How a several dispatch_async to execute sequentially?

Question

I have 2 methods:

-(void)clear
{
  // some code

  dispatch_async( dispatch_get_global_queue(0,0), 
        ^{
              // async_block_1

         });     
}

-(void)download
{
  // some code

  dispatch_async( dispatch_get_global_queue(0,0), 
        ^{
              // async_block_2

         });     
}

And I need call'em in 3rd method:

-(void)relosd
{
   [self clear];
   [self download];
}

How I can guaranted perform first async_block_1 then async_block_2?

Obvious that the following code does not guarantee this:

-(void)reload
{
    dispatch_sync( dispatch_get_global_queue(0,0), 
       ^{
           [self clear];
           [self download];
       });
}

Could you do this as a "NSOperationQueue" thing with "setMaxConcurrentOperationCount:" set to 1? — Michael Dautermann
– Michael Dautermann, Commented May 19, 2014 at 14:17
Michael, yes, I can. But why? CGD also can do it with serial queue. But I need parallel working, that will sinchronized sometimes. — Xandr
– Xandr, Commented Jun 9, 2014 at 14:24

rckoenes · Accepted Answer · 2014-05-19 14:26:11Z

GCD queue can be create to run as a serial queue.

Just create your own queue:

dispatch_queue_t queue = dispatch_queue_create("com.my.download", NULL);

You can do this .m part where you functie are declared something like:

dispatch_queue_t getDownloadQueue() {
    static dispatch_once_t onceToken;
    static dispatch_queue_t ruleQueue;
    dispatch_once(&onceToken, ^{
        ruleQueue = dispatch_queue_create("com.my.download", NULL);
    });

    return ruleQueue;
}

Just place this code outside of your @implementation, after which you can just do something like:

-(void)clear {
  // some code
  dispatch_async( getDownloadQueue(), ^{
    // async_block_1
  });     
}

-(void)download {
  // some code

  dispatch_async(getDownloadQueue(), ^{
    // async_block_2
  });     
}

If you only need the queue in one instance of you class then use a property, this example will create a queue which you can use throughout your app and makes sure that they are executed in order.

Paul.s · Accepted Answer · 2014-05-20 22:14:36Z

2

You are probably after what rckoenes is suggesting but your other options are.

Use NSOperationQueue and set dependencies
Use NSOperationQueue and set maxConcurrentOperationCount
Use dispatch_barrier_async instead of dispatch_async

answered May 20, 2014 at 22:14

Paul.s

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