Understanding dispatch async

Question

I have question around this code   dispatch async dispatch get global queue  DISPATCH QUEUE PRIORITY DEFAULT  0          NSData  data    NSData dataWithContentsOfURL         kLatestKivaLoansURL        self performSelectorOnMainThread  selector fetchedData          withObject data waitUntilDone YES         The first parameter of this code is    dispatch get global queue DISPATCH QUEUE PRIORITY DEFAULT  0     Are we asking this  code to  perform  serial tasks on global queue whose definition itself is that it returns global concurrent queue of a given priority level   What is advantage of using dispatch get global queue over the main queue   I am confused  Could you please help me to understand this better

User · Answer

The main reason you use the default queue over the main queue is to run tasks in the background.

For instance, if I am downloading a file from the internet and I want to update the user on the progress of the download, I will run the download in the priority default queue and update the UI in the main queue asynchronously.

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
    //Background Thread
    dispatch_async(dispatch_get_main_queue(), ^(void){
        //Run UI Updates
    });
});

User · Answer

All of the DISPATCH QUEUE PRIORITY X queues are concurrent queues  meaning they can execute multiple tasks at once   and are FIFO in the sense that tasks within a given queue will begin executing using  first in  first out  order  This is in comparison to the main queue  from dispatch get main queue     which is a serial queue  tasks will begin executing and finish executing in the order in which they are received    So  if you send 1000 dispatch async   blocks to DISPATCH QUEUE PRIORITY DEFAULT  those tasks will start executing in the order you sent them into the queue  Likewise for the HIGH  LOW  and BACKGROUND queues  Anything you send into any of these queues is executed in the background on alternate threads  away from your main application thread  Therefore  these queues are suitable for executing tasks such as background downloading  compression  computation  etc   Note that the order of execution is FIFO on a per-queue basis  So if you send 1000 dispatch async   tasks to the four different concurrent queues  evenly splitting them and sending them to BACKGROUND  LOW  DEFAULT and HIGH in order  ie you schedule the last 250 tasks on the HIGH queue   it s very likely that the first tasks you see starting will be on that HIGH queue as the system has taken your implication that those tasks need to get to the CPU as quickly as possible   Note also that I say  will begin executing in order   but keep in mind that as concurrent queues things won t necessarily FINISH executing in order depending on length of time for each task    As per Apple   https   developer apple com library content documentation General Conceptual ConcurrencyProgrammingGuide OperationQueues OperationQueues html     A concurrent dispatch queue is useful when you have multiple tasks that can run in parallel  A concurrent queue is still a queue in that it dequeues tasks in a first-in  first-out order  however  a concurrent queue may dequeue additional tasks before any previous tasks finish  The actual number of tasks executed by a concurrent queue at any given moment is variable and can change dynamically as conditions in your application change  Many factors affect the number of tasks executed by the concurrent queues  including the number of available cores  the amount of work being done by other processes  and the number and priority of tasks in other serial dispatch queues    Basically  if you send those 1000 dispatch async   blocks to a DEFAULT  HIGH  LOW  or BACKGROUND queue they will all start executing in the order you send them  However  shorter tasks may finish before longer ones  Reasons behind this are if there are available CPU cores or if the current queue tasks are performing computationally non-intensive work  thus making the system think it can dispatch additional tasks in parallel regardless of core count     The level of concurrency is handled entirely by the system and is based on system load and other internally determined factors  This is the beauty of Grand Central Dispatch  the dispatch async   system  - you just make your work units as code blocks  set a priority for them  based on the queue you choose  and let the system handle the rest   So to answer your above question  you are partially correct  You are  asking that code  to perform concurrent tasks on a global concurrent queue at the specified priority level  The code in the block will execute in the background and any additional  similar  code will execute potentially in parallel depending on the system s assessment of available resources   The  main  queue on the other hand  from dispatch get main queue    is a serial queue  not concurrent   Tasks sent to the main queue will always execute in order and will always finish in order  These tasks will also be executed on the UI Thread so it s suitable for updating your UI with progress messages  completion notifications  etc

User · Answer

Swift version This is the Swift version of David s Objective-C answer  You use the global queue to run things in the background and the main queue to update the UI  DispatchQueue global qos   background  async               Background Thread          DispatchQueue main async              Run UI Updates

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