代码
for(int i =0;i<10;i++)
{
NSLog(@"i=%d",i);
dispatch_queue_t mySerialQueue = dispatch_queue_create("com.biostime.xxx", DISPATCH_QUEUE_SERIAL);
__block int d = i;
dispatch_async(mySerialQueue, ^{
NSRunLoop *loop = [NSRunLoop currentRunLoop];
NSLog(@"%p excute1 i = %d d=%d",loop,i,d);
d=9;
NSLog(@"%p excute2 i = %d d=%d",loop,i,d);
});
NSLog(@"i=%d",i);
}
＝＝＝＝＝＝＝＝＝＝＝＝＝
2015-05-29 14:38:49.322 GCDDemo[8558:405173] operation begin
2015-05-29 14:38:49.322 GCDDemo[8558:405173] for begin 0
2015-05-29 14:38:49.322 GCDDemo[8558:405173] mySerialQueue 0x7fb0b051d490
2015-05-29 14:38:49.322 GCDDemo[8558:405173] for end 0
2015-05-29 14:38:49.322 GCDDemo[8558:405173] for begin 1
2015-05-29 14:38:49.322 GCDDemo[8558:405173] mySerialQueue 0x7fb0b05835e0
2015-05-29 14:38:49.322 GCDDemo[8558:405173] for end 1
2015-05-29 14:38:49.322 GCDDemo[8558:405173] for begin 2
2015-05-29 14:38:49.323 GCDDemo[8558:405173] mySerialQueue 0x7fb0b05858b0
2015-05-29 14:38:49.323 GCDDemo[8558:405173] for end 2
2015-05-29 14:38:49.323 GCDDemo[8558:405173] for begin 3
2015-05-29 14:38:49.323 GCDDemo[8558:405208] <NSThread: 0x7fb0b07136e0>{number = 2, name = (null)} === begin
2015-05-29 14:38:49.323 GCDDemo[8558:405209] <NSThread: 0x7fb0b0711f50>{number = 3, name = (null)} === begin
2015-05-29 14:38:49.323 GCDDemo[8558:405207] <NSThread: 0x7fb0b057bde0>{number = 4, name = (null)} === begin
2015-05-29 14:38:49.323 GCDDemo[8558:405173] mySerialQueue 0x7fb0b0509480
2015-05-29 14:38:49.323 GCDDemo[8558:405173] for end 3
2015-05-29 14:38:49.323 GCDDemo[8558:405173] for begin 4
2015-05-29 14:38:49.323 GCDDemo[8558:405173] mySerialQueue 0x7fb0b0516280
2015-05-29 14:38:49.323 GCDDemo[8558:405173] for end 4
2015-05-29 14:38:49.323 GCDDemo[8558:405173] for begin 5
2015-05-29 14:38:49.323 GCDDemo[8558:405215] <NSThread: 0x7fb0b0627e90>{number = 5, name = (null)} === begin
2015-05-29 14:38:49.323 GCDDemo[8558:405216] <NSThread: 0x7fb0b0506f70>{number = 6, name = (null)} === begin
2015-05-29 14:38:49.337 GCDDemo[8558:405173] mySerialQueue 0x7fb0b0610000
2015-05-29 14:38:49.337 GCDDemo[8558:405173] for end 5
2015-05-29 14:38:49.337 GCDDemo[8558:405173] for begin 6
2015-05-29 14:38:49.337 GCDDemo[8558:405173] mySerialQueue 0x7fb0b0623fa0
2015-05-29 14:38:49.337 GCDDemo[8558:405173] for end 6
2015-05-29 14:38:49.337 GCDDemo[8558:405173] for begin 7
2015-05-29 14:38:49.337 GCDDemo[8558:405173] mySerialQueue 0x7fb0b0612e10
2015-05-29 14:38:49.337 GCDDemo[8558:405173] for end 7
2015-05-29 14:38:49.337 GCDDemo[8558:405173] for begin 8
2015-05-29 14:38:49.338 GCDDemo[8558:405173] mySerialQueue 0x7fb0b062a590
2015-05-29 14:38:49.338 GCDDemo[8558:405173] for end 8
2015-05-29 14:38:49.338 GCDDemo[8558:405173] for begin 9
2015-05-29 14:38:49.338 GCDDemo[8558:405217] <NSThread: 0x7fb0b050bbd0>{number = 7, name = (null)} === begin
2015-05-29 14:38:49.338 GCDDemo[8558:405173] mySerialQueue 0x7fb0b0626860
2015-05-29 14:38:49.338 GCDDemo[8558:405173] for end 9
2015-05-29 14:38:49.338 GCDDemo[8558:405218] <NSThread: 0x7fb0b0502770>{number = 8, name = (null)} === begin
2015-05-29 14:38:49.338 GCDDemo[8558:405173] operation end
2015-05-29 14:38:49.338 GCDDemo[8558:405219] <NSThread: 0x7fb0b050e2b0>{number = 9, name = (null)} === begin
2015-05-29 14:38:49.338 GCDDemo[8558:405220] <NSThread: 0x7fb0b05132e0>{number = 10, name = (null)} === begin
2015-05-29 14:38:49.339 GCDDemo[8558:405221] <NSThread: 0x7fb0b04297c0>{number = 11, name = (null)} === begin
2015-05-29 14:38:51.326 GCDDemo[8558:405208] <NSThread: 0x7fb0b07136e0>{number = 2, name = (null)} === end
2015-05-29 14:38:51.326 GCDDemo[8558:405209] <NSThread: 0x7fb0b0711f50>{number = 3, name = (null)} === end
2015-05-29 14:38:51.326 GCDDemo[8558:405207] <NSThread: 0x7fb0b057bde0>{number = 4, name = (null)} === end
2015-05-29 14:38:51.339 GCDDemo[8558:405215] <NSThread: 0x7fb0b0627e90>{number = 5, name = (null)} === end
2015-05-29 14:38:51.339 GCDDemo[8558:405216] <NSThread: 0x7fb0b0506f70>{number = 6, name = (null)} === end
2015-05-29 14:38:51.343 GCDDemo[8558:405217] <NSThread: 0x7fb0b050bbd0>{number = 7, name = (null)} === end
2015-05-29 14:38:51.343 GCDDemo[8558:405220] <NSThread: 0x7fb0b05132e0>{number = 10, name = (null)} === end
2015-05-29 14:38:51.343 GCDDemo[8558:405218] <NSThread: 0x7fb0b0502770>{number = 8, name = (null)} === end
2015-05-29 14:38:51.343 GCDDemo[8558:405221] <NSThread: 0x7fb0b04297c0>{number = 11, name = (null)} === end
2015-05-29 14:38:51.344 GCDDemo[8558:405219] <NSThread: 0x7fb0b050e2b0>{number = 9, name = (null)} === end
解析：
dispatch_queue_create，用DISPATCH_QUEUE_SERIAL参数创建的是串行队列。那么，只要id相同，返回的都是同一个queue吗？
答案是，错误。
从日志中可以看到，系统生成的10个子线程来执行任务。所以，10个block仍然是并行执行。
label的作用，文档中指出，仅仅为了在debug，instrument，crash report中进行线程标识。
所以，label并不是线程的unique id，而是可以多个线程共用同一个id。
请不要被label所误导。