并行、串行、異步、同步的加深理解
單個理解：
并行：就是隊列里面的任務（代碼塊，block）不是一個個執行，而是同時執行，不存在先后順序
串行：隊列里面的任務一個接著一個執行，第一個執行完了才能輪到第二個
異步：具有新開線程的能力
同步：不具備有新開線程的能力，只能在當前線程執行任務（所以任務只能一個挨著一個執行，而不能同時進行）
串起來理解：
并行+異步：就是真正的并發，新開有有多個線程處理任務，任務并發執行（不按順序執行）
串行+異步：新開一個線程，任務一個接一個執行，上一個任務處理完畢，下一個任務才可以被執行
并行+同步：不新開線程，任務一個接一個執行
串行+同步：不新開線程，任務一個接一個執行
? 這里不得不說一下，之前看了這篇博客
http://www.cnblogs.com/ziyi--caolu/p/4900650.html 該作者成功的將我繞進了里面，其實歸根究底，只要是在同步環境中，并行和串行確實就可以這么理解，
即并行+同步 和 串行+同步 都是不新開線程，并且任務一個接一個執行
下面是具體代碼示例：
一：異步并行隊列內回調同步串行隊列
- (void)exampleFirst {
dispatch_queue_t queue = dispatch_queue_create("com.hao123.www", DISPATCH_QUEUE_SERIAL);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---11 %@",[NSThread currentThread]);
dispatch_sync(queue, ^{
//打印結果與主隊列bingxingyibuqiantaozhu函數保持一致
NSLog(@"11 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---22 %@",[NSThread currentThread]);
dispatch_sync(queue, ^{
NSLog(@"22 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---33 %@",[NSThread currentThread]);
dispatch_sync(queue, ^{
NSLog(@"33 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---44 %@",[NSThread currentThread]);
dispatch_sync(queue, ^{
NSLog(@"44 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---55 %@",[NSThread currentThread]);
dispatch_sync(queue, ^{
NSLog(@"55 %@",[NSThread currentThread]);
});
});
}
打印結果
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978456] 外部---44 <NSThread: 0x608000072780>{number = 6, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978364] 外部---11 <NSThread: 0x608000070f40>{number = 3, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978366] 外部---33 <NSThread: 0x600000076380>{number = 5, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978363] 外部---22 <NSThread: 0x600000072f00>{number = 4, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978457] 外部---55 <NSThread: 0x608000073340>{number = 7, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978456] 44 <NSThread: 0x608000072780>{number = 6, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978364] 11 <NSThread: 0x608000070f40>{number = 3, name = (null)}
2017-09-08 14:29:24.408 GcdInsightTest[63582:2978366] 33 <NSThread: 0x600000076380>{number = 5, name = (null)}
2017-09-08 14:29:24.408 GcdInsightTest[63582:2978363] 22 <NSThread: 0x600000072f00>{number = 4, name = (null)}
2017-09-08 14:29:24.408 GcdInsightTest[63582:2978457] 55 <NSThread: 0x608000073340>{number = 7, name = (null)}
二：異步并行內回調同步并行隊列
- (void)exampleSecond {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---11 %@",[NSThread currentThread]);
//對比異步并行回調同步串行，可以看出當外部異步并行隊列未執行完畢時，有可能內部同步并行隊列已經開始執行了
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"11 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---22 %@",[NSThread currentThread]);
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"22 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---33 %@",[NSThread currentThread]);
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"33 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---44 %@",[NSThread currentThread]);
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"44 %@",[NSThread currentThread]);
});
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"外部---55 %@",[NSThread currentThread]);
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(@"55 %@",[NSThread currentThread]);
});
});
}
打印結果
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984344] 外部---11 <NSThread: 0x608000263040>{number = 13, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984353] 外部---22 <NSThread: 0x608000262480>{number = 17, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984111] 外部---33 <NSThread: 0x608000073980>{number = 12, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984345] 外部---55 <NSThread: 0x60000007f1c0>{number = 16, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984346] 外部---44 <NSThread: 0x608000074d40>{number = 14, name = (null)}
2017-09-08 14:34:59.193 GcdInsightTest[64242:2984344] 11 <NSThread: 0x608000263040>{number = 13, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984353] 22 <NSThread: 0x608000262480>{number = 17, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984111] 33 <NSThread: 0x608000073980>{number = 12, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984345] 55 <NSThread: 0x60000007f1c0>{number = 16, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984346] 44 <NSThread: 0x608000074d40>{number = 14, name = (null)}
根據兩段例子的打印結果，可以顯而易見的得出結論：
一：由于外部是異步并行隊列，所以外部打印結果不按順序執行。
二：當外部所有隊列執行完畢，內部隊列才開始執行。
三：其內部的隊列將與外部隊列順序保持一致，無論異步并行隊列內回調的是同步串行隊列，還是同步并行隊列，只有當外部隊列執行完畢，接下來內部隊列才會在同一個線程繼續執行直到任務結束。
根據第三條可以得出結論：
并行+同步：不新開線程，任務一個接一個執行
串行+同步：不新開線程，任務一個接一個執行
這兩句話沒毛病~
