各種方法

1. 獲取主線程隊列

dispatch_queue_t mainQueue = dispatch_get_main_queue();

2. 獲取global子線程隊列

dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

關(guān)于子線程隊列priority钞螟，執(zhí)行完High悔醋，再執(zhí)行Default拢锹，然后是Low谣妻，最后是Backgound。

3. 創(chuàng)建自定義隊列

dispatch_queue_t customQueue = dispatch_queue_create("com.uzai.testQueue", DISPATCH_QUEUE_CONCURRENT);

創(chuàng)建隊列卒稳，serial串行執(zhí)行蹋半，concurrent并行執(zhí)行

4. 異步執(zhí)行

dispatch_async(customQueue, ^{
    NSLog(@"custom serial queue %@",[NSThread currentThread]);
});

5. 同步執(zhí)行

dispatch_sync(customQueue, ^{
    NSLog(@"custom serial queue %@",[NSThread currentThread]);
});

異步執(zhí)行串行隊列，同一子線程串行執(zhí)行
異步執(zhí)行并行隊列充坑，多個子線程并行執(zhí)行
同步執(zhí)行串行隊列减江，主線程串行執(zhí)行
同步執(zhí)行并行隊列染突，主線程串行執(zhí)行

6. 按次數(shù)執(zhí)行block

按照指定次數(shù)將 block 添加到指定的隊列當(dāng)中，dispatch_apply函數(shù)會等待全部處理結(jié)束辈灼。

dispatch_queue_t asyncQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
NSArray *wordsArray = @[@"GCD",@"NSOperations",@"NSOperationQueues",@"algorithms"];
dispatch_apply(wordsArray.count, asyncQueue, ^(size_t currentIndex) {
    NSLog(@"current size -> %@",wordsArray[currentIndex]);
});
NSLog(@"all applies are done");

7. 創(chuàng)建單例

+ (instancetype)shareInstance {
    static id instance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        instance = [[self alloc] init];
    });
    return instance;
}

8. dispatch_group

• 使用dispatch_group_async在多個異步任務(wù)完成后份企，再執(zhí)行dispatch_group_notify中的任務(wù)。

dispatch_queue_t current1 = dispatch_queue_create("current1", DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, current1, ^{
    sleep(3);
    NSLog(@"current 1 finish %@",[NSThread currentThread]);
});

dispatch_group_async(group, current1, ^{
    sleep(3);
    NSLog(@"current 2 finish %@",[NSThread currentThread]);
});

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

//output
//2017-12-07 15:57:59.207 iOSInterviewProblems[64337:3876851] current 1 finish <NSThread: 0x60800026f6c0>{number = 9, name = (null)}
//2017-12-07 15:58:02.282 iOSInterviewProblems[64337:3945895] current 2 finish <NSThread: 0x600000270800>{number = 12, name = (null)}
//2017-12-07 15:58:02.282 iOSInterviewProblems[64337:3875012] all finish

• 使用 dispatch_group_enter巡莹，dispatch_group_leave 獲取多個異步線程執(zhí)行完成時候司志。

1. dispatch_group_enter 在任務(wù)開始執(zhí)行時調(diào)用
2. dispatch_group_leave 在任務(wù)完成時調(diào)用

dispatch_group_t downloadGroup = dispatch_group_create();
dispatch_queue_t downloadQueue = dispatch_queue_create("com.uzai.download", DISPATCH_QUEUE_CONCURRENT);

for (int i = 0; i < 6; i++) {
    dispatch_group_enter(downloadGroup);
    dispatch_async(downloadQueue, ^{
        NSLog(@"now downloading");
        sleep(i);
        dispatch_group_leave(downloadGroup);
        NSLog(@"downloaded");
    });
}

dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{
    NSLog(@"all download done");
});

//output
//    2017-12-07 17:06:24.765 iOSInterviewProblems[65166:3996044] now downloading
//    2017-12-07 17:06:24.765 iOSInterviewProblems[65166:3995991] now downloading
//    2017-12-07 17:06:24.765 iOSInterviewProblems[65166:3995994] now downloading
//    2017-12-07 17:06:24.765 iOSInterviewProblems[65166:3996044] downloaded
//    2017-12-07 17:06:24.765 iOSInterviewProblems[65166:3996047] now downloading
//    2017-12-07 17:06:24.766 iOSInterviewProblems[65166:3996044] now downloading
//    2017-12-07 17:06:24.766 iOSInterviewProblems[65166:3996048] now downloading
//    2017-12-07 17:06:25.840 iOSInterviewProblems[65166:3995991] downloaded
//    2017-12-07 17:06:26.766 iOSInterviewProblems[65166:3995994] downloaded
//    2017-12-07 17:06:27.840 iOSInterviewProblems[65166:3996047] downloaded
//    2017-12-07 17:06:28.766 iOSInterviewProblems[65166:3996044] downloaded
//    2017-12-07 17:06:29.767 iOSInterviewProblems[65166:3996048] downloaded
//    2017-12-07 17:06:29.767 iOSInterviewProblems[65166:3995950] all download done

9. dispatch_sources

GCD提供一系列的dispatch sources，用來監(jiān)控活動的狀況（底層系統(tǒng)活動降宅，例如Unix 信號骂远，VFS 節(jié)點 等等）。并且在上述活動發(fā)生的時候腰根，執(zhí)行提交到隊列的事務(wù)激才。

• 創(chuàng)建GCDTimer

  • gcd timer帶有兩個參數(shù)，interval代表的是觸發(fā)時機(jī)额嘿，leeway代表的是允許的誤差瘸恼。例如設(shè)置interval = 1，leeway = 1册养。代表著1秒觸發(fā)东帅，可以有1秒的偏差。
  • timer無法自動釋放捕儒，需要通過dispatch_source_cancel進(jìn)行釋放冰啃。

//創(chuàng)建并啟動timer
if (self.timerSource == nil) {
    self.timerCountDown = 0;
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    self.timerSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    dispatch_source_set_timer(self.timerSource, dispatch_walltime(NULL, 0), 1 * NSEC_PER_SEC, 1 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(self.timerSource, ^{
        self.timerCountDown++;
        NSLog(@"printting now -> %d",self.timerCountDown);
    });
    dispatch_resume(self.timerSource);
}

//暫停timer
if (self.timerSource) {
    dispatch_suspend(self.timerSource);
}

//取消掉timer
if (self.timerSource) {
    dispatch_source_cancel(self.timerSource);
}

示例參見 github Demo

注意事項

• dispatch_sync 使用
  該方法直到block執(zhí)行完畢才會返回執(zhí)行下一個任務(wù)邓夕。在當(dāng)前隊列調(diào)用會造成死鎖刘莹。