Swift3 GCD全局队列(global)的优先级

Swift 3 在GCD的语法上改变了很多，更加贴近了swift的语法，而且关于队列的优先级也改变了不少。

原先的GCD只有四个优先级，high，default，low，background

然而现在的GCD有六个优先级，background，utility，default，userInitiated，userInteractive，unspecified

经过试验，得出了几个结论，

userInteractive的优先级最高，相当于high

background的优先级最低，相当于原来的background

default的优先级高于unspecified，userInitiated，utility

unspecified的优先级高于userInitiated，utility

userInitiated的优先级高于utility

userInteractive>default>unspecified>userInitiated>utility>background

另附实验代码，若有不正确的地方欢迎指正

for i in 1...10 {
       DispatchQueue.global(qos: DispatchQoS.QoSClass.default).async {
           NSLog("DispatchQoS.QoSClass.default,%d",i)
       }
            
       DispatchQueue.global(qos: DispatchQoS.QoSClass.background).async {
           NSLog("DispatchQoS.QoSClass.background,i)
       }
            
       DispatchQueue.global(qos: DispatchQoS.QoSClass.unspecified).async {
           NSLog("DispatchQoS.QoSClass.unspecified,i)
       }
            
       DispatchQueue.global(qos: DispatchQoS.QoSClass.userInitiated).async {
           NSLog("DispatchQoS.QoSClass.userInitiated,i)
       }
            
       DispatchQueue.global(qos: DispatchQoS.QoSClass.userInteractive).async {
           NSLog("DispatchQoS.QoSClass.userInteractive,i)
       }
            
       DispatchQueue.global(qos: DispatchQoS.QoSClass.utility).async {
           NSLog("DispatchQoS.QoSClass.utility,i)
       }
}

试验数据：

2016-11-15 11:46:12.927070 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,1
2016-11-15 11:46:12.927363 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,2
2016-11-15 11:46:12.927407 GraphicsContext[2099:635171] DispatchQoS.QoSClass.userInteractive,3
2016-11-15 11:46:12.927490 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,4
2016-11-15 11:46:12.927674 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,5
2016-11-15 11:46:12.927780 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,6
2016-11-15 11:46:12.927868 GraphicsContext[2099:635172] DispatchQoS.QoSClass.userInteractive,7
2016-11-15 11:46:12.927871 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,8
2016-11-15 11:46:12.928016 GraphicsContext[2099:635172] DispatchQoS.QoSClass.userInteractive,9
2016-11-15 11:46:12.928138 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInteractive,10
2016-11-15 11:46:12.928719 GraphicsContext[2099:635113] DispatchQoS.QoSClass.default,1
2016-11-15 11:46:12.928817 GraphicsContext[2099:635166] DispatchQoS.QoSClass.unspecified,1
2016-11-15 11:46:12.928924 GraphicsContext[2099:635172] DispatchQoS.QoSClass.userInitiated,1
2016-11-15 11:46:12.929190 GraphicsContext[2099:635173] DispatchQoS.QoSClass.default,2
2016-11-15 11:46:12.929225 GraphicsContext[2099:635171] DispatchQoS.QoSClass.utility,1
2016-11-15 11:46:12.929723 GraphicsContext[2099:635174] DispatchQoS.QoSClass.unspecified,2
2016-11-15 11:46:12.930104 GraphicsContext[2099:635176] DispatchQoS.QoSClass.userInitiated,2
2016-11-15 11:46:12.930131 GraphicsContext[2099:635175] DispatchQoS.QoSClass.utility,2
2016-11-15 11:46:12.930641 GraphicsContext[2099:635177] DispatchQoS.QoSClass.default,3
2016-11-15 11:46:12.931705 GraphicsContext[2099:635179] DispatchQoS.QoSClass.unspecified,3
2016-11-15 11:46:12.931857 GraphicsContext[2099:635180] DispatchQoS.QoSClass.userInitiated,3
2016-11-15 11:46:12.932196 GraphicsContext[2099:635181] DispatchQoS.QoSClass.default,4
2016-11-15 11:46:12.932277 GraphicsContext[2099:635178] DispatchQoS.QoSClass.utility,3
2016-11-15 11:46:12.932779 GraphicsContext[2099:635183] DispatchQoS.QoSClass.unspecified,4
2016-11-15 11:46:12.932889 GraphicsContext[2099:635182] DispatchQoS.QoSClass.userInitiated,4
2016-11-15 11:46:12.933215 GraphicsContext[2099:635185] DispatchQoS.QoSClass.default,5
2016-11-15 11:46:12.933027 GraphicsContext[2099:635184] DispatchQoS.QoSClass.utility,4
2016-11-15 11:46:12.933423 GraphicsContext[2099:635113] DispatchQoS.QoSClass.unspecified,5
2016-11-15 11:46:12.933586 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInitiated,5
2016-11-15 11:46:12.933709 GraphicsContext[2099:635172] DispatchQoS.QoSClass.default,6
2016-11-15 11:46:12.933831 GraphicsContext[2099:635173] DispatchQoS.QoSClass.unspecified,6
2016-11-15 11:46:12.933976 GraphicsContext[2099:635174] DispatchQoS.QoSClass.userInitiated,6
2016-11-15 11:46:12.934095 GraphicsContext[2099:635176] DispatchQoS.QoSClass.default,7
2016-11-15 11:46:12.934242 GraphicsContext[2099:635177] DispatchQoS.QoSClass.unspecified,7
2016-11-15 11:46:12.934356 GraphicsContext[2099:635179] DispatchQoS.QoSClass.userInitiated,7
2016-11-15 11:46:12.934503 GraphicsContext[2099:635180] DispatchQoS.QoSClass.default,8
2016-11-15 11:46:12.934611 GraphicsContext[2099:635171] DispatchQoS.QoSClass.utility,5
2016-11-15 11:46:12.934701 GraphicsContext[2099:635175] DispatchQoS.QoSClass.utility,6
2016-11-15 11:46:12.937166 GraphicsContext[2099:635186] DispatchQoS.QoSClass.unspecified,8
2016-11-15 11:46:12.937476 GraphicsContext[2099:635188] DispatchQoS.QoSClass.userInitiated,8
2016-11-15 11:46:12.937521 GraphicsContext[2099:635187] DispatchQoS.QoSClass.utility,7
2016-11-15 11:46:12.937682 GraphicsContext[2099:635181] DispatchQoS.QoSClass.default,9
2016-11-15 11:46:12.937868 GraphicsContext[2099:635183] DispatchQoS.QoSClass.unspecified,9
2016-11-15 11:46:12.937987 GraphicsContext[2099:635182] DispatchQoS.QoSClass.userInitiated,9
2016-11-15 11:46:12.938110 GraphicsContext[2099:635185] DispatchQoS.QoSClass.default,10
2016-11-15 11:46:12.938262 GraphicsContext[2099:635113] DispatchQoS.QoSClass.unspecified,10
2016-11-15 11:46:12.938381 GraphicsContext[2099:635166] DispatchQoS.QoSClass.userInitiated,10
2016-11-15 11:46:12.938692 GraphicsContext[2099:635178] DispatchQoS.QoSClass.utility,8
2016-11-15 11:46:12.938837 GraphicsContext[2099:635184] DispatchQoS.QoSClass.utility,9
2016-11-15 11:46:12.941483 GraphicsContext[2099:635174] DispatchQoS.QoSClass.utility,10
2016-11-15 11:46:12.949181 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,1
2016-11-15 11:46:12.949734 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,2
2016-11-15 11:46:12.950257 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,3
2016-11-15 11:46:12.950775 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,4
2016-11-15 11:46:12.951348 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,5
2016-11-15 11:46:12.952512 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,6
2016-11-15 11:46:12.953108 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,7
2016-11-15 11:46:12.953659 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,8
2016-11-15 11:46:12.954176 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,9
2016-11-15 11:46:12.954890 GraphicsContext[2099:635115] DispatchQoS.QoSClass.background,10