code performing initialization as a critical section and guarantees it completes before any other thread gets access to the static variable. A critical section is a piece of code that must not execute concurrently, that is, from two threads at once.

The private initializer makes sure that the only PhotoManager is then one assigned to shared.

Brief explanation of Locks

You still have to deal with thread safety when accessing code in the singleton that manipulates shared internal data. You can handle this through methods such as synchronizing data access. In Swift, any variable declared with the let keyword is a constant and, therefore, read-only and thread-safe. Declare the variable with the var keyword however, and it becomes mutable and not thread-safe unless the data type is designed to be so. The Swift collection types like Array and Dictionary are not thread-safe when declared mutable.

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The caller gets a copy of the array and is protected against mutating the original array inappropriately. Passing by value results in a copy of the object, and changes to the copy will not affect the original. By default in Swift, class instances are passed by reference and struct s are passed by value. The Swift collection types are optimized to make copies only when necessary, for instance, when your app modifies an array passed by value for the first time. This is the classic software development Readers-Writers Problem. Dispatch barriers are a group of functions acting as a serial-style bottleneck when working with concurrent queues.

When you submit a DispatchWorkItem to a dispatch queue you can set flags to indicate that it should be the only item executed on the specified queue for that particular time. This means that all items submitted to the queue prior to the dispatch barrier must complete before the DispatchWorkItem will execute. Once finished, the queue returns to its default implementation.

Notice how in normal operation the queue acts just like a normal concurrent queue.

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But when the barrier is executing, it essentially acts like a serial queue. That is, the barrier is the only thing executing. After the barrier finishes, the queue goes back to being a normal concurrent queue. Use caution when using barriers in global background concurrent queues as these queues are shared resources.

Top 80 Thread- Java Interview Questions and Answers (Part 1)

Using barriers in a custom serial queue is redundant as it already executes serially. Using barriers in custom concurrent queue is a great choice for handling thread safety in atomic or critical areas of code. The concurrent queue will allow multiple read operations simultaneously. Open PhotoManager. This initializes concurrentPhotoQueue as a concurrent queue. You set up label with a descriptive name that is helpful during debugging. Typically, you use the reverse DNS style naming convention. This takes care of the write, but you also need to implement the photos read method.

Programming Java threads in the real world, Part 1

To ensure thread safety with your writes, you need to perform reads on the concurrentPhotoQueue queue. Comment on this post. You can also subscribe to this blog by email using the form on the left. Very good tutorial. Nice introduction!

Optimize Query Performance using the Navicat Query Analyzer (Part 1)

A few more concise articles like this and I will feel confident enough to make thread programming a part of my daily routine. It might have finished, it might have output the "hello", but not processed the std::endl yet, or it might not have even started. Hi Anthony, could you please clarify this thing?

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