iOS多線程總結1-GCD

iOS多線程編程

寫在前面，歡迎互相交流，歡迎指出錯誤；

基本概念

進程：計算機中的程序關于某數據集合上的一次運行活動，是系統進行資源分配和調度的基本單位，是操作系統結構的基礎。

線程：是進程中的一個實體，是被系統獨立調度和分派的基本單位，線程自己不擁有系統資源，只擁有一點兒在運行中必不可少的資源，但它可與同屬一個進程的其它線程共享進程所擁有的全部資源。

進程：就是指一個可執行的程序；
線程：指代一個獨立執行的代碼路徑；
參考自小笨狼漫談多線程

進程和線程：

推薦阮一峰的《進程與線程的一個簡單解釋》
http://www.ruanyifeng.com/blog/2013/04/processes_and_threads.html

GCD

推薦閱讀《土土哥的GCD使用經驗與技巧淺談》

任務和隊列

任務：

任務是在線程中執行的；具體分為：

同步任務dispatch_sync;
異步任務dispatch_async;

兩者的區別是：同步sync和異步async的區別在于會不會阻塞當前線程；

隊列：

隊列只是負責任務的調度，而不負責任務的執行；具體分為串行隊列serial queue和并行隊列concurrent queue;

隊列的特點：

先進先出FIFO，排在前面的任務最先執行;
串行隊列：任務按照順序被調度，前一個任務不執行完畢，隊列不會調度;
并行隊列：只要有空閑的線程，隊列就會調度當前任務，交給線程去執行，不需要考慮前面是不是有任務在執行、有沒有執行完畢，只要有線程可以利用，隊列就會調度任務。

討論幾種情況：

Tips：主隊列是全局串行隊列

主隊列main queue, 異步任務async task

特別注意：雖然是異步任務，但是主隊列并不會創建新的線程，任務會在主線程順序執行；

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 ..

主隊列main queue，同步任務sync task，這種情況下，會徹底卡死主線程;

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)

其他串行隊列other serial queue，異步任務async task，這種情況下，會創建新的線程，然后在新的線程上順序執行任務；

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 ..

其他串行隊列other serial queue，同步步任務sync task，這種情況下，任務會在主線程順序執行；

    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 ..

并發隊列concurrent queue, 異步任務async task，這種情況下，會為每個任務新建線程，并放到新線程上執行(注意：線程和任務一一對應)。

dispatch_queue_t queue = dispatch_queue_create("com.investment.CONCURRENT", DISPATCH_QUEUE_CONCURRENT);
// 并發隊列 異步執行
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)}

并發隊列concurrent queue, 同步任務sync task，這種情況下，所有任務會在當前線程順序執行。

    dispatch_queue_t queue = dispatch_queue_create("com.investment.CONCURRENT", DISPATCH_QUEUE_CONCURRENT);
// 并發隊列 異步執行
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}

主隊列：這是一個特殊的串行隊列。所有的UI操作都在這個隊列上;

dispatch_get_main_queue()

全局并行隊列：這是系統提供的一個并發隊列;

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...

通過log我們可以發現，在子線程的任務還沒有完成的情況下；組任務就直接完成，這顯然不是我們想要的結果；現在改進一下：

    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

這個時候就達到我們想要的結果了。

常規用法，這種用法在開發中可以應用的場景不多，更多的情況是上面那種并發隊列多異步任務嵌套的情況。

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 ..");

打印結果：

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

多用于單例

    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
    //單例代碼 
   });

dispatch_after

注意：dispatch_after是延遲提交，不是延遲運行;
作用是：將一個Block在特定的延時以后，加入到指定的隊列中，并執行；

    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

暫停隊列，但是這里的“暫?！?，并不能保證可以立即停止隊列上正在運行的block；dispatch_suspend并不會立即暫停正在運行的block，而是在當前block執行完成后，暫停后續的block執行。

    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...");
    });
    //延時一秒
    NSLog(@"sleep 1 second...");
    [NSThread sleepForTimeInterval:1];
    //掛起隊列
    NSLog(@"suspend...");
    dispatch_suspend(queue);
    //延時10秒
    NSLog(@"sleep 10 second...");
    [NSThread sleepForTimeInterval:10];
    //恢復隊列            
    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

這個方法是同步的! 會阻塞當前線程;

 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

用于線程間同步，比如下面這個生產者消費者模型。

    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

設置隊列的優先級，將參數一的隊列的優先級設置成和參數二的優先級一樣。

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

最后編輯于：2017.12.10 14:06:10

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三个男躁一个女,国精产品一区一手机的秘密,麦子交换系列最经典十句话,欧美国产综合欧美视频

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iOS多線程總結1-GCD

iOS多線程編程

基本概念

GCD

任務和隊列

dispatch_group

dispatch_once

dispatch_after

dispatch_suspend

dispatch_apply

dispatch_semaphore_t

dispatch_set_target_queue

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三个男躁一个女,国精产品一区一手机的秘密,麦子交换系列最经典十句话,欧美 国产 综合 欧美 视频

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iOS多線程編程

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dispatch_apply

dispatch_semaphore_t

dispatch_set_target_queue

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