iOS 线程简述

一、串行队列和并行队列

我们先看一下队列的概念。

队列是先进先出（FIFO）结构的，其主要的任务主要是负责线程的创建、回收工作，不论什么队列和什么任务都不需要程序员参与，减轻了程序员的工作。

队列主要分为：

GCD队列

运行在主线程中的主队列。
3 个不同优先级的后台队列（全局队列）。
一个优先级更低的后台队列（用于 I/O，全局队列property=background）。

自定义队列

串行队列
并行队列。
自定义队列非常强大，建议在开发中使用。在自定义队列中被调度的所有Block最终都将被放入到系统的全局队列中和线程池中。

下面看一张非常重要的图。

img01

摘抄自https://www.jianshu.com/p/cc875ef54aa9

二、队列和执行方式的随机组合

执行方式有同步执行sync和异步执行 async两种
自定义队列有串行队列serial和并行队列concurrent加上系统的主队列mainQueue和全局队列globalQueue，共四种

那么两种执行方式和四种队列共有8种组合，分别为：

1. 串行队列+异步执行

//串行队列+异步执行
-(void)serialAndAsync{
    
    NSLog(@"当前线程--%@",[NSThread currentThread]);
    //第二个参数传NULL或者0也表示串行队列
    dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
    for (int i=0; i<5; i++) {
        
        void(^task)(void) = ^{
            NSLog(@"%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queue, task);
    }
    
}

当前线程--<NSThread: 0x6000026b0240>{number = 1, name = main}
0--<NSThread: 0x600002dc4000>{number = 5, name = (null)}
1--<NSThread: 0x600002dc4000>{number = 5, name = (null)}
2--<NSThread: 0x600002dc4000>{number = 5, name = (null)}
3--<NSThread: 0x600002dc4000>{number = 5, name = (null)}
4--<NSThread: 0x600002dc4000>{number = 5, name = (null)}

结合执行结果示例可知

开辟了新线程
任务是串行执行的

2. 串行队列+同步执行

//串行队列+同步执行
-(void)serialAndSync{
   NSLog(@"当前线程%@",[NSThread currentThread]);
   dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
   for (int i=0; i<5; i++) {
       void(^task)(void) = ^{
           NSLog(@"%d--%@",i,[NSThread currentThread]);
       };
       dispatch_sync(queue, task);
   }
}

当前线程<NSThread: 0x600001fa8e80>{number = 1, name = main}
0--<NSThread: 0x600001fa8e80>{number = 1, name = main}
1--<NSThread: 0x600001fa8e80>{number = 1, name = main}
2--<NSThread: 0x600001fa8e80>{number = 1, name = main}
3--<NSThread: 0x600001fa8e80>{number = 1, name = main}
4--<NSThread: 0x600001fa8e80>{number = 1, name = main}

结合执行结果示例可知

任务在主线程执行
任务是串行执行的

3. 并行队列+同步执行

//并行队列+异步执行
-(void)concurrentAndAsync{
   dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
   for (int i=0; i<5; i++) {
       void(^task)(void) = ^{
           NSLog(@"%d--%@",i,[NSThread currentThread]);
       };
       dispatch_async(queue, task);
   }
}

当前线程<NSThread: 0x600003044f00>{number = 1, name = main}
4--<NSThread: 0x60000304f8c0>{number = 3, name = (null)}
2--<NSThread: 0x60000304d540>{number = 6, name = (null)}
0--<NSThread: 0x600003064540>{number = 7, name = (null)}
1--<NSThread: 0x600003010800>{number = 5, name = (null)}
3--<NSThread: 0x600003064f80>{number = 4, name = (null)}

结合执行结果示例可知

开辟了多条子线程
任务是并行执行的

⚠️注：当我们没有对子线程数量做限制时系统并不会无限量创建子线程

4. 并行队列+同步执行

//并行队列+同步执行
-(void)concurrentAndSync{
   NSLog(@"当前线程%@",[NSThread currentThread]);
   dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
   for (int i=0; i<5; i++) {
       void(^task)(void) = ^{
           NSLog(@"%d--%@",i,[NSThread currentThread]);
       };
       dispatch_sync(queue, task);
   }
}

当前线程<NSThread: 0x6000020b85c0>{number = 1, name = main}
0--<NSThread: 0x6000020b85c0>{number = 1, name = main}
1--<NSThread: 0x6000020b85c0>{number = 1, name = main}
2--<NSThread: 0x6000020b85c0>{number = 1, name = main}
3--<NSThread: 0x6000020b85c0>{number = 1, name = main}
4--<NSThread: 0x6000020b85c0>{number = 1, name = main}

结合执行结果示例可知

运行在主线程
任务是串行执行的

5. 主队列+同步执行

//主队列+同步执行
-(void)mainQueueAndSync{
    NSLog(@"当前线程%@",[NSThread currentThread]);
    dispatch_queue_t queue = dispatch_get_main_queue();
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"%d--%@",i,[NSThread currentThread]);
        };
        dispatch_sync(queue, task);
    }
}

崩溃了

Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

分析原因：

主队列的任务一定会在主线程执行
同步执行dispatch_sync本身也是一个任务

那么对于主线程来说接收了6个任务：

mainQueueAndSync任务
任务1
任务2
任务3
任务4
任务5

对主线程来说先要完成mainQueueAndSync任务才会去执行下一个任务，也就是任务1，但是mainQueueAndSync任务执行完成的前提是任务1到任务5都执行完毕，所以就出现了线程等待，一直等到死...

如何破解线程死锁，那就是将mainQueueAndSync任务放到子线程去执行

//主队列+同步执行
-(void)mainQueueAndSync{
    NSLog(@"当前线程%@",[NSThread currentThread]);
    
    dispatch_queue_t queue = dispatch_get_main_queue();
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"%d--%@",i,[NSThread currentThread]);
        };
        [NSThread detachNewThreadWithBlock:^{
            dispatch_sync(queue, task);
        }];
    }
}

当前线程<NSThread: 0x600000784280>{number = 1, name = main}
0--<NSThread: 0x600000784280>{number = 1, name = main}
1--<NSThread: 0x600000784280>{number = 1, name = main}
2--<NSThread: 0x600000784280>{number = 1, name = main}
3--<NSThread: 0x600000784280>{number = 1, name = main}
4--<NSThread: 0x600000784280>{number = 1, name = main}

6. 主队列+异步执行

//主队列+异步执行
-(void)mainQueueAndAsync{
    NSLog(@"当前线程%@",[NSThread currentThread]);
    dispatch_queue_t queue = dispatch_get_main_queue();
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queue, task);
    }
}

当前线程<NSThread: 0x60000396cec0>{number = 1, name = main}
0--<NSThread: 0x60000396cec0>{number = 1, name = main}
1--<NSThread: 0x60000396cec0>{number = 1, name = main}
2--<NSThread: 0x60000396cec0>{number = 1, name = main}
3--<NSThread: 0x60000396cec0>{number = 1, name = main}
4--<NSThread: 0x60000396cec0>{number = 1, name = main}

结合执行结果示例可知

运行在主线程
任务是串行执行的

7. 全局队列+异步执行

//全局队列+异步执行
-(void)globalQueueAndAsync{
   NSLog(@"当前线程%@",[NSThread currentThread]);
   dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
   for (int i=0; i<5; i++) {
       void(^task)(void) = ^{
           NSLog(@"%d--%@",i,[NSThread currentThread]);
       };
       dispatch_async(queue, task);
   }
}

当前线程<NSThread: 0x60000068cf00>{number = 1, name = main}
0--<NSThread: 0x6000006dc200>{number = 5, name = (null)}
3--<NSThread: 0x60000069cec0>{number = 7, name = (null)}
2--<NSThread: 0x6000006de500>{number = 6, name = (null)}
1--<NSThread: 0x6000006ca400>{number = 4, name = (null)}
4--<NSThread: 0x6000006dc7c0>{number = 8, name = (null)}

结果同并行队列+异步执行一样，全局队列本身就是一个并行队列

8. 全局队列+同步执行

//全局队列+同步执行
-(void)globalQueueAndSync{
   NSLog(@"当前线程%@",[NSThread currentThread]);
   dispatch_queue_t queue =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
   for (int i=0; i<5; i++) {
       void(^task)(void) = ^{
           NSLog(@"%d--%@",i,[NSThread currentThread]);
       };
       dispatch_sync(queue, task);
   };
   
}

当前线程<NSThread: 0x600002b98100>{number = 1, name = main}
0--<NSThread: 0x600002b98100>{number = 1, name = main}
1--<NSThread: 0x600002b98100>{number = 1, name = main}
2--<NSThread: 0x600002b98100>{number = 1, name = main}
3--<NSThread: 0x600002b98100>{number = 1, name = main}
4--<NSThread: 0x600002b98100>{number = 1, name = main}

结果同并行队列+同步执行一样

img02

三、全局队列的优先级

/*!
 * @typedef dispatch_queue_priority_t
 * Type of dispatch_queue_priority
 *
 * @constant DISPATCH_QUEUE_PRIORITY_HIGH
 * Items dispatched to the queue will run at high priority,
 * i.e. the queue will be scheduled for execution before
 * any default priority or low priority queue.
 *
 * @constant DISPATCH_QUEUE_PRIORITY_DEFAULT
 * Items dispatched to the queue will run at the default
 * priority, i.e. the queue will be scheduled for execution
 * after all high priority queues have been scheduled, but
 * before any low priority queues have been scheduled.
 *
 * @constant DISPATCH_QUEUE_PRIORITY_LOW
 * Items dispatched to the queue will run at low priority,
 * i.e. the queue will be scheduled for execution after all
 * default priority and high priority queues have been
 * scheduled.
 *
 * @constant DISPATCH_QUEUE_PRIORITY_BACKGROUND
 * Items dispatched to the queue will run at background priority, i.e. the queue
 * will be scheduled for execution after all higher priority queues have been
 * scheduled and the system will run items on this queue on a thread with
 * background status as per setpriority(2) (i.e. disk I/O is throttled and the
 * thread's scheduling priority is set to lowest value).
 */
#define DISPATCH_QUEUE_PRIORITY_HIGH 2
#define DISPATCH_QUEUE_PRIORITY_DEFAULT 0
#define DISPATCH_QUEUE_PRIORITY_LOW (-2)
#define DISPATCH_QUEUE_PRIORITY_BACKGROUND INT16_MIN

备注信息可以看到全局队列默认设置4种优先级，CPU按照优先级由高到低的顺序依次执行

//全局队列+同步执行之不同优先级
-(void)globalQueueAndSync{
    NSLog(@"当前线程%@",[NSThread currentThread]);
    dispatch_queue_t queueBackground =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_queue_t queueLow =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    dispatch_queue_t queueDefault =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_queue_t queueHigh =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"BACKGROUND%d--%@",i,[NSThread currentThread]);
        };
        dispatch_sync(queueBackground, task);
    };
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"LOW%d--%@",i,[NSThread currentThread]);
        };
        dispatch_sync(queueLow, task);
    };
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"DEFAULT%d--%@",i,[NSThread currentThread]);
        };
        dispatch_sync(queueDefault, task);
    };
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"HIGH%d--%@",i,[NSThread currentThread]);
        };
        dispatch_sync(queueHigh, task);
    };
    
}

当前线程<NSThread: 0x600002f6cd40>{number = 1, name = main}
BACKGROUND0--<NSThread: 0x600002f6cd40>{number = 1, name = main}
BACKGROUND1--<NSThread: 0x600002f6cd40>{number = 1, name = main}
BACKGROUND2--<NSThread: 0x600002f6cd40>{number = 1, name = main}
BACKGROUND3--<NSThread: 0x600002f6cd40>{number = 1, name = main}
BACKGROUND4--<NSThread: 0x600002f6cd40>{number = 1, name = main}
LOW0--<NSThread: 0x600002f6cd40>{number = 1, name = main}
LOW1--<NSThread: 0x600002f6cd40>{number = 1, name = main}
LOW2--<NSThread: 0x600002f6cd40>{number = 1, name = main}
LOW3--<NSThread: 0x600002f6cd40>{number = 1, name = main}
LOW4--<NSThread: 0x600002f6cd40>{number = 1, name = main}
DEFAULT0--<NSThread: 0x600002f6cd40>{number = 1, name = main}
DEFAULT1--<NSThread: 0x600002f6cd40>{number = 1, name = main}
DEFAULT2--<NSThread: 0x600002f6cd40>{number = 1, name = main}
DEFAULT3--<NSThread: 0x600002f6cd40>{number = 1, name = main}
DEFAULT4--<NSThread: 0x600002f6cd40>{number = 1, name = main}
HIGH0--<NSThread: 0x600002f6cd40>{number = 1, name = main}
HIGH1--<NSThread: 0x600002f6cd40>{number = 1, name = main}
HIGH2--<NSThread: 0x600002f6cd40>{number = 1, name = main}
HIGH3--<NSThread: 0x600002f6cd40>{number = 1, name = main}
HIGH4--<NSThread: 0x600002f6cd40>{number = 1, name = main}

同步执行严格按照FIFO顺序执行全局队列中的任务，这个时候优先级并没有起作用

//全局队列+异步执行之不同优先级
-(void)globalQueueAndSync{
    NSLog(@"当前线程%@",[NSThread currentThread]);
    dispatch_queue_t queueBackground =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_queue_t queueLow =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    dispatch_queue_t queueDefault =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_queue_t queueHigh =dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"BACKGROUND%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queueBackground, task);
    };
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"LOW%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queueLow, task);
    };
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"DEFAULT%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queueDefault, task);
    };
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"HIGH%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queueHigh, task);
    };
    
}

这个我猜测执行结果应该按照high-default-low-background的顺序依此执行，但是事实并不是这样

当前线程<NSThread: 0x600000a00500>{number = 1, name = main}
DEFAULT1--<NSThread: 0x600000a14780>{number = 4, name = (null)}
DEFAULT3--<NSThread: 0x600000a1b5c0>{number = 7, name = (null)}
DEFAULT2--<NSThread: 0x600000a511c0>{number = 6, name = (null)}
DEFAULT4--<NSThread: 0x600000a4f800>{number = 8, name = (null)}
DEFAULT0--<NSThread: 0x600000a24680>{number = 5, name = (null)}
HIGH0--<NSThread: 0x600000a14980>{number = 3, name = (null)}
HIGH1--<NSThread: 0x600000a4eb40>{number = 9, name = (null)}
HIGH2--<NSThread: 0x600000a50140>{number = 10, name = (null)}
HIGH4--<NSThread: 0x600000a1b5c0>{number = 7, name = (null)}
HIGH3--<NSThread: 0x600000a4ed80>{number = 11, name = (null)}
LOW0--<NSThread: 0x600000a511c0>{number = 6, name = (null)}
LOW1--<NSThread: 0x600000a085c0>{number = 12, name = (null)}
LOW3--<NSThread: 0x600000a14980>{number = 3, name = (null)}
LOW4--<NSThread: 0x600000a24680>{number = 5, name = (null)}
LOW2--<NSThread: 0x600000a4eb40>{number = 9, name = (null)}
BACKGROUND2--<NSThread: 0x600000a50140>{number = 10, name = (null)}
BACKGROUND0--<NSThread: 0x600000a4f800>{number = 8, name = (null)}
BACKGROUND1--<NSThread: 0x600000a14780>{number = 4, name = (null)}
BACKGROUND3--<NSThread: 0x600000a1b500>{number = 13, name = (null)}
BACKGROUND4--<NSThread: 0x600000a25bc0>{number = 14, name = (null)}

将default和high优先级队列执行顺序调换之后，打印结果也跟着改变了

...
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"HIGH%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queueHigh, task);
    };
    
    for (int i=0; i<5; i++) {
        void(^task)(void) = ^{
            NSLog(@"DEFAULT%d--%@",i,[NSThread currentThread]);
        };
        dispatch_async(queueDefault, task);
    };
...

HIGH1--<NSThread: 0x6000015a59c0>{number = 4, name = (null)}
HIGH2--<NSThread: 0x6000015d8940>{number = 5, name = (null)}
HIGH0--<NSThread: 0x6000015a5040>{number = 3, name = (null)}
HIGH3--<NSThread: 0x6000015d6040>{number = 6, name = (null)}
HIGH4--<NSThread: 0x6000015c1c00>{number = 7, name = (null)}
DEFAULT1--<NSThread: 0x6000015d8700>{number = 9, name = (null)}
DEFAULT0--<NSThread: 0x6000015d6140>{number = 8, name = (null)}
DEFAULT3--<NSThread: 0x6000015a59c0>{number = 4, name = (null)}
DEFAULT2--<NSThread: 0x6000015dd1c0>{number = 10, name = (null)}
DEFAULT4--<NSThread: 0x6000015d8940>{number = 5, name = (null)}

似乎high并没有比defaut优先级高，而是跟执行顺序又关系。
为什么？？？？？
有大佬知道其中原委还望告知🙏

最后编辑于：2020.08.10 12:33:51

人面猴
序言：七十年代末，一起剥皮案震惊了整个滨河市，随后出现的几起案子，更是在滨河造成了极大的恐慌，老刑警刘岩，带你破解...
沈念sama阅读 212,185评论 6赞 493
死咒
序言：滨河连续发生了三起死亡事件，死亡现场离奇诡异，居然都是意外死亡，警方通过查阅死者的电脑和手机，发现死者居然都...
沈念sama阅读 90,445评论 3赞 385
救了他两次的神仙让他今天三更去死
文/潘晓璐我一进店门，熙熙楼的掌柜王于贵愁眉苦脸地迎上来，“玉大人，你说我怎么就摊上这事。” “怎么了？”我有些...
开封第一讲书人阅读 157,684评论 0赞 348
道士缉凶录：失踪的卖姜人
文/不坏的土叔我叫张陵，是天一观的道长。经常有香客问我，道长，这世上最难降的妖魔是什么？我笑而不...
开封第一讲书人阅读 56,564评论 1赞 284
港岛之恋（遗憾婚礼）
正文为了忘掉前任，我火速办了婚礼，结果婚礼上，老公的妹妹穿的比我还像新娘。我一直安慰自己，他们只是感情好，可当我...
茶点故事阅读 65,681评论 6赞 386
恶毒庶女顶嫁案：这布局不是一般人想出来的
文/花漫我一把揭开白布。她就那样静静地躺着，像睡着了一般。火红的嫁衣衬着肌肤如雪。梳的纹丝不乱的头发上，一...
开封第一讲书人阅读 49,874评论 1赞 290
城市分裂传说
那天，我揣着相机与录音，去河边找鬼。笑死，一个胖子当着我的面吹牛，可吹牛的内容都是我干的。我是一名探鬼主播，决...
沈念sama阅读 39,025评论 3赞 408
双鸳鸯连环套：你想象不到人心有多黑
文/苍兰香墨我猛地睁开眼，长吁一口气：“原来是场噩梦啊……” “哼！你这毒妇竟也来了？” 一声冷哼从身侧响起，我...
开封第一讲书人阅读 37,761评论 0赞 268
万荣杀人案实录
序言：老挝万荣一对情侣失踪，失踪者是张志新（化名）和其女友刘颖，没想到半个月后，有当地人在树林里发现了一具尸体，经...
沈念sama阅读 44,217评论 1赞 303
护林员之死
正文独居荒郊野岭守林人离奇死亡，尸身上长有42处带血的脓包…… 初始之章·张勋以下内容为张勋视角年9月15日...
茶点故事阅读 36,545评论 2赞 327
白月光启示录
正文我和宋清朗相恋三年，在试婚纱的时候发现自己被绿了。大学时的朋友给我发了我未婚夫和他白月光在一起吃饭的照片。...
茶点故事阅读 38,694评论 1赞 341
活死人
序言：一个原本活蹦乱跳的男人离奇死亡，死状恐怖，灵堂内的尸体忽然破棺而出，到底是诈尸还是另有隐情，我是刑警宁泽，带...
沈念sama阅读 34,351评论 4赞 332
日本核电站爆炸内幕
正文年R本政府宣布，位于F岛的核电站，受9级特大地震影响，放射性物质发生泄漏。R本人自食恶果不足惜，却给世界环境...
茶点故事阅读 39,988评论 3赞 315
男人毒药：我在死后第九天来索命
文/蒙蒙一、第九天我趴在偏房一处隐蔽的房顶上张望。院中可真热闹，春花似锦、人声如沸。这庄子的主人今日做“春日...
开封第一讲书人阅读 30,778评论 0赞 21
一桩弑父案，背后竟有这般阴谋
文/苍兰香墨我抬头看了看天上的太阳。三九已至，却和暖如春，着一层夹袄步出监牢的瞬间，已是汗流浃背。一阵脚步声响...
开封第一讲书人阅读 32,007评论 1赞 266
情欲美人皮
我被黑心中介骗来泰国打工，没想到刚下飞机就差点儿被人妖公主榨干…… 1. 我叫王不留，地道东北人。一个月前我还...
沈念sama阅读 46,427评论 2赞 360
代替公主和亲
正文我出身青楼，却偏偏与公主长得像，于是被迫代替她去往敌国和亲。传闻我的和亲对象是个残疾皇子，可洞房花烛夜当晚...
茶点故事阅读 43,580评论 2赞 349

iOS 线程简述

一、 串行队列和并行队列

二、队列和执行方式的随机组合

1. 串行队列+异步执行

2. 串行队列+同步执行

3. 并行队列+同步执行

4. 并行队列+同步执行

5. 主队列+同步执行

6. 主队列+异步执行

7. 全局队列+异步执行

8. 全局队列+同步执行

三、 全局队列的优先级

推荐阅读更多精彩内容

一、串行队列和并行队列

三、全局队列的优先级