最近在使用GCD的時候，發(fā)現(xiàn)自己對dispatch_semaphore理解不是那么深刻，所以自己在網(wǎng)上找資料學習dispatch_semaphore，并對dispatch_semaphore有了一些自己的看法和理解。
1.異步任務使用semaphore
NSLog(@"----------------------------開始---------------------------- 線程：%@",[NSThread currentThread]);
dispatch_semaphore_t sema = dispatch_semaphore_create(0);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"---------------------------- 任務一開始 ---------------------------- 線程：%@",[NSThread currentThread]);
[NSThread sleepForTimeInterval:2.0];
NSLog(@"---------------------------- 任務一結(jié)束 ---------------------------- 線程：%@",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"---------------------------- 任務二開始 ---------------------------- 線程：%@",[NSThread currentThread]);
[NSThread sleepForTimeInterval:2.0];
NSLog(@"---------------------------- 任務二結(jié)束 ---------------------------- 線程：%@",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
dispatch_async(dispatch_get_global_queue(0, 0), ^{
NSLog(@"---------------------------- 任務三開始 ---------------------------- 線程：%@",[NSThread currentThread]);
[NSThread sleepForTimeInterval:2.0];
NSLog(@"---------------------------- 任務三結(jié)束 ---------------------------- 線程：%@",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
});
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
NSLog(@"----------------------------結(jié)束---------------------------- 線程：%@",[NSThread currentThread]);
打印結(jié)果如下：
2018-11-20 10:05:48.165624+0800 Demo[2920:724653] ----------------------------開始---------------------------- 線程：<NSThread: 0x282c6b180>{number = 1, name = main}
2018-11-20 10:05:48.166227+0800 Demo[2920:724705] ---------------------------- 任務一開始 ---------------------------- 線程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:50.171470+0800 Demo[2920:724705] ---------------------------- 任務一結(jié)束 ---------------------------- 線程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:50.171953+0800 Demo[2920:724705] ---------------------------- 任務二開始 ---------------------------- 線程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:52.177195+0800 Demo[2920:724705] ---------------------------- 任務二結(jié)束 ---------------------------- 線程：<NSThread: 0x282cebc40>{number = 3, name = (null)}
2018-11-20 10:05:52.177657+0800 Demo[2920:724710] ---------------------------- 任務三開始 ---------------------------- 線程：<NSThread: 0x282ce9380>{number = 4, name = (null)}
2018-11-20 10:05:54.182973+0800 Demo[2920:724710] ---------------------------- 任務三結(jié)束 ---------------------------- 線程：<NSThread: 0x282ce9380>{number = 4, name = (null)}
2018-11-20 10:05:54.183224+0800 Demo[2920:724653] ----------------------------結(jié)束---------------------------- 線程：<NSThread: 0x282c6b180>{number = 1, name = main}
這里我們可以分析到，semaphore確實起到了阻塞等待的作用。dispatch_semaphore_t sema = dispatch_semaphore_create(0)首先創(chuàng)建了一個信號量為0的semaphore，接著開啟了一個異步任務，在任務中dispatch_semaphore_signal(sema)會使信號量+1，然后在主線程中dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)使信號量-1，此時信號量為-1，那么會一直等待。所以主線程會一直等待，直到異步任務執(zhí)行完畢dispatch_semaphore_signal(sema)這里，對信號量+1,然后才會接著往下走。后面的任務以此類推。
可是在我們平常的使用中，更多是配合網(wǎng)絡請求。例如請求B的數(shù)據(jù)依賴于請求A，只有先拿到了請求A的數(shù)據(jù)，才能請求B，當然這個需求解決辦法有很多，但這里我們就只談談怎么用semaphore來解決這個需求。接著我們利用相同思路，結(jié)合AFN使用semaphore：
NSLog(@"----------------- 測試開始 -----------------");
NSDictionary *dict = @{ @"page": @1,
@"pageSize": @20};
dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
//對AFN的簡單封裝 請求一個post任務
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypePOST
withURLString:@"https://api.it120.cc/tz/shop/goods/list" withParameters:dict
withSuccessBlock:^(id result) {
NSLog(@"----------------- 任務一完成 -----------------%@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(semaphore);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
//請求一個get任務
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 任務二完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(semaphore);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(semaphore);
}];
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"----------------- 測試結(jié)束 -----------------");
打印結(jié)果如下：
2018-11-20 10:05:01.794640+0800 Demo[2918:724320] ----------------- 測試開始 -----------------
這里我們發(fā)現(xiàn)，僅僅只打印了測試開始，后面的任務都沒有執(zhí)行。那么究竟是什么原因，導致了主線程的阻塞呢？原來在AFN中，AFN已經(jīng)已經(jīng)將回調(diào)回到了主線程中:
[[CYW_NetworkingManager shareManager] cyw_networkType:NetWorkTypeGET withURLString:@"https://api.it120.cc/tz/shop/goods/category/all" withParameters:nil withSuccessBlock:^(id result) {
NSLog(@"----------------- 當前線程：%@ -----------------",[NSThread currentThread]);
} withFailBlock:^(NSError *error) {
}];
打印結(jié)果如下：
2018-11-20 10:11:32.721403+0800 Demo[2924:725812] ----------------- 當前線程：<NSThread: 0x283e4fa00>{number = 1, name = main} -----------------
而在執(zhí)行到第一個dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)時候，信號量就為0了，將主線程阻塞了，就沒有辦法繼續(xù)往下執(zhí)行dispatch_semaphore_signal(semaphore)。那為什么上面第一種方法可以執(zhí)行呢？是因為上面的任務都是異步的，所以就算主線程阻塞了，在異步任務中還是可以執(zhí)行dispatch_semaphore_signal(semaphore)，也就不存在阻塞問題了。
那么我們怎么去解決這樣的問題呢？這里我們就需要用到GCD的另外一種使用方式了:dispatch_group。對dispatch_group不是很了解的小伙伴可以參考這里，里面對GCD的各種使用方法都有很詳細的講解。
好了，廢話不多說，接下來我們看看怎么用dispatch_group完成dispatch_semaphore和AFN的結(jié)合使用：
2.結(jié)合AFN使用dispatch_semaphore
NSLog(@"----------------- 測試開始 -----------------");
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(@"----------------- 任務一完成 ----------------- %@ -----------------",[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(@"----------------- 任務二完成 ----------------- %@ -----------------",[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(@"----------------- 任務三完成 ----------------- %@ -----------------",[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(@"----------------- 測試結(jié)束 -----------------");
});
打印結(jié)果如下：
2018-11-20 10:31:07.634295+0800 Demo[2942:730510] ----------------- 測試開始 -----------------
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] ----------------- 任務一完成 ----------------- <NSThread: 0x283c01b40>{number = 1, name = main} -----------------
2018-11-20 10:31:08.130937+0800 Demo[2942:730510] ----------------- 任務二完成 ----------------- <NSThread: 0x283c01b40>{number = 1, name = main} -----------------
2018-11-20 10:31:08.229891+0800 Demo[2942:730510] ----------------- 任務三完成 ----------------- <NSThread: 0x283c01b40>{number = 1, name = main} -----------------
2018-11-20 10:31:08.230509+0800 Demo[2942:730510] ----------------- 測試結(jié)束 -----------------
根據(jù)打印的結(jié)果，這里完美解決了A任務、B任務之間的依賴關系的需求。也再次證明AFN的回調(diào)確實在主線程中(AFN的源碼中，也可以找到，有興趣的小伙伴可以研究下源碼)。
那么，為什么用到dispatch_group可以解決問題呢？這里dispatch_group_async(gruop, dispatch_get_global_queue(0, 0)開辟一個新的子線程。而在子線程中，dispatch_semaphore_t sema = dispatch_semaphore_create(0)創(chuàng)建了一個信號量為0的semaphore，緊接著在當前子線程中執(zhí)行dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)，那么會阻塞當前線程，一直等待，直到等待AFN的回調(diào)中dispatch_semaphore_signal(sema)將信號量+1。
如果對以上理解了的話，同樣開啟一個異步任務，也可以將任務一、二、三按順序完成：
NSLog(@"----------------- 測試開始 -----------------");
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(@"----------------- 任務一完成 ----------------- %@ -----------------",[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(@"----------------- 任務二完成 ----------------- %@ -----------------",[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(@"----------------- 任務三完成 ----------------- %@ -----------------",[NSThread currentThread]);
dispatch_semaphore_signal(sema);
} withFailBlock:^(NSError *error) {
dispatch_semaphore_signal(sema);
}];
dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
});
NSLog(@"----------------- 測試結(jié)束 -----------------");
打印結(jié)果如下：
2018-11-20 10:38:48.473069+0800 Demo[2950:732087] ----------------- 測試開始 -----------------
2018-11-20 10:38:48.473251+0800 Demo[2950:732087] ----------------- 測試結(jié)束 -----------------
2018-11-20 10:38:48.917159+0800 Demo[2950:732087] ----------------- 任務一完成 ----------------- <NSThread: 0x282351b40>{number = 1, name = main} -----------------
2018-11-20 10:38:49.005818+0800 Demo[2950:732087] ----------------- 任務二完成 ----------------- <NSThread: 0x282351b40>{number = 1, name = main} -----------------
2018-11-20 10:38:49.092094+0800 Demo[2950:732087] ----------------- 任務三完成 ----------------- <NSThread: 0x282351b40>{number = 1, name = main} -----------------
這里可以看到，雖然并不像使用dispatch_gruop那樣，將調(diào)度組中的任務，執(zhí)行完畢后再去調(diào)用
dispatch_group_notify里面的任務。但任務一、二、三卻也是按順序執(zhí)行的。
以上是我個人對dispatch_semaphore的一點點理解，有錯誤的地方希望大家能指出來。
最后，個人還有點小疑問。既然AFN的回調(diào)已經(jīng)在主線程中了，那么為什么我們還要AFN的回調(diào)中用dispatch_get_main_queue()回到主線程更新UI呢？
