Take You system learning GCD (ii)

Welcome into the coding world of summer. 1. Dispatch_barrier_async

He is explained by the code below.

    dispatch_queue_tQueue = Dispatch_queue_create ("Queue.example", dispatch_queue_concurrent);Dispatch_async(Queue, ^{NSLog(@"-------------------1"); });Dispatch_async(Queue, ^{NSLog(@"-------------------2"); });Dispatch_async(Queue, ^{NSLog(@"-------------------3"); });Dispatch_async(Queue, ^{NSLog(@"-------------------4"); });//Dispatch_barrier_async (queue, ^{        //NSLog (@ "I am a Barrier");        //    });    Dispatch_async(Queue, ^{NSLog(@"-------------------5"); });Dispatch_async(Queue, ^{NSLog(@"-------------------6"); });Dispatch_async(Queue, ^{NSLog(@"-------------------7"); });Dispatch_async(Queue, ^{NSLog(@"-------------------8"); });

When certain features are implemented in the development process, we may need specific representations, such as what I want to do after the completion of the 1234 Execution (5678), so that using Dispatch_barrier_async can be done, Of course, using the group and Dispatch_set_target_queue (less commonly used) in the previous article can also be implemented, but the source code will be very complex, not recommended.

2. Dispatch_sync

The Dispatch_sync function "Async" is asynchronous and non-synchronous, and sync is synchronous.

Suppose a scenario: when executing the main Dispatch queue, use a different thread of the Global Dispatch queue for processing, and immediately use the resulting result code as shown below

dispatch_queue_t0);    dispatch_sync(queue, ^{        //    });

Sync can easily cause a deadlock condition. Below is a list of several
Scenario 1

dispatch_queue_t queue2 = dispatch_get_main_queue();    dispatch_sync(queue2, ^{        //        NSLog(@"----------"// 不会被执行    });

Scenario 2

    dispatch_queue_t queue3 = dispatch_get_main_queue();    dispatch_async(queue3, ^{        //        dispatch_sync(queue3, ^{            NSLog(@"----------");        });    });

Scenario 3

    dispatch_queue_t queue4 = dispatch_queue_create("queue.example", DISPATCH_QUEUE_SERIAL);    dispatch_async(queue4, ^{        //        dispatch_sync(queue4, ^{            //            NSLog(@"---------");        });    });

3. dispatch_apply

Dispatch_apply is the associated API for Dispatch_sync and dispatch Group.
Execute the code in block repeatedly. Examples such as the following

dispatch_queue_t0);    dispatch_apply(5, queue5, ^(size_t i) {        NSLog(@"i= %zu", i);    });

In fact, the dispatch_apply function is the same as the Dispatch_sync function, and will wait for the execution to finish, so it is recommended to use the sample code as follows

    dispatch_queue_tQueue6 = Dispatch_get_global_queue (Dispatch_queue_priority_default,0);//non-synchronous execution in global Dispatch Queue    Dispatch_async(Queue6, ^{//Global Dispatch Queue waits for all processing execution to finish in the Dispatch_apply functionDispatch_apply (5, Queue6, ^ (size_t index) {//            NSLog(@"i =%zu", index); });//The processing in the Dispatch_apply function ends with all execution        //In main Dispatch Queue non-synchronous execution        Dispatch_async(Dispatch_get_main_queue (), ^{//user interface update, etc.            NSLog(@"End");    }); });

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Take You system learning GCD (ii)