CD多线程的使用（三）

        关于Main Dispatch Queue和Global Dispatch Queue。

        上一篇说了创建Dispatch Queue的方法，其实不用dispatch_queue_create函数也可以获取Dispatch Queue，那就是Main Dispatch Queue 和 Global Dispatch Queue。

        顾名思义，Main Dispatch Queue是在主线程中执行的Dispatch Queue，主线程只能有一个，所以Main Dispatch Queue是一个串行的Dispatch Queue，即Serial Dispatch Queue。

        所有追加到Main Dispatch Queue中的处理都是在RunLoop中执行。由于是在主线程中执行，所以要将用户界面的更新等一些必须在主线程中执行的处理追加到Main Dispatch Queue中使用。这和NSObject类的performSelectorOnMainThread实例方法相似。

        Global Dispatch Queue是Concurrent Dispatch Queue。没有必要用dispatch_queue_create函数逐个生成Concurrent Dispatch Queue，只要获取Global Dispatch Queue就可以了。Global Dispatch Queue有四个执行优先级，分别是高优先级（High Priority）、默认优先级（Default Priority）、低优先级（Low Priority）和后台优先级（Background Priority）。

        获取Main Dispatch Queue的方法如下：

/**
 *  获取Main Dispatch Queue
 */
- (void)getMainDispatchQueue {
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    
    dispatch_async(mainQueue, ^{
        NSLog(@"%@: mainQueue  is main thread:%i", [NSThread currentThread], [NSThread isMainThread]);
    });
}

        输出结果：

2015-06-16 20:51:48.710 GCD_Study[19270:607] {name = (null), num = 1}: mainQueue  is main thread:1

        dispatch_get_main_queue函数就是获取Main Dispatch Queue的方法。另外，切记往Main Dispatch Queue中追加任务要用dispatch_async函数，因为dispatch_sync函数在追加任务时，在指定的任务执行结束前该函数是不会返回的，所以如果用dispatch_sync函数往Main Dispatch Queue中追加任务，会导致程序死锁。

        dispatch_get_main_queue函数的定义如下：

/*!
 * @function dispatch_get_main_queue
 *
 * @abstract
 * Returns the default queue that is bound to the main thread.
 *
 * @discussion
 * In order to invoke blocks submitted to the main queue, the application must
 * call dispatch_main(), NSApplicationMain(), or use a CFRunLoop on the main
 * thread.
 *
 * @result
 * Returns the main queue. This queue is created automatically on behalf of
 * the main thread before main() is called.
 */
 __OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_4_0)
DISPATCH_EXPORT struct dispatch_queue_s _dispatch_main_q;
 #define dispatch_get_main_queue() 
        DISPATCH_GLOBAL_OBJECT(dispatch_queue_t, _dispatch_main_q)

        获取Global Dispatch Queue的方法如下：

/**
 *  获取四个优先级的Global Dispatch Queue
 */
- (void) getGlobalDispatchQueue {
    //高优先级  DISPATCH_QUEUE_PRIORITY_HIGH
    dispatch_queue_t highPriorityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    
    //默认优先级 DISPATCH_QUEUE_PRIORITY_DEFAULT
    dispatch_queue_t defaultPriorityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    //低优先级 DISPATCH_QUEUE_PRIORITY_LOW
    dispatch_queue_t lowPriorityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    
    //后台优先级 DISPATCH_QUEUE_PRIORITY_BACKGROUND
    dispatch_queue_t backgroundPriorityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
}

        dispatch_get_global_queue需要传递两个参数，第一个是指定获得的优先级，第二个是一个flogs值，源码中是这样定义的：

/*!
 * @function dispatch_get_global_queue
 *
 * @abstract
 * Returns a well-known global concurrent queue of a given priority level.
 *
 * @discussion
 * The well-known global concurrent queues may not be modified. Calls to
 * dispatch_suspend(), dispatch_resume(), dispatch_set_context(), etc., will
 * have no effect when used with queues returned by this function.
 *
 * @param priority
 * A priority defined in dispatch_queue_priority_t
 *
 * @param flags
 * Reserved for future use. Passing any value other than zero may result in
 * a NULL return value.
 *
 * @result
 * Returns the requested global queue.
 */
__OSX_AVAILABLE_STARTING(__MAC_10_6,__IPHONE_4_0)
DISPATCH_EXPORT DISPATCH_CONST DISPATCH_WARN_RESULT DISPATCH_NOTHROW
dispatch_queue_t
dispatch_get_global_queue(dispatch_queue_priority_t priority,
        unsigned long flags);

        flags是保留着以后用的，现在只能传0，传任何大于零的值只能返回一个NULL。

        另外，对于Main Dispatch Queue和Global Dispatch Queue执行dispatch_retain或者dispatch_release是不会引起变化的，也不会有任何问题。