通过这篇文章，再熟悉一下多线程，这里主要是根据自己的理解，来介绍一下多线程
iOS有三种多线程编程的技术，分别是：
（一）NSThread
（二）Cocoa NSOperation
（三）GCD（全称：Grand Central Dispatch）
GCD是最简单，也是最常用的，所以重点介绍一下GCD，其他两种如果你有兴趣，可以搜索专门的文章，这里不做重点讲解，这篇文章，会重点讲解，GCD的队列加线程四种组合，1 ，串行+同步，2，串行+异步，3，并行+同步，4，并行+异步。dispatch_group管理队列。
1,串行+同步:不开新线程，在原来的线程按顺序一个一个执行
- (void)serialSyn{
//创建一个串行队列，放三个同步任务
dispatch_queue_t queue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL);
dispatch_sync(queue, ^{
NSLog(@"1--------%@",[NSThread currentThread]);
});
dispatch_sync(queue, ^{
NSLog(@"2--------%@",[NSThread currentThread]);
});
dispatch_sync(queue, ^{
NSLog(@"3--------%@",[NSThread currentThread]);
});
}
输出结果
2017-09-15 17:35:51.890 MultithreadingDemo[93486:81180033] 1--------{number = 1, name = main}
2017-09-15 17:35:51.891 MultithreadingDemo[93486:81180033] 2--------{number = 1, name = main}
2017-09-15 17:35:51.892 MultithreadingDemo[93486:81180033] 3--------{number = 1, name = main}
2,串行+异步：开辟一个新线程，在这个新开的线程，按顺序一个一个执行
- (void)serialAsynl{
dispatch_queue_t queue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
NSLog(@"1--------%@",[NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"2--------%@",[NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"3--------%@",[NSThread currentThread]);
});
}
运行结果
2017-09-15 17:39:21.668 MultithreadingDemo[93562:81188424] 1--------{number = 3, name = (null)}
2017-09-15 17:39:21.668 MultithreadingDemo[93562:81188424] 2--------{number = 3, name = (null)}
2017-09-15 17:39:21.669 MultithreadingDemo[93562:81188424] 3--------{number = 3, name = (null)}
3,并行+同步：不开新线程，在原来的线程按顺序一个一个执行
- (void)concurrenSyn{
dispatch_queue_t queue = dispatch_queue_create("concurrent", DISPATCH_QUEUE_CONCURRENT);
dispatch_sync(queue, ^{
NSLog(@"1--------%@",[NSThread currentThread]);
});
dispatch_sync(queue, ^{
NSLog(@"2--------%@",[NSThread currentThread]);
});
dispatch_sync(queue, ^{
NSLog(@"3--------%@",[NSThread currentThread]);
});
}
运行结果
2017-09-15 17:42:50.005 MultithreadingDemo[93632:81196380] 1--------{number = 1, name = main}
2017-09-15 17:42:50.005 MultithreadingDemo[93632:81196380] 2--------{number = 1, name = main}
2017-09-15 17:42:50.006 MultithreadingDemo[93632:81196380] 3--------{number = 1, name = main}
4,并行+异步：开辟多个线程，任务不是按顺序执行的
- (void)concurrentAsyn{
dispatch_queue_t queue =dispatch_queue_create("concurrent",DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"1---%@", [NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"2---%@", [NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"3---%@", [NSThread currentThread]);
});
}
运行结果
2017-09-15 17:51:42.178 MultithreadingDemo[93867:81218630] 3---{number = 5, name = (null)}
2017-09-15 17:51:42.178 MultithreadingDemo[93867:81218631] 2---{number = 4, name = (null)}
2017-09-15 17:51:42.178 MultithreadingDemo[93867:81218633] 1---{number = 3, name = (null)}
从上面运行运行结果，可以看出，串行+同步和并行+同步结果一样，它们的区别就是，
串行+同步：上一个任务执行完之后，再把下一个任务放进队列。
并行+同步：一开始就把所有的任务放入队列，然后按顺序一个一个执行
Dispatch Group
在追加到Dispatch Queue中的多个任务处理完毕之后想执行结束处理，这种需求会经常出现。如果只是使用一个Serial Dispatch Queue（串行队列）时，只要将想执行的处理全部追加到该串行队列中并在最后追加结束处理即可，但是在使用Concurrent Queue 时，可能会同时使用多个Dispatch Queue时，源代码就会变得很复杂。在这种情况下，就可以使用Dispatch Group。
- (void)groupSync{
dispatch_group_t group =dispatch_group_create();
dispatch_queue_t globalQueue=dispatch_get_global_queue(0, 0);
dispatch_queue_t queue=dispatch_get_global_queue(0, 0);
dispatch_group_async(group, globalQueue, ^{
NSLog(@"%@---1结束",[NSThread currentThread]);
});
dispatch_group_async(group, queue, ^{
NSLog(@"%@---2结束",[NSThread currentThread]);
});
dispatch_group_async(group, queue, ^{
NSLog(@"%@---3结束",[NSThread currentThread]);
});
dispatch_group_async(group, globalQueue, ^{
NSLog(@"%@---4结束",[NSThread currentThread]);
});
dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
NSLog(@"%@---全部结束",[NSThread currentThread]);
});
}
运行结果
2017-09-18 11:56:09.395 MultithreadingDemo[46231:89003127]{number = 5, name = (null)}---3结束2017-09-18 11:56:09.395 MultithreadingDemo[46231:89003125]{number = 4, name = (null)}---2结束2017-09-18 11:56:09.395 MultithreadingDemo[46231:89003146]{number = 6, name = (null)}---4结束2017-09-18 11:56:09.395 MultithreadingDemo[46231:89003124]{number = 3, name = (null)}---1结束2017-09-18 11:56:09.395 MultithreadingDemo[46231:89003124]{number = 3, name = (null)}---全部结束
我们可以看到，队列通过Dispatch Group来管理，可以很好的控制所有任务完成，再去处理一件事情，不过我们要注意，如果任务里面要放的是耗时异步任务，比如用AFNetworking请求数据，会有什么输出结果呢
- (void)groupSync{
dispatch_group_t group =dispatch_group_create();
dispatch_queue_t globalQueue=dispatch_get_global_queue(0, 0);
dispatch_queue_t queue=dispatch_get_global_queue(0, 0);
dispatch_group_async(group, globalQueue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(3);
NSLog(@"%@---1结束",[NSThread currentThread]);
});
});
dispatch_group_async(group, queue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(3);
NSLog(@"%@---2结束",[NSThread currentThread]);
});
});
dispatch_group_async(group, queue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
dispatch_after(3, dispatch_get_global_queue(0, 0), ^{
});
});
});
dispatch_group_async(group, globalQueue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(3);
NSLog(@"%@---4结束",[NSThread currentThread]);
});
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"%@---全部结束",[NSThread currentThread]);
});
}
输出结果
2017-09-18 15:56:48.554 MultithreadingDemo[57887:89409814]{number = 1, name = main}---全部结束2017-09-18 15:56:51.576 MultithreadingDemo[57887:89409942]{number = 3, name = (null)}---1结束2017-09-18 15:56:51.576 MultithreadingDemo[57887:89409958]{number = 5, name = (null)}---4结束2017-09-18 15:56:51.576 MultithreadingDemo[57887:89409941]{number = 4, name = (null)}---2结束
我们看到先执行dispatch_group_notify，而不是等上面的任务执行完之后，这时我们就需要用到dispatch_group_enter(group)、dispatch_group_leave(group)
- (void)groupSync{
dispatch_group_t group =dispatch_group_create();
dispatch_queue_t globalQueue=dispatch_get_global_queue(0, 0);
dispatch_queue_t queue=dispatch_get_global_queue(0, 0);
dispatch_group_enter(group);
dispatch_group_async(group, globalQueue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(3);
NSLog(@"%@---1结束",[NSThread currentThread]);
dispatch_group_leave(group);
});
});
dispatch_group_enter(group);
dispatch_group_async(group, queue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(3);
NSLog(@"%@---2结束",[NSThread currentThread]);
dispatch_group_leave(group);
});
});
dispatch_group_enter(group);
dispatch_group_async(group, queue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
dispatch_after(3, dispatch_get_global_queue(0, 0), ^{
});
dispatch_group_leave(group);
});
});
dispatch_group_enter(group);
dispatch_group_async(group, globalQueue, ^{
dispatch_async(dispatch_get_global_queue(0, 0), ^{
sleep(3);
NSLog(@"%@---4结束",[NSThread currentThread]);
dispatch_group_leave(group);
});
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"%@---全部结束",[NSThread currentThread]);
});
}
输出结果
2017-09-18 16:04:29.942 MultithreadingDemo[58192:89430284]{number = 4, name = (null)}---4结束2017-09-18 16:04:29.942 MultithreadingDemo[58192:89430285]{number = 3, name = (null)}---1结束2017-09-18 16:04:29.942 MultithreadingDemo[58192:89430287]{number = 5, name = (null)}---2结束2017-09-18 16:04:29.943 MultithreadingDemo[58192:89430213]{number = 1, name = main}---全部结束
这就达到了我们的目的，实现了异步线程同步的效果
dispatch_barrier_sync和dispatch_barrier_async区别
我们知道，并行+异步是不确定执行顺序的，不知道哪个任务先执行，有时候我们在操作数据库的时候，如果不用线程锁的话，规定，写入数据的时候不能读写，这时候就可以用到dispatch_barrier_sync和dispatch_barrier_async，它们都可以进行阻断操作，阻断前面的任务执行完，再执行下面的任务，从字面意思我们可以看出，一个是同步，一个是异步，他们有什么区别呢，我们可以看一下代码
dispatch_barrier_sync
- (void)barrierSync{
dispatch_queue_t queue = dispatch_queue_create("thread", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"写入数据1----%@",[NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"写入数据2----%@",[NSThread currentThread]);
});
dispatch_barrier_sync(queue, ^{
for (int i = 0; i < 600000000; i++) {
if (i == 200000) {
NSLog(@"0-------------%@",[NSThread currentThread]);
}
}
});
NSLog(@"主线程11111--%@",[NSThread currentThread]);
NSLog(@"主线程22222---%@",[NSThread currentThread]);
dispatch_async(queue, ^{
NSLog(@"读取数据3----%@",[NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"读取数据4----%@",[NSThread currentThread]);
});
}
输出结果
2017-09-18 16:21:56.203 MultithreadingDemo[58621:89473189] 写入数据2----{number = 4, name = (null)}
2017-09-18 16:21:56.203 MultithreadingDemo[58621:89473188] 写入数据1----{number = 3, name = (null)}
2017-09-18 16:21:56.204 MultithreadingDemo[58621:89473087] 0-------------{number = 1, name = main}
2017-09-18 16:21:57.806 MultithreadingDemo[58621:89473087] 主线程11111--{number = 1, name = main}
2017-09-18 16:21:57.807 MultithreadingDemo[58621:89473087] 主线程22222---{number = 1, name = main}
2017-09-18 16:21:57.807 MultithreadingDemo[58621:89473188] 读取数据3----{number = 3, name = (null)}
2017-09-18 16:21:57.807 MultithreadingDemo[58621:89473189] 读取数据4----{number = 4, name = (null)}
dispatch_barrier_async
- (void)barrierAsync{
dispatch_queue_t queue = dispatch_queue_create("thread", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"写入数据1----%@",[NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"写入数据2----%@",[NSThread currentThread]);
});
dispatch_barrier_async(queue, ^{
for (int i = 0; i < 600000000; i++) {
if (i == 200000) {
NSLog(@"0-------------%@",[NSThread currentThread]);
}
}
});
NSLog(@"主线程11111--%@",[NSThread currentThread]);
NSLog(@"主线程22222---%@",[NSThread currentThread]);
dispatch_async(queue, ^{
NSLog(@"读取数据3----%@",[NSThread currentThread]);
});
dispatch_async(queue, ^{
NSLog(@"读取数据4----%@",[NSThread currentThread]);
});
}
输出结果
2017-09-18 16:32:36.304 MultithreadingDemo[58817:89500553] 主线程11111--{number = 1, name = main}
2017-09-18 16:32:36.304 MultithreadingDemo[58817:89500873] 写入数据2----{number = 4, name = (null)}
2017-09-18 16:32:36.304 MultithreadingDemo[58817:89500871] 写入数据1----{number = 3, name = (null)}
2017-09-18 16:32:36.304 MultithreadingDemo[58817:89500553] 主线程22222---{number = 1, name = main}
2017-09-18 16:32:36.305 MultithreadingDemo[58817:89500871] 0-------------{number = 3, name = (null)}
2017-09-18 16:32:37.997 MultithreadingDemo[58817:89500871] 读取数据3----{number = 3, name = (null)}
2017-09-18 16:32:37.997 MultithreadingDemo[58817:89500873] 读取数据4----{number = 4, name = (null)}
从上面（主线程11111--）和（主线程22222---）输出结果我们可以看出dispatch_barrier_sync会阻塞主线程，因为它的任务是在主线程执行，dispatch_barrier_async不会阻塞主线程，因为它是在子线程执行
线程安全
我们知道开辟多个线程是不安全的，因为当多个线程同时执行任务，访问同一个资源，就会出现问题，这时候我们就需要用到线程锁，保证它的安全，有篇文章写写的比较全，大家可以看一下这是文章。链接//www.greatytc.com/p/35dd92bcfe8c
