Use of serial queues

1. Declare member variables

/**
  GCD  Global queues are provided in 
   however , In a third-party framework , For greater flexibility , You usually create queues yourself 
 */
@interface ViewController (){
    //  Serial queue member variables 
    dispatch_queue_t _queue;
}
@end

2. Initialize the creation of serial queues

_queue = dispatch_queue_create("com.me.queue", DISPATCH_QUEUE_SERIAL);

3. test

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event{
    [self demo1];
}

/// Serial queue front set use   Asynchronous normal , Synchronization will hang up , Wait for each other 
-(void) demo1 {
    // Serial queue , Synchronous execution cannot be nested 
    dispatch_sync(_queue, ^{
        NSLog(@"AAAAAA");
        
        dispatch_sync(_queue, ^{
            NSLog(@"BBBBB");
        });
    });
}

/// Demonstration of serial queue 
-(void) demo{
    for (int i = 0; i<10; i++) {
        // Time consuming operations require asynchronous execution 
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            // Database operation needs to be guaranteed at the same time , There can only be one operation 
            // Synchronous execution  -  Serial threads have no ability to open threads 
            // Sync , Able to , Even when executing all database operation tasks asynchronously , It can still guarantee the order 
            dispatch_sync(_queue, ^{
                NSLog(@"%@ %d",NSThread.currentThread, i);
            });
        });
    }
}