Dispatch Queue API 详解

Dispatch Queue

“开发者要做的只是定义想要执行的任务，并追加到适当的Dispatch Queque中”，用代码表述如下：

    dispatch_async(queue, ^{
       /*
        * 想要执行的任务Code
        */
    });

Dispatch Queue在上述代码 就是执行处理等待的队列，开发者可通过下面的代码创建想要执行的队列。

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

Dispatch Queque的分类

1. Serial Dispatch queue：又叫private dispatch queues，同时只执行一个任务。一个Serial queue只占用一个线程，常用于同步访问特定的资源或数据。当你创建多个Serialqueue时，虽然各自是同步，但serial queue之间是并发执行。
2. Concurrent Dispatch queue：又叫global dispatch queue，可以并发的执行多个任务，但执行完成顺序是随机的。系统提供四个全局并发队列，这四个队列有这对应的优先级，用户是不能够创建全局队列的，只能获取。
3. Main Dispatch queue：全局可用的serial queue，在应用程序主线程上执行任务。

dispatch_queue_creat

通过dispatch_queue_creat函数来生产可执行的Dispatch Queue。
该函数有两个参数，第一个自定义的队列名，第二个参数是队列类型，默认NULL或者DISPATCH_QUEUE_SERIAL的是串行，参数为DISPATCH_QUEUE_CONCURRENT为并行队列。

dispatch_queue_t queueCon, queueSerial;
queueCon = dispatch_queue_create("com.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
queueSerial = dispatch_queue_create("com.serial.queue", DISPATCH_QUEUE_SERIAL);

队列优先级

可以通过dipatch_queue_attr_make_with_qos_class或dispatch_set_target_queue方法设置队列的优先级。

dipatch_queue_attr_make_with_qos_class

 * Returns an attribute value which may be provided to dispatch_queue_create()
 * or dispatch_queue_create_with_target(), in order to assign a QOS class and
 * relative priority to the queue.
 *
 * @discussion
 * When specified in this manner, the QOS class and relative priority take
 * precedence over those inherited from the dispatch queue's target queue (if
 * any) as long that does not result in a lower QOS class and relative priority.
 *
 * The global queue priorities map to the following QOS classes:
 *  - DISPATCH_QUEUE_PRIORITY_HIGH:         QOS_CLASS_USER_INITIATED
 *  - DISPATCH_QUEUE_PRIORITY_DEFAULT:      QOS_CLASS_DEFAULT
 *  - DISPATCH_QUEUE_PRIORITY_LOW:          QOS_CLASS_UTILITY
 *  - DISPATCH_QUEUE_PRIORITY_BACKGROUND:   QOS_CLASS_BACKGROUND
 *
 * Example:
 * <code>
 *  dispatch_queue_t queue;
 *  dispatch_queue_attr_t attr;
 *  attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL,
 *          QOS_CLASS_UTILITY, 0);
 *  queue = dispatch_queue_create("com.example.myqueue", attr);
 * </code>
  * @param attr
 * A queue attribute value to be combined with the QOS class, or NULL.
 *
 * @param qos_class
 * A QOS class value:
 *  - QOS_CLASS_USER_INTERACTIVE
 *  - QOS_CLASS_USER_INITIATED
 *  - QOS_CLASS_DEFAULT
 *  - QOS_CLASS_UTILITY
 *  - QOS_CLASS_BACKGROUND
 * Passing any other value results in NULL being returned.
 *
 * @param relative_priority
 * A relative priority within the QOS class. This value is a negative
 * offset from the maximum supported scheduler priority for the given class.
 * Passing a value greater than zero or less than QOS_MIN_RELATIVE_PRIORITY
 * results in NULL being returned.
 *
 * @return
 * Returns an attribute value which may be provided to dispatch_queue_create()
 * and dispatch_queue_create_with_target(), or NULL if an invalid QOS class was
 * requested.
 * The new value combines the attributes specified by the 'attr' parameter and
 * the new QOS class and relative priority.
 */

dispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_UTILITY, -1);
dispatch_queue_t queue = dispatch_queue_create("com.starming.gcddemo.qosqueue", attr);

dispatch_set_target_queue

可以设置优先级，也可以设置队列层级体系，比如让多个串行和并行队列在统一一个串行队列里串行执行。

//dispatch_set_target_queue
dispatch_queue_t queue = dispatch_queue_create("com.starming.gcddemo.settargetqueue",NULL); //需要设置优先级的queue
dispatch_queue_t referQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0); //参考优先级
//设置queue和referQueue的优先级一样
dispatch_set_target_queue(queue, referQueue); 

//让多个串行和并行队列在统一一个串行队列里串行执行
dispatch_queue_t serialQueue = dispatch_queue_create("com.starming.gcddemo.serialqueue", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t firstQueue = dispatch_queue_create("com.starming.gcddemo.firstqueue", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t secondQueue = dispatch_queue_create("com.starming.gcddemo.secondqueue", DISPATCH_QUEUE_CONCURRENT);

dispatch_set_target_queue(firstQueue, serialQueue);
dispatch_set_target_queue(secondQueue, serialQueue);

dispatch_async(firstQueue, ^{
    NSLog(@"1"