GCD

一、Dispatch Queue

//  1.dispatch_queue_create:生成的dispatch_queue_t

dispatch_queue_t mySerialiDispatchQueue = dispatch_queue_create("com.example.gcd.MySerialiDispatchQueue", DISPATCH_QUEUE_SERIAL);

dispatch_queue_t myConcurrentDispatchQueue = dispatch_queue_create("com.example.gcd.MyConcurrentDispatchQueue", DISPATCH_QUEUE_CONCURRENT);

// 2.dispatch_get_main_queue \ dispatch_get_global_queue

dispatch_queue_t mainDispatchQueue = dispatch_get_main_queue();

dispatch_queue_t globalDispatchQueueHigh = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_queue_t globalDispatchQueueDefault = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_queue_t globalDispatchQueueLow = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
dispatch_queue_t globalDispatchQueueBackground = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);

二、基本概念：异步同步串行队列并发队列

dispatch_async: 异步,队列中的任务无需执行完毕，就可执行后续代码
dispatch_sync: 同步，队列中的任务执行完毕之后才可以执行后续代码
mySerialiDispatchQueue: 串行队列，队列中的是有顺序的执行完毕
myConcurrentDispatchQueue: 并发队列，队列中的任务谁执行的快谁先完成

三、dispatch_set_target_queue：为dispatch_queue_create生成的队列设置优先级

dispatch_set_target_queue(mySerialiDispatchQueue, globalDispatchQueueHigh);

四、dispatch_after

//  不是在x秒后执行DISPATCH_AFTER_BLOCK，而是x秒后把DISPATCH_AFTER_BLOCK追加到mainQueue
#define JAX_dispatch_after(x, dispatch_after_block) dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)((x) * NSEC_PER_SEC)), dispatch_get_main_queue(), dispatch_after_block);

JAX_dispatch_after(10.0, ^{
    NSLog(@"123");
})

五、dispatch_group

dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, globalDispatchQueueDefault, ^{
    [NSThread sleepForTimeInterval:3.0];
    NSLog(@"1");
});
dispatch_group_async(group, globalDispatchQueueDefault, ^{
    [NSThread sleepForTimeInterval:5.0];
    NSLog(@"2");
});
dispatch_group_async(group, globalDispatchQueueDefault, ^{
    [NSThread sleepForTimeInterval:1.0];
    NSLog(@"3");
});
//  dispatch_group_notify: 对多个处理全部结束之后再执行结束处理
dispatch_group_notify(group, mainDispatchQueue, ^{
    NSLog(@"main");
});

dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 3.5 * NSEC_PER_SEC);
long result =  dispatch_group_wait(group, time);
if (result == 0) {
    NSLog(@"等待时间里任务全部完成");
} else {
    NSLog(@"等待时间里任有未完成的任务");
}

//3
//1
//等待时间里任有未完成的任务
//2
//main

六、dispatch_barrier_async: 队列中的其他任务执行完成才会执行该任务。该任务完成才会去执行后续追加的任务。

七、dispatch_apply: 按指定的次数将指定的block追加到的dispatchQueue中

NSArray *array = @[@100, @200, @300];

dispatch_async(dispatch_get_global_queue(0, 0), ^{
    
    //  把3个block追加到queue中，执行不分先后顺序
    dispatch_apply(array.count, globalDispatchQueueHigh, ^(size_t index) {
        NSLog(@"%@", array[index]);
    });
    
    //  此时执行所有block执行结束
    
    //  返回主线程刷新UI
    dispatch_async(mainDispatchQueue, ^{
        NSLog(@"updateUI");
    });
    
});

八、dispatch_suspend \ dispatch_resume 队列挂起、恢复

九、dispatch_once: 执行只需运行一次的线程安全代码，单例

+ (instancetype)sharedInstance {
    static JAXClass *sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[self alloc] init];
    });
    return sharedInstance;
}

十、dispatch semaphore

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

//  计数设为1，保证可以访问arry的线程同时只能有一个
dispatch_semaphore_t sempahore = dispatch_semaphore_create(1);
NSMutableArray *array_semaphore = [NSMutableArray array];

for (int i = 0; i < 1000; ++i) {
    dispatch_async(queue, ^{
        
        //  等待sempahore、直到sempahore大于等于1， 一旦计数器大于或等于1，计数器-1
        dispatch_semaphore_wait(sempahore, DISPATCH_TIME_FOREVER);
        
        [array_semaphore addObject:@(i)];
        
        //  计数器+1
        dispatch_semaphore_signal(sempahore);
    });
}

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