前邊已經介紹了GCD的基本概念，下邊來介紹一下GCD佇列的基本用法，以及一些其他注意的地方。

本文參考連結：https://developer.apple.com/library/ios/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html

一：佇列的幾種型別：

GCD offers three kinds of queues:

Main: tasks execute serially on your application’s main thread(主佇列，在你程式的主執行緒中順序執行)
Concurrent: tasks are dequeued in FIFO order, but run concurrently and can finish in any order.(並行佇列，任務遵循先入先出，但同時執行並按照任意順序完成，完成順序取決於任務的複雜度)
Serial: tasks execute one at a time in FIFO order(序列佇列，依次執行，遵循先入先出)
 

二：向主佇列註冊block函式

The main queue is automatically created by the system and associated with your application’s main thread. 

Your application uses one (and only one) of the following three approaches to invoke blocks submitted to the main queue:

Calling dispatch_main
Calling UIApplicationMain (iOS) or NSApplicationMain (OS X)
Using a CFRunLoopRef on the main thread
 

詳解: 主佇列是由系統自動建立的並且與你應用的主執行緒相結合，你的應用只能用下邊的一種方式來把block函式註冊到你的主佇列中。到這裡大家可能不太理解者到底怎麼用呢，下邊來看例項程式碼

@autoreleasepool {
        return UIApplicationMain(argc, argv,nil,NSStringFromClass([AppDelegateclass]));
    }

注意，這裡是用到的就是UIApplicationMain方式,對於iOS要用這種方式

Important: GCD is a C level API; it does not catch exceptions generated by higher level languages. Your application must catch all exceptions before returning from a block submitted to a dispatch queue.
 

注意：GCD的介面是基於C的，它並不能捕捉到有高級別語言產生的exceptions, 你的應用必須在執行把block函式提交到佇列之前捕捉所有的異常。 三. 佇列使用的相關程式碼 建立一個佇列

dispatch_queue_t myQueue = dispatch_queue_create("com.openJailBreak.ios", NULL)

此處要注意的是，第一個引數是一個identifier,用於表示一個佇列，關鍵的是第二個引數，先來看官方原文:

attr
In OS X v10.7 and later, specify DISPATCH_QUEUE_SERIAL (or NULL) to create a serial queue or specify DISPATCH_QUEUE_CONCURRENT to create a concurrent queue. In earlier versions of OS X, you must specify NULL for this parameter.

意思是，如果第二個引數設定為DISPATCH_QUEUE_SERIAL或者NULL，意味著建立了一個序列佇列，如果指定一個DISPATCH_QUEUE_CONCURRENT意味著建立了一個並行佇列。在早期的版本中並行佇列是不可用的，需要設定為NULL,從OS X10.7,iOS 4.3以後才可用，先來看一下官方介面文件：

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

/*!
 * @const 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)
__OSX_AVAILABLE_STARTING(__MAC_10_7,__IPHONE_4_3)
DISPATCH_EXPORT

好了，到這裡我們就該清楚了，現在來用三種方式來建立一個佇列

dispatch_queue_t queue_t_concurrent = dispatch_queue_create("com.openJailBreak.concurrent", DISPATCH_QUEUE_CONCURRENT);//concurrent
dispatch_queue_t queue_t_serial = dispatch_queue_create("com.openJailBreak.serial", DISPATCH_QUEUE_SERIAL);//serial
dispatch_queue_t queue_t_null = dispatch_queue_create("com.openJailBreak.null", NULL); //serilal

下邊進入討論議題，來看原文：

Discussion

Blocks submitted to a serial queue are executed one at a time in FIFO order. Note, however, that blocks submitted to independent queues may be executed concurrently with respect to each other. Blocks submitted to a concurrent queue are dequeued in FIFO order but may run concurrently if resources are available to do so.

When your application no longer needs the dispatch queue, it should release it with the  function. Any pending blocks submitted to a queue hold a reference to that queue, so the queue is not deallocated until all pending blocks have completed.

相信大家都能理解這裡的重點，此處還是要強調一下：

只有提交到序列佇列的block函數出佇列時才會FIFO，而對於獨立的佇列可能會並行執行。提交到並行佇列的block出隊時也是FIFO，但是在執行的過程中可能會根據系統的資源來並行執行。

當你的應用不在需要佇列時，可以使用dispatch_release來釋放，但如果佇列中又任何pending狀態的block，此時是無法dealloc的直到pending block完成。

好了，第一部分就先講到這裡吧，後續會繼續釋出，感謝大家！