如何充分的利用计算机的有效资源，在并发之前使用多线程解决，然而很多东西需要程序员来考虑，如何创建合理数目的线程，如何控制线程之间有效运行互不影响。并发很好的解决了这样的问题。在学习的过程中，我们经常看到并发，串行，异步，同步这样的自研。它们之间有如下4种组合：
1.串行+同步
- (void)serialSync {
dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_SERIAL);
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_sync(serialQueue, ^{
NSLog(@"2 - %@", [NSThread currentThread]);
});
dispatch_sync(serialQueue, ^{
NSLog(@"3 - %@", [NSThread currentThread]);
});
NSLog(@"4 - %@", [NSThread currentThread]);
}
结果：
2017-08-21 08:37:44.619 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.619 Chapter2[1772:68997] 2 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.620 Chapter2[1772:68997] 3 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.620 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}
可以看到该组合，没有创建新的线程，在当前线程执行任务。首先队列是先进先出的原则，当第一个同步任务插入队列时，阻塞线程，需要执行队列中的任务。因为队列中只有它自己，所以执行。同理，第二个第三个...也是这样的情况。
2.串行+异步
- (void)serialAsync {
dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_SERIAL);
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(serialQueue, ^{
NSLog(@"2 - %@", [NSThread currentThread]);
});
dispatch_async(serialQueue, ^{
NSLog(@"3 - %@", [NSThread currentThread]);
});
NSLog(@"4 - %@", [NSThread currentThread]);
}
结果：
2017-08-21 08:37:44.620 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.620 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.621 Chapter2[1772:69315] 2 - <NSThread: 0x608000262480>{number = 3, name = (null)}
2017-08-21 08:37:44.621 Chapter2[1772:69315] 3 - <NSThread: 0x608000262480>{number = 3, name = (null)}
当前组合，只创建了一条新的线程。因为是异步任务，所以在插入队列时，不会阻塞，待插入结束后，然后依次执行队列中的任务。
3.并行+同步
- (void)concurrentSync {
dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_sync(serialQueue, ^{
NSLog(@"2 - %@", [NSThread currentThread]);
});
dispatch_sync(serialQueue, ^{
NSLog(@"3 - %@", [NSThread currentThread]);
});
NSLog(@"4 - %@", [NSThread currentThread]);
}
结果
2017-08-21 08:37:44.621 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.621 Chapter2[1772:68997] 2 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.622 Chapter2[1772:68997] 3 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.622 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}
在当前队列执行任务，没有创建新的线程。
4.并行+异步
- (void)concurrentAsync {
dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"1 - %@", [NSThread currentThread]);
dispatch_async(serialQueue, ^{
NSLog(@"2 - %@", [NSThread currentThread]);
});
dispatch_async(serialQueue, ^{
NSLog(@"3 - %@", [NSThread currentThread]);
});
NSLog(@"4 - %@", [NSThread currentThread]);
}
结果
2017-08-21 08:37:44.624 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.624 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.624 Chapter2[1772:69315] 2 - <NSThread: 0x608000262480>{number = 3, name = (null)}
2017-08-21 08:37:44.624 Chapter2[1772:69312] 3 - <NSThread: 0x600000260000>{number = 4, name = (null)}
当前组合，按照子任务的数目创建相应数目的线程，达到了并发的目的。因为异步，在插入到队列的时候，不会阻塞。弹出队列的执行时，会创建不同的线程去执行子任务，子任务执行的结果的输出是无序的，子任务之间相互不影响。
附图一张，便于理解：
