最近在使用GCD的时候，发现自己对dispatch_semaphore理解不是那么深刻，所以自己在网上找资料学习dispatch_semaphore，并对dispatch_semaphore有了一些自己的看法和理解。
1.异步任务使用semaphore
NSLog(@"----------------------------开始---------------------------- 线程：%@",[NSThread currentThread]);
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"---------------------------- 任务一开始 ---------------------------- 线程：%@",[NSThread currentThread]);
[NSThread sleepForTimeInterval:2.0];
NSLog(@"---------------------------- 任务一结束 ---------------------------- 线程：%@",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"---------------------------- 任务二开始 ---------------------------- 线程：%@",[NSThread currentThread]);
[NSThread sleepForTimeInterval:2.0];
NSLog(@"---------------------------- 任务二结束 ---------------------------- 线程：%@",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"---------------------------- 任务三开始 ---------------------------- 线程：%@",[NSThread currentThread]);
[NSThread sleepForTimeInterval:2.0];
NSLog(@"---------------------------- 任务三结束 ---------------------------- 线程：%@",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
NSLog(@"----------------------------结束---------------------------- 线程：%@",[NSThread currentThread]);
打印结果如下：
2018-11-20 10:05:48.165624+0800 Demo[2920:724653] ----------------------------开始---------------------------- 线程：<NSThread: 0x282c6b180>{number = 1, name = main}
2018-11-20 10:05:48.166227+0800 Demo[2920:724705] ---------------------------- 任务一开始 ---------------------------- 线程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:50.171470+0800 Demo[2920:724705] ---------------------------- 任务一结束 ---------------------------- 线程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:50.171953+0800 Demo[2920:724705] ---------------------------- 任务二开始 ---------------------------- 线程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:52.177195+0800 Demo[2920:724705] ---------------------------- 任务二结束 ---------------------------- 线程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:52.177657+0800 Demo[2920:724710] ---------------------------- 任务三开始 ---------------------------- 线程：<NSThread: 0x282ce9380>{number = 4, name = (null)}
2018-11-20 10:05:54.182973+0800 Demo[2920:724710] ---------------------------- 任务三结束 ---------------------------- 线程：<NSThread: 0x282ce9380>{number = 4, name = (null)}
2018-11-20 10:05:54.183224+0800 Demo[2920:724653] ----------------------------结束---------------------------- 线程：<NSThread: 0x282c6b180>{number = 1, name = main}
这里我们可以分析到，semaphore确实起到了阻塞等待的作用。dispatch_semaphore_t sema = dispatch_semaphore_create(0)首先创建了一个信号量为0的semaphore，接着开启了一个异步任务，在任务中dispatch_semaphore_signal(sema)会使信号量+1，然后在主线程中dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)使信号量-1，如果此时信号量为0，那么会一直等待。所以主线程会一直等待，直到异步任务执行完毕dispatch_semaphore_signal(sema)这里，对信号量+1,然后才会接着往下走。后面的任务以此类推。
可是在我们平常的使用中，更多是配合网络请求。例如请求B的数据依赖于请求A，只有先拿到了请求A的数据，才能请求B，当然这个需求解决办法有很多，但这里我们就只谈谈怎么用semaphore来解决这个需求。接着我们利用相同思路，结合AFN使用semaphore：
NSLog(@"----------------- 测试开始 -----------------");
NSDictionary *dict = @{ @"page": @1,
@"pageSize": @20};
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
//对AFN的简单封装 请求一个post任务
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypePOST
withURLString:@"https://api.it120.cc/tz/shop/goods/list" withParameters:dict
withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务一完成 -----------------%@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(semaphore);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
//请求一个get任务
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务二完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(semaphore);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"----------------- 测试结束 -----------------");
打印结果如下：
2018-11-20 10:05:01.794640+0800 Demo[2918:724320] ----------------- 测试开始 -----------------
这里我们发现，仅仅只打印了测试开始，后面的任务都没有执行。那么究竟是什么原因，导致了主线程的阻塞呢？原来在AFN中，AFN已经已经将回调回到了主线程中:
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 当前线程：%@ -----------------",[NSThread currentThread]);
} withFailBlock:^(NSError *error) {
}];
打印结果如下：
2018-11-20 10:11:32.721403+0800 Demo[2924:725812] ----------------- 当前线程：<NSThread: 0x283e4fa00>{number = 1, name = main} -----------------
而在执行到第一个dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)时候，信号量就为0了，将主线程阻塞了，就没有办法继续往下执行dispatch_semaphore_signal(semaphore)。那为什么上面第一种方法可以执行呢？是因为上面的任务都是异步的，所以就算主线程阻塞了，在异步任务中还是可以执行dispatch_semaphore_signal(semaphore)，也就不存在阻塞问题了。
那么我们怎么去解决这样的问题呢？这里我们就需要用到GCD的另外一种使用方式了:dispatch_group。对dispatch_group不是很了解的小伙伴可以参考这里，里面对GCD的各种使用方法都有很详细的讲解。
好了，废话不多说，接下来我们看看怎么用dispatch_group完成dispatch_semaphore和AFN的结合使用：
2.结合AFN使用dispatch_semaphore
NSLog(@"----------------- 测试开始 -----------------");
dispatch_group_t gruop = dispatch_group_create();
dispatch_group_async(gruop, dispatch_get_global_queue(0, 0), ^{
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
NSLog(@"----------------- gruop:%@ -----------------",[NSThread currentThread]);
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务一完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务二完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务三完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
});
dispatch_group_notify(gruop, dispatch_get_main_queue(), ^{
NSLog(@"----------------- 测试结束 -----------------");
});
打印结果如下：
2018-11-20 10:31:07.634295+0800 Demo[2942:730510] ----------------- 测试开始 -----------------
2018-11-20 10:31:07.636867+0800 Demo[2942:730582] ----------------- gruop:<NSThread: 0x283c8ebc0>{number = 3, name = (null)} -----------------
2018-11-20 10:31:08.057232+0800 Demo[2942:730510] ----------------- 任务一完成 ----------------- <NSThread: 0x283c01b40>{number = 1, name = main} -----------------
2018-11-20 10:31:08.130937+0800 Demo[2942:730510] ----------------- 任务二完成 ----------------- <NSThread: 0x283c01b40>{number = 1, name = main} -----------------
2018-11-20 10:31:08.229891+0800 Demo[2942:730510] ----------------- 任务三完成 ----------------- <NSThread: 0x283c01b40>{number = 1, name = main} -----------------
2018-11-20 10:31:08.230509+0800 Demo[2942:730510] ----------------- 测试结束 -----------------
根据打印的结果，这里完美解决了A任务、B任务之间的依赖关系的需求。也再次证明AFN的回调确实在主线程中(AFN的源码中，也可以找到，有兴趣的小伙伴可以研究下源码)。
那么，为什么用到dispatch_group可以解决问题呢？这里dispatch_group_async(gruop, dispatch_get_global_queue(0, 0)开辟一个新的子线程。而在子线程中，dispatch_semaphore_t sema = dispatch_semaphore_create(0)创建了一个信号量为0的semaphore，紧接着在当前子线程中执行dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)，那么会阻塞当前线程，一直等待，直到等待AFN的回调中dispatch_semaphore_signal(sema)将信号量+1。
如果对以上理解了的话，同样开启一个异步任务，也可以将任务一、二、三按顺序完成：
NSLog(@"----------------- 测试开始 -----------------");
dispatch_async(dispatch_get_global_queue(0, 0), ^{
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务一完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务二完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任务三完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
});
NSLog(@"----------------- 测试结束 -----------------");
打印结果如下：
2018-11-20 10:38:48.473069+0800 Demo[2950:732087] ----------------- 测试开始 -----------------
2018-11-20 10:38:48.473251+0800 Demo[2950:732087] ----------------- 测试结束 -----------------
2018-11-20 10:38:48.917159+0800 Demo[2950:732087] ----------------- 任务一完成 ----------------- <NSThread: 0x282351b40>{number = 1, name = main} -----------------
2018-11-20 10:38:49.005818+0800 Demo[2950:732087] ----------------- 任务二完成 ----------------- <NSThread: 0x282351b40>{number = 1, name = main} -----------------
2018-11-20 10:38:49.092094+0800 Demo[2950:732087] ----------------- 任务三完成 ----------------- <NSThread: 0x282351b40>{number = 1, name = main} -----------------
这里可以看到，虽然并不像使用dispatch_gruop那样，将调度组中的任务，执行完毕后再去调用
dispatch_group_notify里面的任务。但任务一、二、三却也是按顺序执行的。
以上是我个人对dispatch_semaphore的一点点理解，有错误的地方希望大家能指出来。
最后，个人还有点小疑问。既然AFN的回调已经在主线程中了，那么为什么我们还要AFN的回调中用dispatch_get_main_queue()回到主线程更新UI呢？
