GCD全解-02-dispatch_queue

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队列分类

串行队列 :

DISPATCH_QUEUE_SERIAL

@discussion A dispatch queue that invokes blocks serially in FIFO order.
#define DISPATCH_QUEUE_SERIAL NULL

dispatch_get_main_queue();

@discussion Returns the default queue that is bound to the main thread.

dispatch_queue_t dispatch_get_main_queue(void){
    return DISPATCH_GLOBAL_OBJECT(dispatch_queue_t, _dispatch_main_q);
}

并行队列:

DISPATCH_QUEUE_CONCURRENT

@discussion A dispatch queue that may invoke blocks concurrently and supports barrier blocks submitted with the dispatch barrier API.

#define DISPATCH_QUEUE_CONCURRENT \
        DISPATCH_GLOBAL_OBJECT(dispatch_queue_attr_t, _dispatch_queue_attr_concurrent)

dispatch_get_global_queue(queue_priority, 0);

@discussion Returns a well-known global concurrent queue of a given quality of service class.The well-known global concurrent queues may not be modified. 

@param identifier
A quality of service class defined in qos_class_t or a priority defined in dispatch_queue_priority_t.

@param flags
Reserved for future use. 默认传入0就好了

dispatch_queue_t dispatch_get_global_queue(long identifier, unsigned long flags);

quality of service & queue_priority

The global concurrent queues are identified by their priority, which 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

创建Queue

dispatch_queue_create(@”描述字符串”，DISPATCH_QUEUE_CONCURRENT / DISPATCH_QUEUE_SERIAL )

* Creates a new dispatch queue to which blocks may be submitted.
 * @discussion
    传入DISPATCH_QUEUE_SERIAL or a NULL创建的是顺序执行的串行队列

    传入DISPATCH_QUEUE_CONCURRENT创建的是并发队（有点像是global concurrent queues）

    Block会引用Queue，当Block执行完只有Queue才会被释放
 * When a dispatch queue is no longer needed, it should be released with
 * dispatch_release(). Note that any pending blocks submitted to a queue will
 * hold a reference to that queue. Therefore a queue will not be deallocated
 * until all pending blocks have finished.

 * @param label
     A string label to attach to the queue. This parameter is optional and may be NULL.
 * @param attr
     A predefined attribute such as DISPATCH_QUEUE_SERIAL, DISPATCH_QUEUE_CONCURRENT.

dispatch_queue_t dispatch_queue_create(const char *_Nullable label, dispatch_queue_attr_t _Nullable attr);

//获取主队列
dispatch_queue_t mainQueue = dispatch_get_main_queue();

//串行队列
DISPATCH_QUEUE_SERIAL == NULL
dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_SERIAL);

//并行队列
dispatch_queue_t queue3 = dispatch_queue_create("queue3", DISPATCH_QUEUE_CONCURRENT);

//全局并发队列
dispatch_queue_t queue4 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);