dispatch_semaphore使用

dispatch_semaphore是GCD采用線程同步的一種方式，與他相關(guān)的共有三個(gè)參數(shù):

dispatch_semaphore_create
dispatch_semaphore_signal
dispatch_semaphore_wait

dispatch_semaphore_create 創(chuàng)建信號(hào)量
dispatch_semaphore_create(long value); 給信號(hào)量初始一個(gè)值噩死，當(dāng)傳遞的值小于0焙压，信號(hào)量將初始化失敗返回NULL怕篷。

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_semaphore_t semaphore1 = dispatch_semaphore_create(2);

dispatch_semaphore_signal 發(fā)送信號(hào)量
long dispatch_semaphore_signal(dispatch_semaphore_t dsema)
這個(gè)函數(shù)會(huì)使傳入的信號(hào)量dsema的值加1；

    dispatch_semaphore_signal(semaphore);

dispatch_semaphore_wait 等待信號(hào)量
long dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout)；
這個(gè)函數(shù)會(huì)使傳入的信號(hào)量dsema的值減1揍障，如果傳入信號(hào)量的值等于0脉课，函數(shù)將持續(xù)等待不返回救军。

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

這個(gè)函數(shù)的作用是這樣的财异，如果dsema信號(hào)量的值大于0，該函數(shù)所處線程就繼續(xù)執(zhí)行下面的語句唱遭，并且將信號(hào)量的值減1戳寸；
　　如果desema的值為0，那么這個(gè)函數(shù)就阻塞當(dāng)前線程等待timeout（注意timeout的類型為dispatch_time_t拷泽，
　　不能直接傳入整形或float型數(shù)）疫鹊，如果等待的期間desema的值被dispatch_semaphore_signal函數(shù)加1了，
　　且該函數(shù)（即dispatch_semaphore_wait）所處線程獲得了信號(hào)量司致，那么就繼續(xù)向下執(zhí)行并將信號(hào)量減1拆吆。
　　如果等待期間沒有獲取到信號(hào)量或者信號(hào)量的值一直為0，那么等到timeout時(shí)脂矫，其所處線程自動(dòng)執(zhí)行其后語句枣耀。

信號(hào)量的初始值，可以用來控制線程并發(fā)訪問的最大數(shù)量庭再。

信號(hào)量的初時(shí)值為1捞奕，代表同時(shí)只允許1條線程訪問資源，保證線程同步拄轻。

iOS線程同步方案性能比較
性能從高到低排序
os_unfair_lock // 缺點(diǎn)：iOS10才支持
OSSpinLock　　// 缺點(diǎn)：可能出現(xiàn)優(yōu)先級(jí)反轉(zhuǎn) 已經(jīng)不再安全蘋果也不推薦使用
dispatch_semaphore // 推薦使用
pthread_mutex　　// 優(yōu)點(diǎn)：跨平臺(tái) 互斥鎖（普通鎖）推薦使用
dispatch_queue(DISPATCH_QUEUE_SERIAL)　// c
NSLock 　　// oc
NSCondition　　 // oc
pthread_mutex(recursive) // 遞歸鎖
NSRecursiveLock　　// oc
NSConditionLock // oc
@synchronized // 遞歸鎖 oc

從上可以知道線程同步除了os_unfair_lock 和
OSSpinLock之外颅围，dispatch_semaphore的性能是很好的極力推薦使用。

dispatch_semaphore.jpg

如以上假如創(chuàng)建了A哺眯、B 谷浅、C、D四個(gè)子線程奶卓，假如A線程先執(zhí)行了35行代碼后一疯，此時(shí)信號(hào)量的值-1也就是1，并往下繼續(xù)執(zhí)行夺姑。隨后B線程也執(zhí)行了35行代碼后墩邀，此時(shí)信號(hào)量的值-1，也就是0盏浙。C線程眉睹、D線程都執(zhí)行35行代碼時(shí)，此時(shí)信號(hào)量的值已經(jīng)是0了废膘，C線程和D線程就會(huì)進(jìn)入休眠等待中竹海，此時(shí)就卡住35行代碼處。假如第11秒時(shí)丐黄，線程A斋配、B執(zhí)行完了第39行代碼后，信號(hào)量+1,+1此時(shí)信號(hào)量就是2了，這樣C線程艰争、D線程會(huì)被喚醒繼續(xù)執(zhí)行坏瞄。這樣信號(hào)量的初始值，可以用來控制線程并發(fā)訪問的最大數(shù)量甩卓。同理當(dāng)我們設(shè)置信號(hào)量的初時(shí)值為1時(shí)鸠匀，就可以實(shí)現(xiàn)線程同步。

dispatch_semaphore 使用案例

控制線程并發(fā)數(shù)

//定義一個(gè)信號(hào)量逾柿，初始化為10
dispatch_semaphore_t semaphore = dispatch_semaphore_create(10);
    
//同時(shí)執(zhí)行100個(gè)任務(wù)
 for (int i = 0; i < 100; i++)
 {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            
    //當(dāng)前信號(hào)量-1
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            
    NSLog(@"任務(wù)%d執(zhí)行",i+1);
            
    NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:kUrlString]];

    dispatch_async(dispatch_get_main_queue(), ^{
        //TODO:刷新界面
    });
            
    //當(dāng)前信號(hào)量+1
    dispatch_semaphore_signal(semaphore);
            
    });
}

線程安全

通過信號(hào)量可以用來控制線程并發(fā)訪問的最大數(shù)量缀棍，當(dāng)我們設(shè)置信號(hào)量的初時(shí)值為1時(shí)，就可以實(shí)現(xiàn)線程同步机错。

- (void)viewDidLoad {
    [super viewDidLoad];
    semaphore =  dispatch_semaphore_create(1);
    self.array = [NSMutableArray array];
    for (int i=0; i<100; i++) {
        NSThread *thread = [[NSThread alloc]initWithTarget:self selector:@selector(test) object:nil];
        [thread start];
    }
}

- (void)test{
    NSLog(@"測試開始");
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    sleep(3);
    [self.array addObject:@"0"];
    dispatch_semaphore_signal(semaphore);
    NSLog(@"測試");
}

self.array直接在多個(gè)線程上進(jìn)行做修改操作是不安全的睦柴，我可以通過信息量同步加鎖保證其線程安全。

多個(gè)網(wǎng)絡(luò)請求同步

1毡熏、首先通過網(wǎng)絡(luò)請求一獲取用戶useid，之后用userid為參數(shù)發(fā)起網(wǎng)絡(luò)請求二侣诵。

#pragma mark - 網(wǎng)絡(luò)請求一
- (void)getuserId:(dispatch_semaphore_t)semaphore{
   AFHTTPSessionManager *sessionmanger=[[AFHTTPSessionManager alloc]init];
   sessionmanger.responseSerializer=[AFHTTPResponseSerializer serializer];
   [sessionmanger POST:@"https://www.baidu.com/" parameters:nil constructingBodyWithBlock:nil progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
         NSLog(@"請求成功1%@", [NSThread currentThread]);
        useid=@"1234";
       dispatch_semaphore_signal(semaphore);
   } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
       NSLog(@"%@",error);
        dispatch_semaphore_signal(semaphore);
   }];
   
}

#pragma mark - 網(wǎng)絡(luò)請求二
- (void)requestwithuserid:(NSString *)userid{
   NSDictionary *parms=[NSMutableDictionary dictionary];
   [parms setValue:userid forKey:@"userid"];
   AFHTTPSessionManager *sessionmanger=[[AFHTTPSessionManager alloc]init];
   sessionmanger.responseSerializer=[AFHTTPResponseSerializer serializer];
   [sessionmanger POST:@"https://www.baidu.com/" parameters:userid constructingBodyWithBlock:nil progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
       NSLog(@"請求成功2");
   } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
       NSLog(@"%@",error);
   }];
}

#pragma mark - 使用信號(hào)量實(shí)現(xiàn)
- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_semaphore_t semaphore= dispatch_semaphore_create(0); // 創(chuàng)建信號(hào)量
    [self getuserId:semaphore];//獲取用戶useid
 
dispatch_semaphore_wait(semaphore,DISPATCH_TIME_FOREVER);//當(dāng)前信號(hào)量為0痢法，一直等待阻塞線程
    [self requestwithuserid:useid];
}

command+R運(yùn)行一下，沒有任何反應(yīng)杜顺。
原因分析：線程卡住了财搁。代碼執(zhí)行到dispatch_semaphore_wait(semaphore,DISPATCH_TIME_FOREVER)因?yàn)樾盘?hào)量為0，當(dāng)前線程會(huì)被阻塞躬络。而當(dāng)前線程是主線程尖奔，網(wǎng)絡(luò)請求一成功后回調(diào)到主線程，因?yàn)橹骶€程被阻塞造成信號(hào)量無法釋放穷当，一直卡住提茁。

解決方案就是開啟一個(gè)異步線程

- (void)viewDidLoad {
    [super viewDidLoad];
    //創(chuàng)建一個(gè)并行隊(duì)列
    dispatch_queue_t queque = dispatch_queue_create("GoyakodCreated", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queque, ^{
        dispatch_semaphore_t semaphore= dispatch_semaphore_create(0); // 創(chuàng)建信號(hào)量
        [self getuserId:semaphore];
        dispatch_semaphore_wait(semaphore,DISPATCH_TIME_FOREVER);
        [self requestwithuserid:useid];
    });
}

2、多個(gè)網(wǎng)絡(luò)請求后刷新UI

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    
    dispatch_group_t group = dispatch_group_create();
    // 創(chuàng)建信號(hào)量
   semaphore = dispatch_semaphore_create(0);
    // 創(chuàng)建全局并行
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    dispatch_group_async(group, queue, ^{

        [self request1];

    });
    
    dispatch_group_async(group, queue, ^{
       
        [self request2];
 
    });
    
    dispatch_group_notify(group, queue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        //在這里 進(jìn)行請求后的方法馁菜，回到主線程
        dispatch_async(dispatch_get_main_queue(), ^{
            
            NSLog(@"更新UI");
            
        });
    });
    NSLog(@"12344");
    
}

- (void)request1{
    AFHTTPSessionManager *manger=[[AFHTTPSessionManager alloc]init];
    manger.responseSerializer=[AFHTTPResponseSerializer serializer];
    [manger POST:@"https://www.baidu.com" parameters:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"請求成功1");
        dispatch_semaphore_signal(semaphore);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
         dispatch_semaphore_signal(semaphore);
    }];

}
- (void)request2{
    AFHTTPSessionManager *manger=[[AFHTTPSessionManager alloc]init];
    manger.responseSerializer=[AFHTTPResponseSerializer serializer];
    [manger POST:@"https://www.baidu.com" parameters:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"請求成功2");
        dispatch_semaphore_signal(semaphore);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
         dispatch_semaphore_signal(semaphore);
    }];
   
}

或者這樣

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    
    dispatch_group_t group = dispatch_group_create();
   
    // 創(chuàng)建全局并行
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    dispatch_group_async(group, queue, ^{

        [self request1];

    });
    
    dispatch_group_async(group, queue, ^{
       
        [self request2];
 
    });
    
    dispatch_group_notify(group, queue, ^{

        //在這里 進(jìn)行請求后的方法茴扁，回到主線程
        dispatch_async(dispatch_get_main_queue(), ^{
            
            NSLog(@"更新UI");
            
        });
    });
    NSLog(@"12344");
    
}
- (void)request1{
    // 創(chuàng)建信號(hào)量
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    AFHTTPSessionManager *manger=[[AFHTTPSessionManager alloc]init];
    manger.responseSerializer=[AFHTTPResponseSerializer serializer];
    [manger POST:@"https://www.baidu.com" parameters:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"請求成功1");
         NSLog(@"%ld", dispatch_semaphore_signal(semaphore));
       ;
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
         dispatch_semaphore_signal(semaphore);
        NSLog(@"%@",semaphore);
    }];
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}
- (void)request2{
    // 創(chuàng)建信號(hào)量
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    AFHTTPSessionManager *manger=[[AFHTTPSessionManager alloc]init];
    manger.responseSerializer=[AFHTTPResponseSerializer serializer];
    [manger POST:@"https://www.baidu.com" parameters:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        NSLog(@"請求成功2");
        dispatch_semaphore_signal(semaphore);
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
         dispatch_semaphore_signal(semaphore);
    }];
     dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
}
@end

參考：CodeVicent