iOS与OS X之GCD

GCD中有兩種queue,分別為下面兩種

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生成Dispatch Queue的方法

一:dispatch_queue_create

生成Serial Dispatch Queue

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生成Concurrent Dispatch Queue

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生成的Dispatch Queue必須由程序員負責釋放,這是因為Dispatch Queue並沒有像 Block那樣具有作為<OC對象>來處理的技術,代碼如下

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在通過函數或方法獲取Dispatch Queue 以及其他名稱中含有 create 的API生成的對象時,有必要通過 dispatch_retain函數持有,並在不需要時 dispatch_release釋放

二:Main Dispatch Queue/Global Dispatch Queue

    //Main Dispatch Queue 的獲取方法
    dispatch_queue_t mainDispatchQueue = dispatch_get_main_queue();
    //Global Dispatch Queue(高優先級)的獲取方法
    dispatch_queue_t golbalDispatchQueueHigh = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
    //Global Dispatch Queue(默認優先級)
    dispatch_queue_t golbalDispatchQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    //Global Dispatch Queue(低優先級)
    dispatch_queue_t golbalDispatchQueueLow = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
    //Global Dispatch Queue(後臺優先級)
    dispatch_queue_t golbalDispatchQueueBackground = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

對於Main Dispatch Queue和Golbal Dispatch Queue執行dispatch_retain 和 dispatch_release函數不會引起任何變化,也不會有任何問題
dispatch_set_target_queue

dispatch_queue_create生成的queue都與默認優先級的global dispatch queue相同執行優先級的線程,如需將其變為後臺優先級可參考下面代碼

  dispatch_queue_t mySerialDispatchQueue = dispatch_queue_create("com.atsmart.gcd.MySerialDispatchQueue", NULL);
    dispatch_queue_t golbalDispatchQueueBackground = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_set_target_queue(mySerialDispatchQueue, golbalDispatchQueueBackground);

如果在多個serial dispatch queue 中用 dispatch_set_target_queue函數指定某一個serial dispatch queue,那麼原本可以並行執行的多個serial dispatch queue,在目標serial dispatch queue上也只能執行一個處理,示意圖如下(如果有需求需要將多個queue按順序執行可用此方法)

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dispatch_afer

iOS多线程中performSelector: 和dispatch_time的不同

    dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 3 * NSEC_PER_SEC);
    dispatch_after(time, dispatch_get_main_queue(), ^{
        NSLog(@"");
    });

上面代碼不代表在3秒後就執行,而是3秒後加入到主隊列中

dispatch_group

 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{NSLog(@"blk0");});
    dispatch_group_async(group, queue, ^{NSLog(@"blk1");});
    dispatch_group_async(group, queue, ^{NSLog(@"blk2");});
    dispatch_group_async(group, queue, ^{NSLog(@"blk3");});
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"done");
    });

queue是並行隊列,所以blk0,blk1,blk2,blk3不能保證順序執行,但是可以保證done是最後打印出來
也可以僅僅等待全部任務結束
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

dispatch_group_wait的另一種用法,隔多少時間判斷裏面任務是否執行完畢

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dispatch_barrier_async

對於數據庫,我們在寫入的時候要保證不能有多個動作同時進行,因為會

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{NSLog(@"blk0_for_reading");});
    dispatch_group_async(group, queue, ^{NSLog(@"blk1_for_reading");});
    dispatch_barrier_async(queue, ^{
        NSLog(@"writing");
    });
    dispatch_group_async(group, queue, ^{NSLog(@"blk2_for_reading");});
    dispatch_group_async(group, queue, ^{NSLog(@"blk3_for_readinglk3");});
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"done");
    });

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dispatch_sync

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容易發生死鎖的幾個例子:

1,主線程正在執行代碼,你又等待主線程執行完畢,所以死鎖

    dispatch_queue_t queue = dispatch_get_main_queue();
    dispatch_sync(queue, ^{
        NSLog(@"hello??");
    });

    dispatch_queue_t queue = dispatch_queue_create("xxx", NULL);
    dispatch_async(queue, ^{
       dispatch_sync(queue, ^{
           NSLog(@"hello?");
       });
    });

- dispatch_apply

指定次數將Block加入指定Queue中,並等待全部處理執行完畢,示例代碼如下

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        dispatch_apply([array count], queue, ^(size_t index) {
            
            NSLog(@"%zu:%@",index,[array objectAtIndex:index]);
        });
        //dispatch_apply函數中處理全部執行完畢
        dispatch_async(dispatch_get_main_queue(), ^{
            //可執行用戶更新操作等
            NSLog(@"done");
        });
    });

dispatch_suspend/dispatch_resume

可以掛起和重新恢復執行queue

- dispatch semaphore

下面代碼會因為同時很多線程操作數組導致系統奔潰

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    NSMutableArray *array = [[NSMutableArray alloc]init];
    for (int i = 0; i<1000; ++i) {
        dispatch_async(queue, ^{
            [array addObject:[NSNumber numberWithInt:i]];
        });
    }

改進如下

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);//保證可訪問array的線程只能有一個
    NSMutableArray *array = [[NSMutableArray alloc]init];
    for (int i = 0; i<100000; ++i) {
        dispatch_async(queue, ^{
            //等待semaphore,直到semaphore值大於或者等於1
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            //第一次我們設置為1所以可以直接下來執行,然後會semaphore減1,現在變為0
            [array addObject:[NSNumber numberWithInt:i]];
            //執行完後還是0,需要進行加1操作,semaphore
            dispatch_semaphore_signal(semaphore);
            //semaphore為1後,才可以進行下一次循環
        });
    }

- dispath_once

一般用來初始化操作,保證在多線程環境下也能安全

Dispatch I/O

GCD高级用法－Dispatch I/O