GCD的使用

1.同步 主线程 串行 死锁

// MARK: 同步 主线程 串行 死锁
- (void)syncMain {
    NSLog(@"0");
    dispatch_sync(dispatch_get_main_queue(), ^{
        NSLog(@"1 %@",[NSThread currentThread]);
    });
    NSLog(@"2 %@",[NSThread currentThread]);
    // 0
}

2.异步 主线程

// MARK: 1.异步 主线程(一般用做刷新UI操作) FIFO:(先进先出)
- (void)asyncMain {
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    dispatch_async(mainQueue, ^{
        NSLog(@"1 %@",[NSThread currentThread]);
    });
    dispatch_async(mainQueue, ^{
        NSLog(@"2 %@",[NSThread currentThread]);
    });
    dispatch_async(mainQueue, ^{
        NSLog(@"3 %@",[NSThread currentThread]);
    });
    // 1 2 3
}

3.线程通信

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
    // 耗时的操作  
    dispatch_async(dispatch_get_main_queue(), ^{  
        // 更新界面  
    });  
});

4.延迟执行

// GCD延迟执行
double delayTime = 2.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayTime * NSEC_PER_SEC);
dispatch_after(popTime, dispatch_get_main_queue(), ^{
    
});

// performSelector 延迟执行
[self performSelector:@selector(doSomeThing) withObject:self afterDelay:2];

// NSTimer 延迟执行 repeats:是否重复,设置为yes = 每2秒执行一次test
[NSTimer scheduledTimerWithTimeInterval:2.0 target:self selector:@selector(test) userInfo:nil repeats:NO];

5.组

// MARK: group_async
- (void)group_async {
    dispatch_group_t group = dispatch_group_create(); 
    dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0); 

    dispatch_group_async(group, globalQueue, ^{
        NSLog(@"1  %@",[NSThread currentThread]);
    });
    dispatch_group_async(group, globalQueue, ^{
        NSLog(@"2  %@",[NSThread currentThread]);
    });
    dispatch_group_async(group, globalQueue, ^{
        NSLog(@"3  %@",[NSThread currentThread]);
    });
    
    dispatch_group_notify(group, globalQueue, ^{

    });
    // 1 2 3 无序
}

6. 栅栏方法 dispatch_barrier_async

有时需要异步执行两组操作，而且第一组操作执行完之后才能开始执行第二组操作。如果没有dispatch_barrier_async这个方法，这4个任务执行是没有先后顺序的，完全随机，那么加上这个栅栏方法之后，会严格按照先随机顺序执行完栅栏之前的所有任务，然后再执行栅栏方法之后的任务。

dispatch_barrier_async 函数只有在 DISPATCH_QUEUE_CONCURRENT 队列中才起作用,
在全局并发队列 dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) 中无效

// MARK: 栅栏方法 dispatch_barrier_async
- (void)dispatch_barrier_async {
    // 这里不能使用 dispatch_get_global_queue
    dispatch_queue_t queue = dispatch_queue_create("abc", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_async(queue, ^{
        NSLog(@"1  %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"2  %@", [NSThread currentThread]);
    });
    
    // 子线程
    dispatch_barrier_async(queue, ^{ 
        NSLog(@"/ %@", [NSThread currentThread]);
    });
    
    // 主线程
//    dispatch_barrier_sync(queue, ^{
//        NSLog(@"// %@", [NSThread currentThread]);
//    });
    
    dispatch_async(queue, ^{
        NSLog(@"3  %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"4  %@", [NSThread currentThread]);
    });
    //  1 2 / 3 4
}