Swift3 GCD队列优先级说明

从ios8开始，苹果引入了一个新的概念 QoS（quality of service），用于指定GCD队列的优先级。

swift3之前：只有4个优先级

high > default > low > background

swift3之后：调整为6个优先级

userInteractive > default > unspecified > userInitiated > utility > background

其中：userInteractive优先级最高，background最低

现有和之前的大概对应关系：

* DISPATCH_QUEUE_PRIORITY_HIGH:         .userInteractive
* DISPATCH_QUEUE_PRIORITY_DEFAULT:      .default
* DISPATCH_QUEUE_PRIORITY_LOW:          .utility
* DISPATCH_QUEUE_PRIORITY_BACKGROUND:   .background

我们使用时：

系统提供两个方法：

1、使用global(),系统会默认给予Default优先级

2、使用global(qos:xxx)自定义自己想要的优先级

测试代码：（摘自网络，侵权删）

1、我们对每个优先级进行打印：

for i in ... {
       DispatchQueue.global(qos: DispatchQoS.QoSClass.default).async {
           NSLog("DispatchQoS.QoSClass.default, %d", i)
       }

       DispatchQueue.global(qos: DispatchQoS.QoSClass.background).async {
           NSLog("DispatchQoS.QoSClass.background, %d", i)
       }

       DispatchQueue.global(qos: DispatchQoS.QoSClass.unspecified).async {
           NSLog("DispatchQoS.QoSClass.unspecified, %d", i)
       }

       DispatchQueue.global(qos: DispatchQoS.QoSClass.userInitiated).async {
           NSLog("DispatchQoS.QoSClass.userInitiated, %d", i)
       }

       DispatchQueue.global(qos: DispatchQoS.QoSClass.userInteractive).async {
           NSLog("DispatchQoS.QoSClass.userInteractive, %d", i)
       }

       DispatchQueue.global(qos: DispatchQoS.QoSClass.utility).async {
           NSLog("DispatchQoS.QoSClass.utility, %d", i)
       }
}

打印结果：

-- ::12.927070 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927363 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927407 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927490 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927674 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927780 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927868 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.927871 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.928016 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.928138 GraphicsContext[:] DispatchQoS.QoSClass.userInteractive,
-- ::12.928719 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.928817 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.928924 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.929190 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.929225 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.929723 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.930104 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.930131 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.930641 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.931705 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.931857 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.932196 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.932277 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.932779 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.932889 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.933215 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.933027 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.933423 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.933586 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.933709 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.933831 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.933976 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.934095 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.934242 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.934356 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.934503 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.934611 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.934701 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.937166 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.937476 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.937521 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.937682 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.937868 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.937987 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.938110 GraphicsContext[:] DispatchQoS.QoSClass.default,
-- ::12.938262 GraphicsContext[:] DispatchQoS.QoSClass.unspecified,
-- ::12.938381 GraphicsContext[:] DispatchQoS.QoSClass.userInitiated,
-- ::12.938692 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.938837 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.941483 GraphicsContext[:] DispatchQoS.QoSClass.utility,
-- ::12.949181 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.949734 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.950257 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.950775 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.951348 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.952512 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.953108 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.953659 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.954176 GraphicsContext[:] DispatchQoS.QoSClass.background,
-- ::12.954890 GraphicsContext[:] DispatchQoS.QoSClass.background, 

结果分析：

其中：

1、userinteractive 10个全部第一轮就打印出来，优先级最高。

2、background 10个全部最后才打印出来，优先级最低。

3、中间几个大致为 default > unspecified > userInitiated > utility

～