GCD面试

各种队列的执行结果

规则：

只有异步，非主队列，才会开启新线程

死锁:

使用sync函数往当前串形队列添加任务，会卡住当前串形队列

主队列开启同步线程 1.队列的特点：排队，FIFO，先进先出 2.dispatch_sync需要马上当前线程完成 queue先进先出原理 包含（viewDidLoad，dispatch_sync两个任务队列）完成dispatch_sync需要viewDidLoad完成，dispatch_sync没法完成执行viewDidLoad就没法执行完

- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_get_main_queue();//主队列都放在主线程
    dispatch_sync(queue, ^{//同步
        for (int i = 0; i < 110; i++) {
            NSLog(@"执行任务1 %@",[NSThread currentThread]);
        }   
    });
}

    dispatch_queue_t queue = dispatch_queue_create( "myQueue", DISPATCH_QUEUE_SERIAL);//串行队列 队列的类型执行方式
    dispatch_async(queue, ^{//任务1
        NSLog(@"执行1");
        dispatch_sync(queue, ^{//任务2
            NSLog(@"这里也将产生死锁  任务1执行完才能执行");
        });
        NSLog(@"执行2");
    });

###以下代码打印结果

- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        NSLog(@"1");
        [self performSelector:@selector(test) withObject:self afterDelay:0];//本质添加runloop里面添加定时器（字线程默认没有启动rumloop）
        NSLog(@"3");
        //开启runloop
        //[[NSRunLoop currentRunLoop]addPort:[[NSPort alloc]init] forMode:NSDefaultRunLoopMode];
        //[[NSRunLoop currentRunLoop]runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];

    });
}
- (void)test {
    NSLog(@"2");
}
打印结果
2020-03-23 15:21:06.960451+0800 多线程[78991:1518734] 1
2020-03-23 15:21:06.961140+0800 多线程[78991:1518734] 3

- (void)test {
    NSLog(@"2");
}
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    NSThread *th = [[NSThread alloc]initWithBlock:^{
        NSLog(@"1");
        [[NSRunLoop currentRunLoop]addPort:[[NSPort alloc]init] forMode:NSDefaultRunLoopMode];
        [[NSRunLoop currentRunLoop]runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    }];
    [th start];
    [self performSelector:@selector(test) onThread:th withObject:nil waitUntilDone:YES];
}

#先执行任务1 任务2 最后执行任务3:

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    //创建队列数祖
    dispatch_group_t grounp = dispatch_group_create();
    //创建并发队列
    dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
    
    //创建异步任务
    dispatch_group_async(grounp, queue, ^{
        for (int i = 0; i < 5; i++) {
            NSLog(@"任务1===%@",[NSThread currentThread]);
        }
        
    });
    //等前面的任务执行完，会自动执行这个任务
    dispatch_group_async(grounp, queue, ^{
        for (int i = 0; i < 5; i++) {
            NSLog(@"任务2===%@",[NSThread currentThread]);
        }
    });
    dispatch_group_notify(grounp, queue, ^{
        for (int i = 0; i < 5; i++) {
            NSLog(@"任务3===%@",[NSThread currentThread]);
        }
    });
}