iOS多线程总结1-GCD

作者: wtqhy14615 | 来源:发表于2017-09-20 12:06 被阅读41次

iOS多线程编程

写在前面，欢迎互相交流，欢迎指出错误；

基本概念

进程：计算机中的程序关于某数据集合上的一次运行活动，是系统进行资源分配和调度的基本单位，是操作系统结构的基础。

线程：是进程中的一个实体，是被系统独立调度和分派的基本单位，线程自己不拥有系统资源，只拥有一点儿在运行中必不可少的资源，但它可与同属一个进程的其它线程共享进程所拥有的全部资源。

进程：就是指一个可执行的程序；
线程：指代一个独立执行的代码路径；
参考自小笨狼漫谈多线程

进程和线程：

推荐阮一峰的《进程与线程的一个简单解释》
http://www.ruanyifeng.com/blog/2013/04/processes_and_threads.html

GCD

推荐阅读《土土哥的GCD使用经验与技巧浅谈》

任务和队列

任务：

任务是在线程中执行的；具体分为：

同步任务dispatch_sync;
异步任务dispatch_async;

两者的区别是：同步sync和异步async的区别在于会不会阻塞当前线程；

队列：

队列只是负责任务的调度，而不负责任务的执行；具体分为串行队列serial queue和并行队列concurrent queue;

队列的特点：

先进先出FIFO，排在前面的任务最先执行;
串行队列：任务按照顺序被调度，前一个任务不执行完毕，队列不会调度;
并行队列：只要有空闲的线程，队列就会调度当前任务，交给线程去执行，不需要考虑前面是不是有任务在执行、有没有执行完毕，只要有线程可以利用，队列就会调度任务。

讨论几种情况：

Tips：主队列是全局串行队列

主队列main queue, 异步任务async task

特别注意：虽然是异步任务，但是主队列并不会创建新的线程，任务会在主线程顺序执行；

dispatch_queue_t queue = dispatch_get_main_queue();
dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation1 ..");
});

dispatch_async(queue, ^{
    sleep(2);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation2 ..");
});

dispatch_async(queue, ^{
    sleep(1);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation3 ..");
});

2017-09-20 10:42:46.153353 iOSSyncDemo[15164:4338648] <NSThread: 0x17407cf40>{number = 1, name = main}
2017-09-20 10:42:46.153575 iOSSyncDemo[15164:4338648] operation1 ..
2017-09-20 10:42:48.155361 iOSSyncDemo[15164:4338648] <NSThread: 0x17407cf40>{number = 1, name = main}
2017-09-20 10:42:48.155553 iOSSyncDemo[15164:4338648] operation2 ..
2017-09-20 10:42:49.157272 iOSSyncDemo[15164:4338648] <NSThread: 0x17407cf40>{number = 1, name = main}
2017-09-20 10:42:49.157467 iOSSyncDemo[15164:4338648] operation3 ..

主队列main queue，同步任务sync task，这种情况下，会彻底卡死主线程;

dispatch_queue_t queue = dispatch_get_main_queue();
NSLog(@"start ...");
dispatch_sync(queue, ^{
    sleep(2);
    NSLog(@"operation1 ...");
});
NSLog(@"finished");

  2017-09-20 10:47:51.087949 iOSSyncDemo[15169:4341123] start ...
(lldb)

其他串行队列other serial queue，异步任务async task，这种情况下，会创建新的线程，然后在新的线程上顺序执行任务；

dispatch_queue_t queue = dispatch_queue_create("com.investment.SERIAL", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation1 ..");
});

dispatch_async(queue, ^{
    sleep(2);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation2 ..");
});

dispatch_async(queue, ^{
    sleep(1);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation3 ..");
})；

    2017-09-20 10:21:01.822707 iOSSyncDemo[15148:4335599] <NSThread: 0x174260240>{number = 3, name = (null)}
2017-09-20 10:21:01.822949 iOSSyncDemo[15148:4335599] operation1 ..
2017-09-20 10:21:02.825276 iOSSyncDemo[15148:4335599] <NSThread: 0x174260240>{number = 3, name = (null)}
2017-09-20 10:21:02.825468 iOSSyncDemo[15148:4335599] operation2 ..
2017-09-20 10:21:03.831180 iOSSyncDemo[15148:4335599] <NSThread: 0x174260240>{number = 3, name = (null)}
2017-09-20 10:21:03.831372 iOSSyncDemo[15148:4335599] operation3 ..

其他串行队列other serial queue，同步步任务sync task，这种情况下，任务会在主线程顺序执行；

    dispatch_queue_t queue = dispatch_queue_create("com.investment.SERIAL", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation1 ..");
});

dispatch_async(queue, ^{
    sleep(2);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation2 ..");
});

dispatch_async(queue, ^{
    sleep(1);
    NSLog(@"%@", [NSThread currentThread]);
    NSLog(@"operation3 ..");
});

    2017-09-20 10:57:52.900571 iOSSyncDemo[15178:4342726] <NSThread: 0x174077c80>{number = 1, name = main}
2017-09-20 10:57:52.900780 iOSSyncDemo[15178:4342726] operation1 ..
2017-09-20 10:57:54.902541 iOSSyncDemo[15178:4342726] <NSThread: 0x174077c80>{number = 1, name = main}
2017-09-20 10:57:54.902735 iOSSyncDemo[15178:4342726] operation2 ..
2017-09-20 10:57:55.904303 iOSSyncDemo[15178:4342726] <NSThread: 0x174077c80>{number = 1, name = main}
2017-09-20 10:57:55.904500 iOSSyncDemo[15178:4342726] operation3 ..

并发队列concurrent queue, 异步任务async task，这种情况下，会为每个任务新建线程，并放到新线程上执行(注意：线程和任务一一对应)。

dispatch_queue_t queue = dispatch_queue_create("com.investment.CONCURRENT", DISPATCH_QUEUE_CONCURRENT);
// 并发队列 异步执行
dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"concurrentQueue async task1: %@", [NSThread currentThread]);
});

dispatch_async(queue, ^{
    sleep(2);
    NSLog(@"concurrentQueue async task2: %@", [NSThread currentThread]);
});

dispatch_async(queue, ^{
    sleep(1);
    NSLog(@"concurrentQueue async task3: %@", [NSThread currentThread]);
});

dispatch_async(queue, ^{
    NSLog(@"concurrentQueue async task4: %@", [NSThread currentThread]);
});

    2017-09-20 11:09:27.772133 iOSSyncDemo[15186:4344364] concurrentQueue async task4: <NSThread: 0x1742673c0>{number = 3, name = (null)}
2017-09-20 11:09:28.777422 iOSSyncDemo[15186:4344365] concurrentQueue async task3: <NSThread: 0x1700785c0>{number = 4, name = (null)}
2017-09-20 11:09:29.777229 iOSSyncDemo[15186:4344366] concurrentQueue async task2: <NSThread: 0x170262440>{number = 5, name = (null)}
2017-09-20 11:09:30.777152 iOSSyncDemo[15186:4344363] concurrentQueue async task1: <NSThread: 0x170262480>{number = 6, name = (null)}

并发队列concurrent queue, 同步任务sync task，这种情况下，所有任务会在当前线程顺序执行。

    dispatch_queue_t queue = dispatch_queue_create("com.investment.CONCURRENT", DISPATCH_QUEUE_CONCURRENT);
// 并发队列 异步执行
dispatch_sync(queue, ^{
    sleep(3);
    NSLog(@"concurrentQueue async task1: %@", [NSThread currentThread]);
});

dispatch_sync(queue, ^{
    sleep(2);
    NSLog(@"concurrentQueue async task2: %@", [NSThread currentThread]);
});

dispatch_sync(queue, ^{
    sleep(1);
    NSLog(@"concurrentQueue async task3: %@", [NSThread currentThread]);
});

dispatch_sync(queue, ^{
    NSLog(@"concurrentQueue async task4: %@", [NSThread currentThread]);
});

    2017-09-20 11:11:57.385978 iOSSyncDemo[15189:4344846] concurrentQueue async task1: <NSThread: 0x174076300>{number = 1, name = main}
2017-09-20 11:11:59.387875 iOSSyncDemo[15189:4344846] concurrentQueue async task2: <NSThread: 0x174076300>{number = 1, name = main}
2017-09-20 11:12:00.389544 iOSSyncDemo[15189:4344846] concurrentQueue async task3: <NSThread: 0x174076300>{number = 1, name = main}
2017-09-20 11:12:00.389850 iOSSyncDemo[15189:4344846] concurrentQueue async task4: <NSThread: 0x174076300>{number = 1, name = main}

主队列：这是一个特殊的串行队列。所有的UI操作都在这个队列上;

dispatch_get_main_queue()

全局并行队列：这是系统提供的一个并发队列;

dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

dispatch_group

dispatch_group_enter和dispatch_group_leave

    dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_async(group, queue, ^{
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"group task1 executing...");
    });
    NSLog(@"group task1 finished");
});

dispatch_group_async(group, queue, ^{
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"group task2 executing...");
    });
    NSLog(@"group task2 finished");
});

dispatch_group_async(group, queue, ^{
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"group task3 executing...");
    });
    NSLog(@"group task3 finished");
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"all group task finished");
});

2017-09-20 11:20:48.762150 iOSSyncDemo[15197:4347820] group task1 finished
2017-09-20 11:20:48.762258 iOSSyncDemo[15197:4347820] group task2 finished
2017-09-20 11:20:48.762629 iOSSyncDemo[15197:4347820] group task3 finished
2017-09-20 11:20:48.770034 iOSSyncDemo[15197:4347795] all group task finished
2017-09-20 11:20:50.768016 iOSSyncDemo[15197:4347817] group task1 executing...
2017-09-20 11:20:50.769131 iOSSyncDemo[15197:4347820] group task2 executing...
2017-09-20 11:20:50.769255 iOSSyncDemo[15197:4347818] group task3 executing...

通过log我们可以发现，在子线程的任务还没有完成的情况下；组任务就直接完成，这显然不是我们想要的结果；现在改进一下：

    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"group task1 executing...");
        sleep(3);
        NSLog(@"group task1 finished");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"group task2 executing...");
        sleep(2);
        NSLog(@"group task2 finished");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        NSLog(@"group task3 executing...");
        sleep(1);
        NSLog(@"group task3 finished");
        dispatch_group_leave(group);
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"all group task finished");
    });

2017-09-20 11:30:32.554690 iOSSyncDemo[15212:4350152] group task1 executing...
2017-09-20 11:30:32.554844 iOSSyncDemo[15212:4350156] group task2 executing...
2017-09-20 11:30:32.555487 iOSSyncDemo[15212:4350154] group task3 executing...
2017-09-20 11:30:33.560874 iOSSyncDemo[15212:4350154] group task3 finished
2017-09-20 11:30:34.560076 iOSSyncDemo[15212:4350156] group task2 finished
2017-09-20 11:30:35.560065 iOSSyncDemo[15212:4350152] group task1 finished
2017-09-20 11:30:35.560380 iOSSyncDemo[15212:4350130] all group task finished

这个时候就达到我们想要的结果了。

常规用法，这种用法在开发中可以应用的场景不多，更多的情况是上面那种并发队列多异步任务嵌套的情况。

dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_async(group, queue, ^{
    sleep(2);
    NSLog(@"group task1 finished");
});

dispatch_group_async(group, queue, ^{
    sleep(2);
    NSLog(@"group task2 finished");
});

dispatch_group_async(group, queue, ^{
    sleep(2);
    NSLog(@"group task3 finished");
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"all group task finished");
});

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

NSLog(@"after wait ..");

打印结果：

2017-09-19 18:15:48.088 iOSSyncDemo[17731:1209181] group task3 finished
2017-09-19 18:15:48.088 iOSSyncDemo[17731:1209183] group task1 finished
2017-09-19 18:15:48.088 iOSSyncDemo[17731:1209180] group task2 finished
2017-09-19 18:15:48.089 iOSSyncDemo[15192:4346855] after wait ..
2017-09-19 18:15:48.089 iOSSyncDemo[17731:1208970] all group task finished

dispatch_once

多用于单例

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
    //单例代码 
   });

dispatch_after

注意：dispatch_after是延迟提交，不是延迟运行;
作用是：将一个Block在特定的延时以后，加入到指定的队列中，并执行；

    dispatch_queue_t queue = dispatch_queue_create("com.investment.concurrent", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"Begin add operation...");
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:4];
        NSLog(@"operation1 done...");
    });
    
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:4];
        NSLog(@"operation2 done...");
    });
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), queue, ^{
        NSLog(@"after operation");
    });

2017-09-20 09:58:06.633353 iOSSyncDemo[15132:4331075] Begin add operation...
2017-09-20 09:58:08.818947 iOSSyncDemo[15132:4331094] after operation
2017-09-20 09:58:10.639082 iOSSyncDemo[15132:4331095] operation1 done...
2017-09-20 09:58:10.639383 iOSSyncDemo[15132:4331097] operation2 done...

dispatch_suspend

暂停队列，但是这里的“暂停”，并不能保证可以立即停止队列上正在运行的block；dispatch_suspend并不会立即暂停正在运行的block，而是在当前block执行完成后，暂停后续的block执行。

    dispatch_queue_t queue = dispatch_queue_create("com.investment.SERIAL", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:5];
        NSLog(@"operation 1 executing...");
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:5];
        NSLog(@"operation 2 executing...");
    });
    //延时一秒
    NSLog(@"sleep 1 second...");
    [NSThread sleepForTimeInterval:1];
    //挂起队列
    NSLog(@"suspend...");
    dispatch_suspend(queue);
    //延时10秒
    NSLog(@"sleep 10 second...");
    [NSThread sleepForTimeInterval:10];
    //恢复队列            
    NSLog(@"resume...");
    dispatch_resume(queue);

2017-09-20 12:02:56.875441 iOSSyncDemo[15232:4356001] sleep 1 second...
2017-09-20 12:02:57.877435 iOSSyncDemo[15232:4356001] suspend...
2017-09-20 12:02:57.877667 iOSSyncDemo[15232:4356001] sleep 10 second...
2017-09-20 12:03:01.880958 iOSSyncDemo[15232:4356027] operation 1 executing...
2017-09-20 12:03:07.878927 iOSSyncDemo[15232:4356001] resume...
2017-09-20 12:03:12.890141 iOSSyncDemo[15232:4356024] operation 2 executing...

dispatch_apply

这个方法是同步的! 会阻塞当前线程;

 dispatch_queue_t queue = dispatch_queue_create("com.investment.CONCURRENT", DISPATCH_QUEUE_CONCURRENT);
    dispatch_apply(3, queue, ^(size_t i) {
        sleep(1);
        NSLog(@"%@", [NSThread currentThread]);
        NSLog(@"loop: %zu", i);
    });
    NSLog(@"after apply operation...");

2017-09-20 09:51:09.230843 iOSSyncDemo[15125:4329829] <NSThread: 0x17406fac0>{number = 1, name = main}
2017-09-20 09:51:09.231062 iOSSyncDemo[15125:4329829] loop: 0
2017-09-20 09:51:09.233557 iOSSyncDemo[15125:4329856] <NSThread: 0x17007dbc0>{number = 3, name = (null)}
2017-09-20 09:51:09.233747 iOSSyncDemo[15125:4329856] loop: 1
2017-09-20 09:51:10.232754 iOSSyncDemo[15125:4329829] <NSThread: 0x17406fac0>{number = 1, name = main}
2017-09-20 09:51:10.232955 iOSSyncDemo[15125:4329829] loop: 2
2017-09-20 09:51:10.233043 iOSSyncDemo[15125:4329829] after apply operation...

dispatch_semaphore_t

用于线程间同步，比如下面这个生产者消费者模型。

    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_queue_create("com.investment.concurrent", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        for (int i = 0; i < 3; i ++) {
            sleep(2);
            NSLog(@"create one product");
            dispatch_semaphore_signal(sem);
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 100; i ++) {
            dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
            NSLog(@"consume one product");
        }
    });

2017-09-20 12:25:19.951850 iOSSyncDemo[15250:4360783] create one product
2017-09-20 12:25:19.952206 iOSSyncDemo[15250:4360780] consume one product
2017-09-20 12:25:21.955972 iOSSyncDemo[15250:4360783] create one product
2017-09-20 12:25:21.956282 iOSSyncDemo[15250:4360780] consume one product
2017-09-20 12:25:23.961401 iOSSyncDemo[15250:4360783] create one product
2017-09-20 12:25:23.961720 iOSSyncDemo[15250:4360780] consume one product

dispatch_set_target_queue

设置队列的优先级，将参数一的队列的优先级设置成和参数二的优先级一样。

dispatch_set_target_queue(queue, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0));

iOS多线程总结1-GCD

iOS多线程编程

基本概念

GCD

任务和队列

dispatch_group

dispatch_once

dispatch_after

dispatch_suspend

dispatch_apply

dispatch_semaphore_t

dispatch_set_target_queue

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