Objective-C 多線程: Grand Central Dispatch(GCD)

Serial	sync嵌套會死鎖, 外部的sync block會等待內部的sync block完成才結束, 但內部的永遠執行不到
	GCD_TEST[45149:303] HELLO
	GCD_TEST[45149:1a03] async:{name = (null), num = 2}
	-
Concurrent	怎麼嵌套都不會死鎖, 因為block沒有順序依賴關係
	GCD_TEST[45167:303] HELLO
	GCD_TEST[45167:303] sync:{name = (null), num = 1}
	GCD_TEST[45167:1503] async:{name = (null), num = 2}
	GCD_TEST[45167:1503] sync-async:{name = (null), num = 2}
	GCD_TEST[45167:1a03] async-async:{name = (null), num = 3}
	GCD_TEST[45167:303] sync-sync:{name = (null), num = 1}
	GCD_TEST[45167:1503] sync-async:{name = (null), num = 2}
	-
Main	async 和 sync 相同, 都在主線程, 怎麼嵌套都會死鎖, 只用sync也無法執行, 因為main queue一直有任務在執行
	GCD_TEST[45083:303] HELLO
	-
Global	系統共用的queue, 效果同Concurrent
	GCD_TEST[45187:303] HELLO
	GCD_TEST[45187:303] sync:{name = (null), num = 1}
	GCD_TEST[45187:1a03] async:{name = (null), num = 2}
	GCD_TEST[45187:1a03] sync-async:{name = (null), num = 2}
	GCD_TEST[45187:1f03] async-async:{name = (null), num = 3}
	GCD_TEST[45187:303] sync-sync:{name = (null), num = 1}
	GCD_TEST[45187:1a03] sync-async:{name = (null), num = 2}