先貼上打印函數(shù)：
- (void)p_printThread:(int)index {
NSLog(@"%@-------%d", [NSThread currentThread], index);
}
1. Group
Group作用：在并發(fā)異步隊列中，等待執(zhí)行一系列任務后轩猩，再執(zhí)行一個任務卷扮。實現(xiàn)方式有兩種：notify和wait。
notify示例代碼：
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, concurrentQueue, ^{
[self p_printThread:1];
});
dispatch_group_async(group, concurrentQueue, ^{
[self p_printThread:2];
});
dispatch_group_async(group, concurrentQueue, ^{
[self p_printThread:3];
});
dispatch_group_async(group, concurrentQueue, ^{
[self p_printThread:4];
});
// 使用dispatch_group_wait均践，可以阻塞線程晤锹，等待group的任務執(zhí)行完畢，才能繼續(xù)執(zhí)行后續(xù)任務
// 使用dispatch_group_notify彤委，不會阻塞線程（group外的線程執(zhí)行順序不受影響）鞭铆，而且可以在執(zhí)行完成group的任務后進行操作
dispatch_group_notify(group, concurrentQueue, ^{
[self p_printThread:0];
});
打印結(jié)果：
<NSThread: 0x610000260d80>{number = 4, name = (null)}-------2
<NSThread: 0x608000261400>{number = 5, name = (null)}-------3
<NSThread: 0x618000264f40>{number = 3, name = (null)}-------1
<NSThread: 0x610000260e40>{number = 6, name = (null)}-------4
<NSThread: 0x610000260e40>{number = 6, name = (null)}-------0
wait示例代碼：
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
for (int i=0; i<4; i++) {
dispatch_group_enter(group);
dispatch_async(concurrentQueue, ^{
[self p_printThread:i+1];
});
dispatch_group_leave(group);
}
// 使用dispatch_group_wait，可以阻塞線程焦影，等待group的任務執(zhí)行完畢
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
dispatch_async(dispatch_get_main_queue(), ^{
// 主線程處理
[self p_printThread:0];
});
打印結(jié)果：
<NSThread: 0x600000076600>{number = 4, name = (null)}-------2
<NSThread: 0x600000076740>{number = 6, name = (null)}-------4
<NSThread: 0x610000073280>{number = 3, name = (null)}-------1
<NSThread: 0x608000076380>{number = 5, name = (null)}-------3
<NSThread: 0x60000007b240>{number = 1, name = main}-------0
結(jié)論：運行多次车遂，任務1封断、2、3舶担、4的順序不定澄港，但任務0總是最后一個。應用場景：單界面多請求完成后柄沮，主線程刷新界面;notify和wait的區(qū)別：wait會阻塞線程回梧，notify不會阻塞線程，較好一點
2. Barrier
Barrier作用：Barrier在并發(fā)異步隊列中起到承上啟下的作用（Barrier就像名字一樣祖搓，類似一個柵欄把前后的任務隔開了）狱意。Barrier前后任務執(zhí)行順序為：Barrier前面的線程（多個的話，仍是并行）--->barrier線程--->barrier后面的線程（多個的話拯欧，仍是并行）详囤。需要注意的是：Barrier在全局并發(fā)隊列不起作用，只有在自己創(chuàng)建的并發(fā)隊列才起作用镐作。
示例代碼：
// barrier在dispatch_get_global_queue創(chuàng)建的并行隊列中不起作用藏姐，需要使用dispatch_queue_create來創(chuàng)建的并行隊列才可以
//dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t concurrentQueue = dispatch_queue_create("com.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(concurrentQueue, ^(){
[self p_printThread:1];
});
dispatch_async(concurrentQueue, ^(){
[self p_printThread:2];
});
dispatch_async(concurrentQueue, ^(){
[self p_printThread:3];
});
dispatch_barrier_async(concurrentQueue, ^(){
[self p_printThread:0];
});
dispatch_async(concurrentQueue, ^(){
[self p_printThread:4];
});
dispatch_async(concurrentQueue, ^(){
[self p_printThread:5];
});
dispatch_async(concurrentQueue, ^(){
[self p_printThread:6];
});
打印結(jié)果：
<NSThread: 0x600000079300>{number = 4, name = (null)}-------2
<NSThread: 0x6000000790c0>{number = 5, name = (null)}-------3
<NSThread: 0x610000076100>{number = 3, name = (null)}-------1
<NSThread: 0x610000076100>{number = 3, name = (null)}-------0
<NSThread: 0x610000076100>{number = 3, name = (null)}-------4
<NSThread: 0x600000079300>{number = 4, name = (null)}-------6
<NSThread: 0x600000078d40>{number = 6, name = (null)}-------5
結(jié)論：運行多次后可以看出，Barrier的任務0總是在任務1该贾、2羔杨、3之后，而且總是在任務4杨蛋、5兜材、6之前，但是任務1逞力、2曙寡、3的執(zhí)行順序不定，任務4寇荧、5举庶、6的執(zhí)行順序也不定。Barrier的任務就像“柵欄”一樣隔開了前后的任務揩抡。
3. Semaphore
Semaphore作用：信號量可以用來控制同時訪問資源的線程數(shù)量：比如系統(tǒng)只有兩個資源可以用户侥，有三個線程要訪問，那么只能允許兩個線程同時訪問捅膘，第三個線程應當?shù)却Y源被釋放后再訪問添祸。使用信號量可以實現(xiàn)類似于NSOperationQueue里的并發(fā)控制：信號數(shù)>0，執(zhí)行任務寻仗；信號數(shù)<=0刃泌，阻塞線程
示例代碼：
// 下面的例子只允許3個線程同時執(zhí)行
dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);// 創(chuàng)建一個信號量，初始值為3
dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i=0; i<10; i++) {
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);// 計數(shù)－1
dispatch_async(concurrentQueue, ^{
[self p_printThread:i];
sleep(5);
dispatch_semaphore_signal(semaphore);// 計數(shù)＋1
});
}
打印結(jié)果：
<NSThread: 0x608000077b80>{number = 5, name = (null)}-------2
<NSThread: 0x610000074fc0>{number = 3, name = (null)}-------0
<NSThread: 0x6000000787c0>{number = 4, name = (null)}-------1
<NSThread: 0x6000000787c0>{number = 4, name = (null)}-------3
<NSThread: 0x610000074fc0>{number = 3, name = (null)}-------4
<NSThread: 0x608000077b80>{number = 5, name = (null)}-------5
<NSThread: 0x6000000787c0>{number = 4, name = (null)}-------7
<NSThread: 0x608000077b80>{number = 5, name = (null)}-------6
<NSThread: 0x610000074fc0>{number = 3, name = (null)}-------8
<NSThread: 0x610000074fc0>{number = 3, name = (null)}-------9
結(jié)論：由于信號量的值為3，每次只能同時執(zhí)行3個線程里的任務
4. NSOperationQueue
NSOperationQueue作用：
1.將NSOperation添加到NSOperationQueue耙替，使其異步執(zhí)行 === GCD并行異步隊列亚侠。2.NSOperationQueue可以設置依賴關系 ~= GCD Group notify / GCD Group Wait / GCD Barrier。
3.可以設置最大并發(fā)數(shù)量（同時執(zhí)行的線程數(shù)） === GCD 信號量
示例代碼：
NSOperationQueue *operationQueue = [NSOperationQueue new];
NSBlockOperation *blockOperation1 = [NSBlockOperation blockOperationWithBlock:^{
[self p_printThread:1];
sleep(5);
}];
NSBlockOperation *blockOperation2 = [NSBlockOperation blockOperationWithBlock:^{
[self p_printThread:2];
sleep(5);
}];
NSBlockOperation *blockOperation3 = [NSBlockOperation blockOperationWithBlock:^{
[self p_printThread:3];
sleep(5);
}];
NSBlockOperation *blockOperation4 = [NSBlockOperation blockOperationWithBlock:^{
[self p_printThread:4];
sleep(5);
}];
// 2.NSOperationQueue可以設置依賴關系 ~= GCD Group notify / GCD Group Wait / GCD Barrier
[blockOperation1 addDependency:blockOperation2];
// 3.可以設置最大并發(fā)數(shù)量（同時執(zhí)行的線程數(shù)） === GCD 信號量
operationQueue.maxConcurrentOperationCount = 1;
[operationQueue addOperations:@[blockOperation1, blockOperation2, blockOperation3, blockOperation4] waitUntilFinished:YES];
打印結(jié)果：
<NSThread: 0x600000078300>{number = 3, name = (null)}-------2
<NSThread: 0x600000078300>{number = 3, name = (null)}-------1
<NSThread: 0x60000007aa80>{number = 4, name = (null)}-------3
<NSThread: 0x60000007aa80>{number = 4, name = (null)}-------4
結(jié)論：運行多次可以看出俗扇，每次只能執(zhí)行一個線程里的任務（這個類似信號量的控制）硝烂；雖然任務1、2铜幽、3滞谢、4的順序不定，但是任務1總是在任務2的后面（類似group的notify和wait）除抛。對比可以看出NSOperationQueue使用更方便一些狮杨，語法也更接近Objective-C。
